Regulatory Guide 5.23: Difference between revisions

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{{Adams
{{Adams
| number = ML13064A072
| number = ML003740013
| issue date = 05/31/1974
| issue date = 02/29/1984
| title = in Situ Assay of Plutonium Residual Holdup
| title = (Task SG 045-4), Revision 1, in Situ Assay of Plutonium Residual Holdup
| author name =  
| author name =  
| author affiliation = US Atomic Energy Commission (AEC)
| author affiliation = NRC/RES
| addressee name =  
| addressee name =  
| addressee affiliation =  
| addressee affiliation =  
Line 10: Line 10:
| license number =  
| license number =  
| contact person =  
| contact person =  
| document report number = RG-5.023, Rev 1
| document report number = RG-5.23 Rev 1
| document type = Regulatory Guide
| document type = Regulatory Guide
| page count = 16
| page count = 20
}}
}}
{{#Wiki_filter:May 1974 U.S. ATOMIC ENERGY COMMISSION
{{#Wiki_filter:Revision 1*
                                REGULAT(OR Y G U I D[E
                                                                                                                                    February 1984 U.S. NUCLEAR REGULATORY COMMISSION
                                'DIRECTORATE OF REGULATORY STANDARDS
                          REGULATORY GUIDE
                                                                      REGULATORY GUIDE 5.23 IN SITU ASSAY OF PLUTONIUM RESIDUAL HOLDUP
                          OFFICE OF NUCLEAR REGULATORY RESEARCH
                                                        REGULATORY GUIDE 5.23 (Task SG 0454)
                                  IN SITU ASSAY OF PLUTONIUM RESIDUAL HOLDUP
                                                                            information obtained in the holdup survey can be used to


==A. INTRODUCTION==
==A. INTRODUCTION==
2. When the limit of error of Pu holdup is not compatible with constraints on the overall LEMUF, the Part 70, "Special Nuclear Material," of Title 10 of                           information obtained in the holdup survey can be used the Code of Federal Regulations requires licensees                                   to locate principal Pu accumulations and to assure that authorized to possess more than one kilogram of                                       other areas of the process contain less than the detectable plutornium to calculate a material balance based on a                                amount of plutonium. Once located, substantial accu- measured physical inventory at intervals not to exceed                               mulations can be recovered, transforming the plutonium two months. Further, these licensees are required to                                 to a more accurately measurable inventory component.
locate principal plutonium accumulations and to ensure that other areas of the process contain less than the Part 70, "Domestic Licensing of Special Nuclear                         detectable amount of plutonium. Once located, substantial Material," of Title 10 of the Code of Federal Regulations                   accumulations can be recovered, transforming the pluto requires licensees authorized to possess more than 1 kilo                    nium to a more accurately measurable inventory compo gram of plutonium to calculate a material balance based                     nent. Having reduced the amount of plutonium holdup, on a measured physical inventory at intervals not to                         the standard error on the remeasurement of the remaining exceed 2 months. Further, these licensees are required to                     holdup may be sufficiently reduced to be compatible with conduct their nuclear material physical inventories in                        overall SEID requirements.


conduct their nuclear material physical inventories in                              Having reduced the amount of plutonium holdup, the compliance with specific requirements set forth in Part                             limit of error on the remeasurement of the remaining
compliance with specific requirements set forth in Part 70.
    70. Inventory procedures acceptable to the Regulatory                                holdup may be sufficiently reduced to be compatible staff are detailed in Regulatory Guide 5.13, "Conduct of                            with overall LEMUF requirements.


Nuclear Material Physical Inventories."
Inventory procedures acceptable to the NRC staff are                              Any guidance in this document related to information detailed in Regulatory Guide 5.13, "Conduct of Nuclear                       collection activities has been cleared under OMB Clearance Material Physical Inventories."                                               No. 3150-0009.
 
Plutonium residual holdup is defined as the plutonium                                             


==B. DISCUSSION==
==B. DISCUSSION==
Plutonium residual holdup is defined as the
inventory component remaining in and about process equipment and handling areas after these collection areas                        Plutonium accumulates in cracks, pores, and zones of have been prepared for inventory. Whenever possible,                        poor circulation within process equipment. The walls of process equipment should be designed' and operated so as                    process vessels and associated plumbing often become to minimize the amount of holdup. In this guide, proce                      coated with plutonium during solution processing. Surfaces dures acceptable to the NRC staff for the in situ assay of                  internal and adjacent to process equipment, especially the plutonium residual holdup are described.                                glovebox walls and floors, accumulate deposits of pluto nium that can become appreciable. Plutonium also accu Assay information may be used in one of two ways:                      mulates in air filters and associated ductwork. The absolute amounts of plutonium holdup must be small for efficient
,*  plutonium inventory component remaining in and about                                       Plutonium accumulates in cracks, pores, and zones process equipment and handling areas after those                                      of poor circulation within process equipment. The walls collection areas have been prepared for inventory.                                    of process vessels and associated plumbing often become Whenever possible, process equipment should be                                        coated with plutonium during solution processing.
      1. When the standard error (estimator) of plutonium                    processing and proper hazards control. However, the total holdup is compatible2 with constraints on the overall                        holdup can be large relative to the plant inventory differ standard error of the inventory difference (SEID), the mate                  ence (ID) but have no significant impact on the ID if it rial balance can be computed using the measured contents                    remains reasonably constant. It is the change in the of plutonium holdup. Additional cleanout and recovery for                    holdup between beginning inventory and ending inventory accountability will then not be necessary.                                  that may impact the ID.
 
2. When the standard error of plutonium holdup is not                        The measurement procedures described in this guide compatible with constraints on the overall SEID, the                        involve the detection of gamma rays and neutrons that are spontaneously emitted by the plutonium isotopes. Because are    the gamma rays of interest are emitted by the major isotope, Design features to minimize holdup in process equipment
        1 the subject of a series of regulatory guides (5.8, 5.25, and
                                                                      5.42).    23gpU, gamma ray assay is the preferred method whenever          239 its acceptance criteria are satisfied. The amount of                  pu
        2 Compatibility exists when the contribution of the standard to error of the holdup to the total plant SEID is not large enough cause the overall SEID to exceed allowed limit
 
====s. If the plant SEID====
    exceeds allowed limits because of an excessive contribution from          The substantial number of changes in this revision has made it the holdup standard error, compatibility does not exist and the            impractical to indicate the changes with lines in the margin.


designed* and operated so as to minimize the amount of                               Surfaces internal and adjacent to process equipment, holdup. In this guide, procedures are detailed for the in                            especially glove box walls and floors, accumulate situ assay of the residual plutonium holdup.                                          deposits of plutonium which can become appreciable.
remedial steps of paragraph 2 needto be taken.


Plutonium also accumulates in air filters and associated Assay information can be used in one of two ways:                            ductwork. The absolute amounts of plutonium holdup must be small for efficient processing and proper hazards I. When the limit of error of plutonium holdup is                                   control. However, the total amount of plutonium compatible with constraints on the overall limit of error                            holdup may be significant in the context of the tolerable on the facility MUF (LEMUF), the material balance can                                facility MUF.
Comments should be sent to the Secretary of the Commission, USNRC REGULATORY GUIDES                                   U.S. Nuclear Regulatory Commission,         Washington, D.C. 20555, Attention: Docketing and Service Branch.


be computed using the measured contents of Pu holdup.
Regulatory Guides are issued to describe and make available to the public methods acceptable to the NRC staff of Implementing                    The guides are issued in the following ten broad divisions:
  specific parts of the Commission's regulations, to delineate        tech niques used by the staff in evaluating specific problems or postu            1. Power Reactors                  6. Products lated accidents    or to provide guidance  to applicants.  Regulatory        2. Research and Test Reactors        7. Transportation Guides are not substitutes for regulations, and compliance          with    3. Fuels and Materials Facilities 8. Occupational Health them is not required. Methods and solutions different  from those set        4. Environmental and Siting          9. Antitrust and Financial Review out in the guides will be acceptable if they  provide  a  basis  for  the    5. Materials and Plant Protection 10. General findings requisite to the issuance or continuance of a permit or                                                                        Government license by the Commission.                                                    Copies of issued guides may be purchased at the current  guides in spe Printing Office price. A subscription service for future          Office.


Additional cleanout and recovery for accountability will                                    The measurement procedures detailed in this guide then not be necessary.                                                              are based on the controlled observation of gamma rays and neutrons which are spontaneously emitted by the
from                                                              Printing This guide was Issued after consideration of comments received                cific divisions is available through  the Government prices may the public. Comments and suggestions for improvements          in these      Information on the subscription service and current GPO    Commission, guides are encouraged at all times, and guides will new Informaas be revised,          be obtained by writing the U.S.       Nuclear  Regulatory appropriate, to accommodate comments and to reflect                          Washington, D.C. 20555, Attention: Publications Sales Manager.
    "Design features to minimize holdup in process equipment are                        plutonium isotopes. Because the gamma rays of interest the subject of a seriý of rgulatory guides.                                        are emitted by Pu-239, garnma ray assay is the preferred Copies of published guides may be obtained by rsquast indicating the divisions USAEý REGULATORY GUIDES                                      dosircd to the US. Atomic Enrgty Commission, Washington, D.C. 2054'.
                                                                                        Attention: Director of Regulatory Standards. Comments and suggestions for Regulatory Guides we issued to describe and make avaiille to the public inmprovements in thes guides ere encouraged and should be sent to the Secretary methods acceptable to the AEC Regulatory staff of implementing specific parts of  of the Commission, U.S. Atomic Energy Commission. Washington. D.C. 20645.


the Commission's regulations, to delineate techniques .-.ed by the staff in        Attention: Chief. Public Promedinga Staff.
tion or experience.


evaluating specific problems or postulated accidents: or to provlde guidance to epplicents. Regulatory Guides we not substitutes for regulations arnd  comoliancs  The guidas ea issued in the following ton broad divisions:
holdup in a piece of equipment is proportional to the              relatively uniform cross section. When a collection zone measured intensity of the emitted gamma rays after suitable        contains a complex item of equipment with significant corrections are made for attenuation by intervening mate            self-shielding properties, the uncertainty in the holdup, rials, for self-attenuation by plutonium, for scattering, for      measurement may be primarily due to attenuation of K
    with them is not required. Methods and solutions different from those sit out in the guides will be acceptable if they provide a basls for the findings requisot to                                              S. Produects
geometrical factors, and for background radiation.                 radiation in the internal structure. In such cases, neutron assay from the outside and thermoluminescent dosimeter If plutonium is held up in furnaces, grinders, or other        assay from the inside may be applicable.
                                                                                          1. Power eactors the issuanc or continuance of a pearmil or licemniby the Comnission.                 2. Resmrch and Test Reactors             


===7. Transportation===
heavy equipment that is too dense to permit the escape of gamma rays, an assay based on spontaneous fission neutrons              If delineation of collection zones is not possible, two from the even isotopes of plutonium may be possible. This          alternatives are experiments with mockup geometries or technique requires knowledge of the isotopic composition            complex numerical calculations.
                                                                                          3. Fuels and Materials Facilities        S. Occupational l'slooh


* Published guidet will be revised periodically, as appropriate, to accommodatei        4. Envwonnmental and Siting              9. Antitrust Revow S. Materials and Plant Protection        10. General comments end to reflict new information or experience.
of the plutonium, some knowledge of its chemical form, and knowledge of the presence of other radionuclide                2. APPLICABLE METHODS AND INSTRUMENTS
impurities.


assay method whenever its acceptance criteria are                   system with sufficient resolution to measure the activity satisfied. To accomplish either gamma ray, or neutron              from one or'two~isolopes o-Thinterest.
Two considerations are critical to the selection of Thermoluminescent dosimetry is a third technique that          methods and instruments. First, to perform an assay, one can be used to measure holdup from the inside of large              must ensure that the plutonium radiations reach the detector pieces of equipment. This technique is also useful for              and are detected. Second, the observed response must be carrying out measurements in an unobtrusive manner                  attributable to the collection zone being assayed. Therefore, outside normal plant operating hours.                              the assay scheme is developed around penetrating radiations, and the detector is collimated to provide for sufficient For all three techniques, the proportionality factors          directionality in the response to resolve a collection zone between amount of holdup and detector response are best            from its neighboring zones and from the background.


assay, it is essential to consider the facility in terms of a Gamma ray assay has an ' advantage , over series of zones which can be independently assayed.
determined prior to the holdup measurement by assays of            Finally, some effort may be necessary to employ external
                                                                    "shadow shielding" to block radiation being produced in known quantities of plutonium distributed in well-defined and representative geometries, as discussed below.                  adjacent collection zones from the field of view of the collimated detector.


neutron assay in that the emissions are primarily from Such zones are designated as "collection zones."
1. DELINEATION OF COLLECTION ZONES
                                                                      the principal isotopes qf linterest. -Because of the high emission rate of gammna rays, a detection sensitivity of
                                                                    2.1 Gamma Ray Assay Typical plutonium process facilities comprise a number of interconnected gloveboxes that contain work areas and                 Under closely controlled conditions, the measured most process equipment, in-process storage areas, and              plutonium gamma ray spectrum can be interpreted in terms self-contained process equipment. Also, solution process            of the abundance of each gamma ray emitter present in the ing requires tanks, plumbing, and pumping equipment,                sample. Because of the large number of gamma rays (Refs. 1 which are often located in close proximity to the glovebox          and 2) present, many regions of the observed spectrum are lines. Finally, storage areas for feed, scrap and waste, and         characterized by overlapping lines. To accomplish the assay, final product are often located in close proximity to the          it is necessary to select an appropriate spectral region and plutonium process area.                                             provide a detection system with sufficient resolution to measure the activity from the isotopes of interest.
  1.    Delineation of Collection Zones less than one gram is generally attainable..
      Typical plutonium process facilities comprise a                           The most useful portion of the spec trum for number of interconnected glove boxes which contain                   holdup assay is the Pu-239 gamma ray complex in the work areas and most process equipment, in-process                   375-440 keV range. The-yields of these lines are given in storage areas, and self-contained process equipment.                 Table B.l.


Also, solution processing requires tanks, plumbing, and                                                  Table B.1 pumping equipment, which are often located in close proximity to.the glove box lines. Finally, storage areas                  PROMINENT GAMMA',RAYS FROM Pu-239 in for feed, scrap and waste, and final product are also                                ENERGY RANGE 375-440 keV
To accomplish the holdup measurements, it is essential to consider the facility in terms of a series of zones that can         Gamma ray assay has an advantage over neutron assay in be independently assayed. Such zones are designated                 that the emissions are primarily from the principal isotopes
often located in close proximity to the plutonium process area.                                                              Energy                                    Intensity  (- /sec-g Pu-239)
  "collection zones." Each plutonium-processing facility can          of interest. Because of the hiih emission rate of gamma be conceptually divided into a series of contiguous collection       rays, a detection sensitivity of less than 1 gram is generally attainable.
      Each facility can be divided into a series of                         375.0 ........................                      3.59  x J04.l collection zones on the basis of a logical understanding                  380.2 ......................                      0.70    x 10
of process activities. Individual glove boxes can be                       382.7 .......................                        0.59  x 104 subzoned to improve assay performance, but for most                        392 ..5 ................                ......      0.26  x,104 applications, individual glove boxes are -examples of                      393.1 ..........                 ....    .....    1.01 x104 .
suitable size areas for discrete collection zones.                         413.7 ......................                        3.43 x I04
                                                                            422.6 ...............                              ..0.27 x 104 Gamma ray assay for plutonium holdup measurement is practical when a collection zone consists                                                        Total          9.85 x 104 of a single structure of relatively uniform cross section.


When a collection zone contains an item of equipment                              2.1.1 -'Gamma Ray Detection Instruments.
zones on the basis of process activities and collection geometries. Individual machines, filters, pipes, tanks, The most useful portion of the spectrum for holdup gloveboxes, or surface areas that can be isolated from one another may be suitable discrete collection zones. Great            assay is the 2 3 9 pu gamma ray complex in the 375- to
                                                                      45 0-keV range. 3 The yields of these lines are given in care is needed to define all collection zones so that (1) the Table 1.


having significant shielding properties and capable of contributing to the holdup, the uncertainty in the                                     Gamma, ray detection-systems consist of a holdup prediction based on the observed response may                  scintillation or -semiconductor detector sensitive to become primarily due to attenuating the radiations in                 gamma rays and . appropriate -.electronics. 3 Required the internal structure. In such cases, neutron assay is              electronics include lat least a single-ýchannel analyzer and applicable.                                                           a timer-scaler unit.- A second :single- channel analyzer used to determine the background radiation correction is
assay of the zone can be performed with a minimum of interference from nearby zones, (2) the detector can be positioned reproducibly and in such a way that the radiation being measured experiences a minimum, or easily predicted, attenuation in the apparatus being measured, and (3) the                  3
2.    Applicable Methods and Instruments                              a time-saving feature. Battery powered systems are commercially. available and can provide operational Two ,considerations are critical to the selection of           convenience, particularly in this application.
                                                                              1n typical Nal detectors (with energy resolution of 10 percent distribution of material in the zone can be represented by          at 414 keV), the 414-keV photopeak will produce counts in the one of the distribution geometries used in the calibration          approximate energy range of 373 to 455 keV. Thus, an energy window from 375 to 450 keV will include most of the 414-keV
  procedure described below.                                           full-energy counts for a variety of detector resolutions. Further more, suca, window setting will include a significant fraction of the
                                                                      375-keV        P9Pgamma rays (see Table 1), but will also exclude Gamma ray assay for plutonium holdup is practical                    'mosorftihe otentially interfering 332-keV gamma rays from when the collection zone consists of a single structure of
                                                                5.23-2


methods and instruments. First, to perform an assay, the plutonium radiations must reach the detector, and be                                  The detection efficiency and res6lution of detected. Second, the observed response must be                        good Nal(Tl) detectors is'generally adequate for this attributable to the collection zone being assayed.                    application. CdTe, Ge(L), and-intrinsic 'Ge:detectors Therefore, the assay scheme is developed around                      have better resolution than Nal(TI) but: cost more, are penetrating radiations and the detector is collimated to              generally less available, and are more difficult to operate.
Table 1                                    2 inches (5 cm) with a thickness of 2 inches is recommended.


provide for sufficient directionality in the response to resolve a collection zone from its neighbor zones and                            -' -  The 332.3 keV- gamma-ray from U-237, a from the background.                                                  short-lived (6.75 d) daughter -of Pu-241, is usually the principal interference for. Pu-239 assay by Nal detection
For germanium detectors, a moderate-volume coaxial
      2.1 Gamma Ray Assay                                            of the 375-440 keV complex. If the U-237 is in equilibrium with Pu-241, the intensity of this gamma ray Under closely controlled conditions, the                  is 1.15 x 106 7t/sec-g Pu 124l.
                                                        23 9                detector is recommended.


measured plutonium gamma ray spectrum can be interpreted in terms of the abundance of each gamma                                    Since this gamma ray is also emitted inthe ray emitter present in the sample. Because of the large              decay of Am-241., the. interference from this decay number of gamma rays', 2 present, many regions of the                branch may also be important in case -of preferential observed spectrum are characterized by overlapping                    americium holdups. To avoid this interference when lines. To accomplish the assay, it is necessary to select an          using Nal detectors, the assay-energy window is adjusted appropriate spectral region and provide a detection                  to span the range from 390 to 440 keV.
PROMINENT GAMMA RAYS FROM                            pu IN 375-    to 450-keV    ENERGY      RANGE
                                                                                2.1.2 Collimatorsand Absorbers for Gamma Rays Intensity
                                            2 39 Pu)                            A shaped shield constructed of any heavy-element Energy (y/sec-g material is appropriate for gamma ray collimation. For cost,
                      375.0    3.59  x  104                              availability, and ease of fabrication, lead is recommended.


5.23-2
380.2    0.70  x  104                              Less than 2 percent of all 400-keV gamma rays striking a
                      382.7    0.59  x  104                              1.5-cm-thick sheet of lead will pass through without suffer
                                0.26  x  104                              ing an energy loss.


Detector dimensions are selected to                        2.1.4 Calibration    Source for Gamma Ray provide a high probability for detecting the appropriate                        Assay gamma rays. The geometric detection efficiency increases as the square of the detector radius; however,                        To calibrate a collection zone, the observed the weight of the gamma ray shielding material required          assay -response is compared to the response obtained to collimate the detector also increases ;when larger            when the zone contains a known amount of plutonium.
392.5
                      393.1      1.01  x  104
                      413.7    3.43  x  104                                  The collimator will be most effective when it is con
                      422.6    0.27  x  104                              centric about the crystal and photomultiplier and com pletely covers the photomultiplier bas


detectors are used. The crystal depth is chosen such that most of the gamma rays of interest will lose all their                          Because of the complexity of the assay, the energy within the crystal;                                      response is assumed to be linear. To be representative of typical holdup situations, the calibration standard is To reduce the pile-up of low energy              prepared as an encapsulated disk with a bed thickness of radiations, the crystal face can be covered with an              less than 0.2 cm. Care must be exercised in the appropriate shield (e.g., 0.075 cm cadmium). This                preparation of the calibration standard to ensure that procedure will reduce counter dead time effects without          the amount encapsulated of total plutonium, Pu-239, significantly affecting assay results.                           and the amount of Amn-241, is known. It is important to measure the gamma ray attenuation 'through the
====e. Extending the====
            2.1.2 Collimators for Gamma Rays                      encapsulating material and correct the calibration standard response to compensate for that attenuation.
                                  9.85 x 104                                collimator forward of the crystal at least a distance equal to Total half the diameter of the crystal, and preferably the full diameter, is recommended (Ref. 5). Making this distance variable to reproducible settings will permit adjustment over a range of collection zone sizes. However, it is highly
    2.1.1 Gamma Ray Detection Instruments                                    desirable to select collection zones and counting geometries so that one collimator setting will suffice for all measure Gamma ray detection systems consist of a scintillation                  ments. This will simplify the calibration procedures because or semiconductor gamma ray detector and appropriate                          the calibration constants depend strongly on the dimensions electronics (Refs. 3 and 4). Required electronics include at                and placement of the collimator aperture.


A shaped shield constructed of any dense          The amount of plutonium encapsulated in 'the gamma material is appropriate for gamma ray collimation. For          ray calibration standard is selected to be representative cost, availability, and ease of fabrication, lead is            of typical accumulations.
least a single-channel analyzer and a timer-scaler unit. A
second single-channel analyzer viewing the same detector                        The collimator not only defines the effective field of pulses used to determine the background radiation correc                    view but also shields the detector from unwanted radiation.


recommended. Less ,than 2% of all 400 keV gamma rays striking a 1.5-cm-thick sheet of lead will pass through              2.2 Neutron Assay without having suffered an energy loss.
tion is a timesaving feature. A number of portable battery                    To effectively accomplish this latter purpose, the collimator powered systems are commercially available for this applica                  material must also cover the rear of the detector. This is tion.


Neutrons are emitted in the spontaneous fission The collimator will be most effective when        of Pu-238, Pu-240, and Pu-242 and through the it is concentric about the crystal and photomultiplier          interaction of emitted a particles with certain light and completely covers the photomultiplier base.                  nuclei. These neutrons suffer little attenuation in passing Extending the collimator forward of the crystal at least a      through uranium or plutonium or through most distance equal to half the diameter of the crystal, and          structural and containment materials. Glove box preferably the full diameter, is recommended. 4 Making          windows may reduce the energy of emerging neutrons, this distance variable to reproducible settings will permit      but because of their regular and constant shape, their adjustment over a range of collection zone sizes.                effect can generally be factored into the assay calibration.
usually easy to achieve with portable Nal detectors but requires more effort when germanium detectors are used.


2.1.3 Check Source for Gamma Ray Assay To be useful for the assay of plutonium holdup, It is important to check the operation of        the neutron production rate per gram of plutonium must the detection system prior to each inventory sequence.
The detection efficiency andresolution (10 to 15 percent)                    Intensive 50- to 100-keV X-ray radiation and 60-keV
of NaI(Tl) is generally adequate for holdup measurements.                   241 Am    gamma ray radiation are often emitted by process CdTe, Ge(Li), and high-purity germanium (also known as                      equipment, and this radiation can tie up the detector intrinsic germanium) detectors have better resolution than                  electronics unnecessarily. A 1.5-mm-thick layer of lead (on NaI(Tl) but are more costly and more difficult to operate.                  the outside) and a 0.75-mm-thick layer of cadmium (on the For more information on Ge(Li) and intrinsic germanium                      inside) may be placed against the front face of the detector detectors, see Regulatory Guide 5.9, "Guidelines for to alleviate this problem. This graded energy shield will Germanium Spectroscopy Systems for Measurement of absorb most of the low-energy photons incident on the Special Nuclear Material," and the references cited therein.                detector without substantially reducing the number of gamma rays detected in the 375- to 450-keV range.


be known. The spontaneous fission contribution to the Either recalibrating one or more collection zones and           total neutron production can be computed from basic comparing the results'to previous analyses or testing the        nuclear data, once the isotopic composition of the instrument with an appropriate check source is                   contained plutonium has been determined. Computing appropriate. When the performance remains within the             the (an) contribution requires a knowledge of the expected value,, the previous calibration data are               chemical form of the plutonium and the amount and, assumed to be valid. If not, theenergy window may have          distribution of certain high (an) yield target materials.
237 The 332.3-keV gamma ray from                    U, a short-lived
                              24 1                  the principal inter          2.1.3 Check Source for Gamma Ray Assay
  (6.75 days) daughter of          pu,  is usually ference  for  2 39 Pu  assay  by  Nal  detection  of  the 375- to
                                        7                                        It is important to check the operation of the detection
  450-keV      complex.    If  the 2 3 U is in equilibrium with
  24 1 pu, the intensity of this gamma ray is 1.15 x 106                      system each time the instrumentation is moved or otherwise disturbed (e.g., power outage) during the course of each y/sec-g 2 pu. Since this gamma ray is also emitted in the
              4 1 inventory sequence. Either recalibrating one or more decay  of  241 Am, the interference from this decay branch collection zones and comparing the results to previous may also be important in case of preferential americium                      analyses or testing the instrument with a check source is holdups. To avoid this interference when using Nal detectors,                appropriate. When the response remains within the expected the assay energy window is adjusted to span the range from                  value, the previous calibration data are assumed to be valid.


shifted, or the unit may be in need of repair and recalibration.
390 to 450 keV for plutonium holdup with high americium                    If not, the energy window may have shifted, or the unit content.


The background count rate from neutron detectors may be a substantial part of the observed An appropriate check source enables the activity, often corresponding to as much as 20 g of stability of the assay instrument to be tested at any plutonium in typical holdup assays. Thus, neutron assay location. Such a source can be prepared by implanting a is primarily applicable to the measurement of significant small encapsulated plutonium source (containing -0.5 g accumulations of plutonium.
may be in need of repair and recalibration.


Pu) in the face of a plug of shielding material. The plug is shaped to fit and close the collimator channel, and the source is positioned to be adjacent to the crystal when                    The measured neutron yield from prepared the plug is in place.                                            calibration standards is used to calibrate each neutron assay collection zone. In the Appendix, a method is The check source is fabricated in a manner        given to calculate the anticipated neutron yield. This to ensure its internal stability. Other than .radiations          method provides the ability to calculate the neutron increasing from the ingrowth of Am-241, the emission              yield when the isotopic or impurity composition of the rate of the check source should remain constant.                  plutonium holdup is different from that of the
An appropriate check source enables the stability of the Detector crystal dimensions are selected to provide a assay instrument to be tested at any location. Such a source high probability of detecting gamma rays from the 375- to can be prepared by implanting a small encapsulated plutonium
                                                          5.23-3
  450-keV complex and a low probability of detecting sample (containing '.,0.5 gram of plutonium) in the face of high-energy radiation. For NaI, a crystal diameter of
                                                                        5.23-3


calibration standards. The method can be used to                by stopping neutrons coming to the detector from all calculate a ratio of the neutron production rate of the          directions --except the . -.desired one. The cadmium unknown material to the standard material neutron                surrounding the detector will...stop essentially all production rate. The yield from the holdup material is          neutrons striking, the, detector with energies below 0.4 then determined by multiplying the measured "known"              eV. By adding moderator material around the.outside of material yield by the computed ratio.                            the,.<detector in -all -directions except .for the collimator channel, neutrons, coming from unwanted directions will
a plug of shielding material. The plug is shaped to fit and           low gamma ray sensitivity in the detectors are important.
          2.2.1 Neutron Detection Instruments                    lose energy~in 'this shield and will be absorbed in the Cd cover. For each six inches of polyethylene added, the To effectively employ the spontaneous            collimator assembly provides a factor of approximately neutron yield as a measure of plutonium holdup, it is            ten: in -the directionality of the response. An example of necessary to detect the neutrons in the vresence of a            a collimated ,neutron detector assembly for plutonium more intense gamma ray background and to collimate              holdup assay is shownin Figure B-I.


the detector so that the only neutrons being counted are emanating from the collection zone under assay.                                  The weight of the .combined detector and collimator assembly. can easily exceed requirements for a
close the collimator channel, and the source is positioned to        Gas-filled proportional counters containing He or BF 3 are be adjacent to the crystal when the plug is in place.                 suitable for this purpose. Typical fill pressures are 1 to 4 atmospheres. One advantage of 3 He for in-plant applications The check source is fabricated in a manner that will              is that the operating voltage of 3He counters is about ensure its internal stability. Other than radiations increasing      75 percent of that required for BF 3 counters.
              _,Holdup assay -is performed under in-plant        hand-held detector probe. 4 For this reason, and to conditions where ruggedness, high detection efficiency,          provide for reproducible positioning at each assay, a and high (-y,n) rejection, performance in the detectors is      sturdy cart housing both the detector/collimator and the important. He-3 has one advantage over BF 3 detector*            associated-'electronics is recommended; Further, as the tubes in that the operating voltage for He-3' tubes does        items to be assayed will be at different, heights, the not increase as rapidly with increased gas pressure.            ability to raise .and lower. the assembly to reproducible settings is ,recommended to expedite the assay and To increase the efficiency of the system,        reduce the possibility of errors.


detector gas pressure in the tubes may be increased or multiple detectors can be connected in parallel to feed a                    2.2.3 Check Source for Neutron Assay common preamplifier.
from the ingrowth of 2 4 1 Am, the emission rate of the check source should remain constant.                                      The efficiency of 3 He and BF 3 counters increases as the energy of the neutrons decreases. Embedding gas-filled
2.2 Neutron Assay                                                    counters in polyethylene to moderate the incoming neutrons to thermal or epithermal energies will improve their effi Neutrons are emitted in the spontaneous fission of 238Pu,        ciency. A nearly optimum design can be obtained by center
24&deg;Pu, and 242Pu and through the interaction of emitted              ing the counters in 10 cm of polyethylene with 2 to 3 cm alpha particles with certain light nucleL These neutrons              of polyethylene between adjacent counters.


To ensure the proper operation of the He-3 and BF3 detectors have efficiencies          neutron assay system prior to making an assay, it is which increase as 'the energy of the neutrons decrease.           necessary to test the response of the instrument. An To take advantage of this characteristic, the detectors          appropriate , neutron . assay check source can be can be surrounded by a 'neutron moderating material              measured, Por one or more :collection zones can be (see Figure B1I). Polyethylene is recommended. The                recalibrated and compared to the results of previous thickness of the moderator is important. When the                calibrations.
suffer little attenuation in passing through uranium or plutonium or through most structural and containment                    To shield the detector from low-energy neutrons that materials. Glovebox windows may reduce the energy of                may produce a complicated response pattern, the modera emerging neutrons, but, because of their regular and con              tor material is covered with a thermal neutron absorber.


moderating distance is short, a fraction of the higher energy neutrons pass through the gas chamber without                              An appropriate neutron assay check source being detected. Conversely, when the moderating                  can be prepared by implanting a small encapsulated distance is too long, a substantial number of low-energy          plutonium source (containing about 5 g Pu).into the face neutrons are absorbed by the hydrogen contained in the            of a plug. of neutron moderating material .(see Figure moderator. A balance -between these, two effects is              B-2). The plug is .fabricated to fit and close the reached when -the spacing between adjacent tubes is              collimator channel.
stant shape, their effect can generally be factored into the         Cadmium sheeting approximately 0.075 cm thick may be assay calibration.                                                   used for this application.


approximately one-inch of polyethylene, and the -spacing between the front of the unit and the detectors and the                    - 2.2.4 Calibration Source for, Neutron Assay back of the unit and the detectors is approximately I1%
To be useful for the assay of plutonium holdup, the                   2.2.2 Collimatorsfor Neutron Detectors neutron production rate per gram of plutonium must be known. The spontaneous fission contribution to the total                  To assay a specific collection zone in the presence of neutron production can be computed from basic nuclear                other distributed sources of plutonium, it is necessary to data after the isotopic composition of the contained                  collimate the detector. This is accomplished by stopping plutonium has been determined. Computing the (ca,n)                  neutrons coming to the detector from all directions except contribution requires a knowledge of the chemical form of            the desired one. The cadmium surrounding the detector will the plutonium and the amount and distribution of certain              stop essentially all neutrons striking the detector with high-(ca,n)-yield target materials.                                  energies below 0.4 eV. By adding moderator material around the outside of the detector in all directions except The background count rate from neutron detectors may              for the collimator channel, neutrons coming from be a substantial part of the observed activity, often corre          unwanted directions will lose energy in this shield and will sponding to as much as 20 grams of plutonium in typical              be absorbed in the cadmium cover. For each 6 inches holdup assays. Thus, neutron assay is primarily applicable            (15 cm) of polyethylene added, the collimator assembly to the measurement of significant accumulations of pluto              provides a factor of approximately 10 in the directionality nium.                                                                 of the response.
inch when one-inch-diameter tubes are used, and                                  To calibrate a neutron assay collection approximately one inch when two-inch-diameter tubes              zone, the observed response is compared to the response are used.                                                         obtained when the zone contains an additional known amount of plutonium. Neutron assay is less sensitive to
              'To -shield the detector, from low-energy          attenuation than.is.,gamma ray assay.. It is important to neutrons which may produce a complicated response                know how:..much plutonium is dencapsulated in the pattern, the moderator material is covered .with a                neutron assay calibration standard, and the isotopic thermal neutron absorber. Cadmium sheeting approxi-              composition of that plutonium.


mately .0.075 cm thick can be used for this application.
The measured neutron yield from prepared calibration                  An example of a collimated neutron detector assembly standards is used to calibrate each neutron assay collection          for plutonium holdup assay is shown in Figure 1. This zone. In the appendix to this guide, a method is given to            assembly has a polyethylene shield thickness of 6 inches calculate the anticipated neutron yield. This method                  (15 cm) and a directionality of 10 to 1. The combined provides the ability to calculate the neutron yield when the          weight of the detector and collimator exceeds the require isotopic or impurity composition of the plutonium holdup              ments for a hand-held probe. For this reason and to provide is different from that of the calibration standards. The              for reproducible positioning at each assay, a sturdy cart method can be used to calculate a ratio of the neutron                housing such a detector and its associated electronics is production rate of the unknown material to the neutron                recommended. In order to assay items at different heights, production rate of the standard material. The yield from              the capability to raise and lower the assembly to reproduc the holdup material is then determined by multiplying the            ible settings is needed.


The spontaneous neutron production rate
measured "known" material yield by the computed ratio.
          2.2.2 Collimators for Neutron Detectors                from typical reactor plutonium is significantly less than the production rate of 375440 keV gamma rays. To To assay a specific collection zone in the       provide. ,.an adequate response for calibration, it is presence of other distributed sources of plutonium, it is       therefore necessary, to encapsulate a larger amount of necessary to collimate the detector. This is accomplished        plutonium. in the neutron assay calibration standard.
 
An example of a small commercially available hand-held
    2.2.1 Neutron Detection Instruments                              detector is given in References 5 and 6. This Shielded Neutron Assay Probe (SNAP) is 12 inches (30.5 cm) high To effectively employ the spontaneous neutron yield as            and 10 inches (25.4 cm) in diameter and contains two 3He a measure of plutonium holdup, it is necessary to detect the          detectors. It includes a 2-inch-thick (5 cm) polyethylene neutrons in the presence of a more intense gamma ray                  shield that provides a directionality of 3 to
 
===1. The SNAP===
background and to collimate the detector so that neu                  has been used to measure plutonium holdup, UO2 F2 trons emanating from the collection zone under assay are              holdup, and UF 6 enrichment. It is recommended for the preferentially detected.                                              assay of well-defined concentrations of plutonium in pumps, grinders, pipe elbows, or other items of equipment Holdup assay is performed under in-plant conditions              where portability and accessibility are more important than where ruggedness, portability, high neutron efficiency, and          directionality.


5.23-4
5.23-4


COLLIMATED NEUTRON DETECTOR ASSEMBLY FOR PLUTONIUM HOLDUP ASSAY
DETECTOR CABLE ACCESS CHANNEL
          DETECTOR CABLE ACCESS CHANNEL
                      (TOP SECTION ONLY) r-.
                (TOP SECTION ONLY) --.-
                                                                                                                    117
                                                                                                                                -T-
                                                                1 I5.1cm        15.1cm  3" IT
                                                                                                                                17.5cm
                                                                                        4.5cm FRONT VIEW                                -- ,c      +      TOP VIEW
                                                          - 5-1cm            1cmI              "  I
                    +
                                                                                                                                I
                                                                                                       N DETECTOR
                                                                                                              TOP VIEW
                                                                                                      "HANNELS
                          FRONT VIEW
                                                            "I                             2.6cmnDIAl METER   (TYP)
                                                                                                       4.5cm
                                                              I   I        I   I
                                                                  ..          ..              .      \NEUTRO N DETECTOR
                                                                      IIFRONT I
                                                                                                          TUBE C HANNELS
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                                                                                                      VIEW
                                                        I               II                I I           " ,
                                            6 3c I     I   I   IIl    I
                                                                    I     . I             I I
                                                  m  W9mITH0,7I        CDIUMSI        E
                                                                    I I      I          I~
                    4 DETECTOR TUBE SUBASSEMBLY
                                                                     I   I I I                 !
                    4 Assembly includes three BF 3 or 3He tubes (2.54 cm diameter). Unit can be modified to increase or decrease the number of tubes. Moderator thickness of 15 cm provides u 10:1 directionality. Addi tional polyethylene can be added to improve directionality (e.g., 30 cm polyethylene provides *, 100:1 directionality). Components are bolted or strapped to remain in a fixed configuration.
                                                      I          I       I I' I I
 
                                                            I I I I I t lI.
Figure 1. Collimated Neutron Detector Assembly for Plutonium Holdup Assay
                                                  5.23-5
 
3. ISOLATION OF COLLECTION ZONES
     A third example of holdup measurement by neutron detection is given in Reference 7. In this case, a completely
                                                          3              To ensure that each collection zone is independently uncollimated polyethylene slab containing a row of He assayed, it is necessary to shield the detector from all detectors was suspended in midair in some of the processing        radiations except those radiations emanating from the rooms of an industrial plutonium facility. The response of collection zone being assayed. This is principally accom the detector was found to be proportional to the total              plished through the use of the collimators described in room holdup if the plutonium was reasonably uniformly Sections B.2.1.2 and B.2.2.2. Two additional means exist to distributed and if the room was isolated from external isolate a collection zone, detector positioning and shadow sources. The calibration procedure for the use of. this            shielding.
 
detector will not be described here. However, it is recom mended as a means for quickly verifying total room holdup            3.1 Detector Positioning when measurements of the holdup in individual items or equipment are not needed.
 
An unobstructed side view of a collection zone is pre
                                                                     +/-erred. When plutonium is located behind the zone under
    2.2.3 Check Source for Neutron Assay                            assay in another collection zone or a storage facility, an additional background assay may be performed with the To ensure the proper operation of the neutron assay            detector above or below the collection zone and pointing at system prior to making an assay, it is necessary to test the        the material behind the zone under assay. It is important to response of the instrument. An appropriate neutron assay            prevent, or account for, moving objects within the field of check source can be measured, or the detector response for          view. If this is not done, variations in shielding and scatter one or more collection zones can be recalibrated and                ing can affect the measurement.
 
compared to the results of previous calibrations.
 
3.2 Shadow Shielding An appropriate neutron assay check source can be prepared by implanting a small encapsulated plutonium                    It may not be possible to avoid interfering radiations by source (containing about 5 grams of plutonium) into                  collimator design or by choosing the detector position for the face of a plug of neutron moderating material (see              assay. In such cases, it may be possible to move a shield Figure 2). The plug is fabricated to fit and close the colli panel between the source of interfering radiations and the mator channel. When the response from the check source              collimator zone under assay. If the shield panel is very thick remains within the expected value, the previous calibration          and its dimensions match or exceed the back side of the data are assumed to be valid.                                      collection zone under assay, no interfering radiations will penetrate through the shadow shield to the detector. While such characteristics are desirable, the size of such a shield
   2.3 Thermoluminescent Dosimeter (TLD) Assay                        would limit its transportability. A rectangular panel mounted on wheels as an upright panel and containing %5 cm of neu Crystals of LiF, CaF 2 , CaSe 4 , or other compounds can        tron moderator (e.g., benelex, WEP, or polyethylehe) and store energy at manganese or dysprosium impurity centers            ,0.5 cm of lead sheet is recommended. To use such a when they are struck by gamma or neutron radiation. At              panel, it is necessary to measure the response of the collec some later time, the crystals can be heated rapidly to              tion zone with and without the shield in place. Also, the several hundred degrees centigrade to induce thermo                 gamma and neutron transmission factors of the shield itself luminescence. The light output at this time is proportional          must be measured beforehand with a representative pluto to the amount of radiation received. Thermoluminescent              nium sample. From these measurements, the assay of the detectors that are primarily gamma sensitive, graded X-ray          collection zone can be corrected for background radiation shields, read-out instrumentation, and other accessories are        transmitted through the shield.
 
commercially available.
 
4. CALIBRATION FOR HOLDUP MEASUREMENTS
      TLDs have been used to measure the holdup in glove boxes by placing them at regular intervals on the outside          4.1 Basic Counting Geometries surfaces. The TLDs are left in place overnight in order to accumulate a measurable dose. Accuracies of +20 percent                There are three fundamental counting geometries that relative to cleanout values are reported for plutonium of          can be used to represent most collection zones. These known isotopic composition. TLDs have also been used to            geometries are distinguished by the spatial distribution of measure the holdup in the interior of large furnaces that are       the source material and the resulting dependence of the not accessible by other means. For both of these examples,          detector counting rate on the source-to-detector distance, r.
 
calibration requires either careful dose and geometry calculations or mockups of the actual collection zone.                  4.1.1 Point Source Because their use is relatively new and only a few published references exist (Refs. 8 and 9), TLDs will not be discussed            If the material being assayed is distributed over an area further in this guide. However, they could be useful for            with dimensions that are small compared with the source- special applications.
 
5.23-6
 
COLLIMATOR
                        TOP VIEW
                                            -CHECK SOURCE
                                                COVER
                                              CHECK SOURCE
                      FRONT VIEW
Figure 2. Neutron Collimator Channel Plug and Check Source
                          5.23-7


I      II             I        I
to-detector distance and if the material resides entirely                  sources although neutron assay is usually restricted to within the detector field of view, the zone can be treated as             dense, isolated items of equipment that can be represented a point source. The detector count rate for a point source                 as point sources. For both neutron and gamma measure varies inversely as the square of the source-to-detector                    ments, corrections to the point and line source calibrations distance (count rate is proportional to l/r 2 ). Any equip                 for different detector distances are made using the 1/r 2 or ment measured at great distances or any small pieces of                    1 /r count-rate dependence, respectivel
                                                                                      I .1                 FRONT VIEW
                                            &#xa2;r   39cm                              I          I
                                        68                                I I        II
                                                                        I I      I        I*
                                                                        I I      I        I
                                                                        I I II                 I.


.1I    I      I I            I.
====y. For further detailed====
                                                                    4 equipment or equipment parts fall in this category.                      discussion of the measurement of detector responses for these basic geometries, see Reference 10.


I I I I                I
4.1.2 Line Source For gamma ray assay, the calibration of the point source If the material being assayed is distributed along a linear            response can be accomplished with a well-characterized path so that only a segment of that distribution length is                 encapsulated standard plutonium foil smaller in size than contained in the detector field of view, the zone can be                  the detector collimator opening. This foil can also serve as treated as a line source. The detector count rate for a line              the check source for verification of the continued stability source varies inversely as the source-to-detector distance                of the instrument settings in the field. It is important that (count rate is proportional to I/r). Examples of this type of              care be taken in the preparation of this calibration standard holdup geometry include isolated sections of piping and                    to ensure that the amount of encapsulated 2 3 9 pu is well long, narrow ducts or columns.                                            known. It is also important to measure the gamma ray attenuation through the encapsulating material and the
                                                                I                II        -I
      4.1.3 Area Source                                                      self-attenuation of the plutonium foil and to correct the calibration standard response to compensate for these If the material being assayed is spread over an area so                effects. Enough 2 3 9 Pu needs to be encapsulated in this large that it covers the full field of view of the detector for            standard to provide count rates that will ensure good a range of source-to-detector distances, the zone can be                  statistical precision of the calibration in a reasonable period assayed as an area source. As long as the material being                    of time.
                                                                        I I            I          "                 FIGURE B-1 I         I      lIl, I    I 11 I      II        1      14.l POLYETHYLENE BLOCK, COVERED ALL SIDES
                                                  'WITH 0.0756m CADMIUM SHEET
                                                DETECTOR TUBE SUBASSEMBLY
NEUTRON DETECTOR/COLLIMATOR ASSEMBLY. ASSEMBLY INCLUDES THREE BF3 OR He-3 TUBES
(2.54cm DIAMETER) UNIT CAN BE MODIFIED TO INCREASE ORDECREASE THE NUMBER OF TUBES.


MODERATOR THICKNESS OF 15cm PROVIDES,10:? DIRECTIONALITY. ADDITIONAL POLYETHYLENE
viewed is uniformly distributed, the detector count rate will be independent of the source-to-detector distance.
CAN BE ADDED TO IMPROVE DIRECTIONALITY Io.p., 30*m POLYETHYLENE PROVIDES~100:1 DIRECTIONALITY). COMPONENTS ARE BOLTED OR STRAPPED TO REMAIN IN A FIXED CONFIGURATION.


5.23-5
However, for holdup applications, uniform material distribu                    For neutron assay, it is probably necessary to encapsulate tion is rare; so the source-to-detector distance can affect the            a larger amount of material in the calibration standard instrument response and needs to be specified. Furthermore,                because the spontaneous neutron production rate is signifi when there are several measurement locations covering a                    cantly less than the 375- to 450-keV gamma ray production large area (such as a floor), it is important to maintain the              rate. A quantity of 50 to 100 grams of plutonium is ade same source-to-detector distance (even if material distribu                quate for most applications. Again, it is important to know tion is uniform within a given measurement area) so that                  the exact quantity and isotopic composition of the pluto the number of measurement areas needed to cover the                        nium. Also, the neutron calibration standard may generate entire area remains constant. Examples of this type of assay              more neutrons than directly attributable to the spontaneous geometry include floors, walls, glovebox floors, and large                fission and (a ,n) reactions. Because a relatively large quantity rectangular ducting.                                                      of PuO 2 Ls encapsulated in the neutron assay calibration standard, some spontaneous fission or (a,n) neutrons may
4.2 Calibration of Detector Response                                      be absorbed in 239pu or 24 1pu nuclei, producing additional neutrons through the induced fission reactio


MODERATOR\              NEUTRON COLLIMATOR                to the same geometry as found in the neutron assay
====n. The amount====
                                /CHANNEL  PLUG                  calibration standard. Each test sample is transferred to an empty glove box and positioned next to the window for measurement. The neutron assay probe is positioned as close as possible to the sample but outside the glove box. After the measurement is made, that sample is transferred from the glove box and the next sample is
    4.2.1 Mockup of Known Material Distributions                          of multiplication depends in a complex manner on the amount and distribution of PuO 2 and on the surrounding When a gamma ray assay is used and a collimator setting                medium (Ref. 11). For 50 grams distributed in the bottom has been selected, the detector responses for the three basic              of a 4-inch-diameter (10 cm) can, a self-multiplication of source distribution geometries listed above need to be                      0.5 percent of the total neutron output would be typical.
                          1       CHECK SOURCE                  transferred in and positioned in the identical location for measurement. A plot of counts minus background as a TOP' VIEW
                                                                  function of PuO2 mass is made and the points visually fitted using a French curve. If there is no multiplication, a straight line can be drawn through the. origin connecting all points. Multiplication is indicated when the curve turns upward, indicating an increase in counts per gram as the mass of PuO 2 increases. A correction term is obtained by determining the increase in counts CHECK SOURCE        per gram at the mass value corresponding to the neutron COVER            assay calibration standard mass. This increase is readily determined by plotting the straight line through the origin and the lowest mass sample response and reading the difference in counts between the two lines at the abscissa coordinate corresponding to the neutron assay CHECK SOURCE
                                                                  calibration standard mass. All measurements relating to this standaid are thereafter reduced by the ratio of the difference in counts to the observed counts.


FRONT VIEW
determined. For the point source, the response is expressed                At 100 grams, 1 to 2 percent may be expected. Thus, this as (counts per minute)/gram of 2 3Pu at a specified source                  effect is typically smaller than other errors associated with to-detector distance. For the line source, the response is                 holdup measurements and can be neglected if the standard expressed as (counts per minute)/(gram of 2 3 9 pu per unit                contains 100 grams or less of well-distributed material. The length) at a specified source-to-detector distance. For the                 chemical and isotopic composition of the plutonium will area source1 the response is expressed as (counts per minute)/              have a larger effect, as described in the appendix to this (gram of 39pu per unit area) at a specified source-to                      guide.
                                                                3.     Isolation of Collection Zones FIGURE B-2     NEUTRON COLLIMATOR CHANNEL                            To ensure that each collection zone is PLUG AND CHECK SOURCE                        independently assayed, it is necessary to screen all radiations from the detector except those radiations emanating from the collection zone being assayed. This While the amount needed is best determined through an          is principally accomplished through the use of the evaluation of typical accumulations, 100 g Pu is                collimators described in Sections B.2.1.2 and B.2.2.2.


adequate for most applications.                                 Two additional means exist to further isolate a collection zone.
detector distance. When neutron assay is used, the response for a point source is expressed as (counts per minute)/gram of 24&deg;Pu effective at a specified source-to-detector distance.                 The measurement of the line source response is best Calculation of 2 4 &deg;pu effective from the plutonium isotopic              accomplished by constructing a cylindrical surface distribu composition is described in the appendix to this guide.                    tion of plutonium with the aid of large foils. It is also Analogous expressions can be given for line and area                      possible to establish the line source response using a point
    4Caution: small deposits of plutonium could exhibit very large        source, as described in Reference 4. The line source geom gamma ray self-attenuation and could therefore require great care in      etry is closest to that of the pipes and ducts likely to be analysis or could require neutron assay.                                  encountered in actual measurements.


The neutron assay calibration standard may generate more neutrons than directly attributable to the spontaneous fission and (an) reactions. Because a              3.1 Detector Positioning relatively large quantity of PuO2 is encapsulated in the neutron assay calibration standard, some of the                              An unobstructed side view of a collection zone spontaneous fission or (an) neutrons may be absorbed in          is preferred. When plutonium is located behind the zone Pu-239 or Pu-241 nuclei, producing additional neutrons            under assay in another collection *zone or a storage through the induced fission reaction. The amount of              facility, either consider positioning the detector above or multiplication depends in a complex manner on the                below the collection zone, or consider the use of shadow amount and distribution of PuO 2 and on the surrounding          shielding.
5.23-8


medium. The potentially significant calibration error arising by having too large a neutron yield per gram of                 3.2 Shadow Shielding plutonium will be corrected in the long term through assay verification tests. In the initial phase of assaying                  It may not be possible to avoid interfering holdup, a rough correction for this yield can be                 radiations through the collimator design or through measured by preparing two additional PuO2 sources                choosing the detector position for assay. In such cases, it containing 1/3 and 2/3 of the neutron assay calibration         may be possible to move a shield panel between the standard mass. These samples need not be encapsulated,          source of interfering radiations and the collimator zone as they will be measured only once and can then be              under assay. If the shield panel is very thick and its returned to the process stream.                                  dimensions match or exceed the back side of the The PUO2 used in this test is taken from        collection zone under assay, no interfering radiations the same batch used to prepare the neutron assay                will penetrate through the shadow shield to the detector.
measurements of the detector responses can be made. This The area source response can be measured with the same procedure would be a valuable supplement to calibration plutonium foils laid flat to simulate the expected distribu data obtained from mockups of standard counting geom tion on surfaces such as walls and floors. The area response etries and comparisons with cleanout recovery data.


calibration standard. After weighing out the proper              While such characteristics are desirable, the size of such a quantities, the PuO 2 is put into containers having close        shield would limit its transportabilit
can also be established using a point source. The point source is measured at different radial distances from the
                                                                        5. HOLDUP MEASUREMENTS AND STANDARD ERROR
center of the field of view of the collimated detector. The response at each radial distance is weighted by the area of a The measurement of holdup in a complex plant environ concentric ring at that radius. From these weighted re ment can involve a very large number of measurements. In a sponses, it is then possible to calculate the area of a circular stable plant environment where the process behavior is well region of uniform plutonium deposition that would yield this    known and well characterized, it may be possible to arrange the same total response as the point source. From of 23 9 Pu  per    the holdup measurement program so that:
equivalent area, the  expected  response/(gram unit  area)  can be  derived.  Further  useful  details on  this a. Careful and extensive holdup measurements are made procedure may be found in Reference 12. For both line and                      infrequently (e.g., annually) and area calibrations, the self-attenuation of the foils or point sources also needs to be taken into account.


====y. A rectangular====
b. At more frequent intervals (e.g., at inventory times),
                                                          5.23-6
                                                                                careful measurements are made in known problem There may be special material distribution geometries areas, and "spot check" measurements are made in in the facility that are not readily represented by one of the the other, less used, zones where accumulations are three basic configurations described above. These special                      known to be low.


panel containing -5          cm of neutron moderator (e.g.,                 The calibration obtained through this procedure is benelex, WEP, or polyethylene) and -0.5 cm lead sheet                  recommended until a history of comparisons between is recommended, mounted on wheels as an upright                        predicted and recovered holdup quantities is developed, panel. To use such a panel, two measurements are                        as described in Section B.5 of this~guide.
geometries may be mocked up as carefully as possible with large plutonium foils and point sources to produce a usable Such management of measurement resources can result in a detector response calibration for these special cases. Examples very effective holdup measurement program at minimum of special cases might be concave or convex equipment costs surfaces or the internal volume of a rectangular cavity (see Ref. 10). Because material particle sizes (or material deposit        5.1 Holdup Measurements thicknesses) have a significant effect on the self-attenuation of the gamma ray signals, it is important to use (whenever In performing the holdup measurements, one must be practical) well-characterized process material for preparing aware of the large variability in holdup assays arising calibration standards and to duplicate to the extent possible primarily from variability in the measurement conditions process holdup distribution relative to particle size or thick (e.g., background, geometry, gamma ray or neutron attenua ness. Furthermore, holdup in floors is often deposited at tion, material distribution). Accordingly, it is important various depths into the floor, rather than on the surface.


required.*                          --
to perform the assays from as many vantage points as Thus, calibration standards for such measurements need to possible for each collection zone. If this is impractical on a incorporate the appropriate geometry and matrix effects.
                                                                              4.1 Detector Positioning R1 - Rcz + Rlnterference                            (1)                  To calibrate each collection zone, the best position or series of positions is selected to observe the R2 = Rcz + TRinterference                            (2)        collection zone with .the least amount of interference from principal structural components. It is important to where                                                                  view the collection zone with the detector located between the collection zone and all areas used for Pu R1 is the assay response obtained before the shadow            storage during inventory. A three-dimensional approach shield is moved into position,                                  can be investigated, positioning the detector on top of or below the collection zone if it is not possible to have an R2 is the assay response obtained with the shadow              unobstructed, interference-free side view of the shield in position,                                            collection zone. The use of shadow 'shielding can be explored if it is not possible to get a clear view of each Rez is the response component attributable to the              collection zone for assay.


collection zone under assay, On the basis of a detailed examination of the Rinterference is the response component                        physical layout of the facility, some preliminary attributable to the interfering radiations, and                measurements are made to determine optimum detector positions for holdup assay. Once the assay positions for T is the transmission through the shadow shield.              the detector and shadow shields -are established, Note that T represents a measured transmission-T.r            permanently marking the assay positions will. facilitate for gamma rays or Tn for neutrons. Ty. and Tn are              subsequent measurements.
routine basis because of time or space constraints, one Core samples of a floor may be needed to establish typical might consider multiple measurements initially on a collec concentrations at various floor depths.


measured by counting radiations from any arbitrary source of plutonium with the shield between the                    4.2 Calibration Sources source and detector and again with the shadow shield removed:                                                          Since this assay is to measure the amount of plutonium holdup, it is appropriate to use plutonium as T    = (R.?,) shield in/(R,,) shield out            (3)      the calibration standard material. Further, as the plutonium holdup will generally be distributed over a T, = (Rn) shield in/(Rn) shield out                  (4)        large surface area, it is recommended that the gamma ray calibration standard be fabricated to resemble this To correct for the interference, subtract R2 from R 1 ,                characteristic, as described in Section B.2 of this guide.
tion zone, followed by fewer routine measurements at repre sentative assay sites. Careful thought in the selection of Calibration of the holdup measurement system using this measurement points and measurement strategy will mini procedure is recommended until a history of comparisons mize ambiguities in the interpretation of the data.


and solve for 'Rlnterference:
between predicted and recovered holdup quantities is developed. If it is possible to take holdup measurements
                              (R2 - R)                        ("              4.3 Calibration Procedures Rlnterference        (1 -T)          "            (5)                  Once the principal items containing plutonium have been removed and the detector located in its assay To ensure that this correction is sufficiently accurate, it             position, the response from a calibration standard may be necessary to extend the length of the normal                    combined with the plutonium already held up is counting period .to accumulate sufficient counting                      obtained. When the collection zone is appropriately statistics (1% statistics are generally adequate for this              isolated, two factors influence the observed response application).                                                           from the calibration standard:
                                                                              5.1.1 Selection of Collection Zones and Detector before and after the cleanout of a piece of shut-down                            Positions process equipment, they can be used to establish this comparison history and improve the accuracy of the Location and configuration of collection zones are calibration for each collection zone.
                                                                        I.  the location of the calibration standard within the
4.      Calibration of Collection Zones collection zone, and Euch collection zone is independently calibrated, as            2.  the shielding of radiations from the calibration hackground-lfaclors and the compos*tion of each zone                        standard caused by the items comprising the vary widely from zone to zone. A collection zone is best                    collection zone.


calihlaled throngh the in situ measurementi of' known                              The gcomelric response variation is measured t'iilih)lU in lanltdads. When such a program is not by observing lie response from-one calibration standard piositlve,. Ihli callibration can it , based on the calculalion with the other standard removed from the collection of Ithe anticipated response or through measuring a zone under investigation. The calibration standard mockup ot the collection zone of interest. 5 response is measured with the standard positioned in various parts of the collection zon
established on the basis of a detailed physical examination and a radiation survey of the physical layout of the facility.


====e. avoiding internal====
4.2.2 Measurement of CalibrationSources in Actual Preliminary measurements are needed to determine the ProcessEquipment optimum detector positions for the holdup assays. If nonuniform distribution of material in a collection zone is One method for calibrating detector response to holdup suspected or if the process apparatus is sufficiently compli radiation in process equipment is to place a known calibra cated to require extensive attenuation corrections for tion source in various positions in that equipment and certain counting geometries, multiple measurements are record the detector responses. In this way, the overall advisable for the collection zone. More than one detector detector response (including all corrections for attenuation position may be necessary. In the cases where radiation and geometry) is determined empirically. Unfortunately, surveys have pointed out zones of high holdup collection, this procedure is impractical, if not impossible, in process extra care will be necessary in the holdup measurements for equipment already in operation. However, if those respon those zones to minimize their contribution to the overall sible for holdup assays are made aware of occasions when holdup variability. Where radiation surveys show little hold new equipment is brought into the plant for installation in up, proportionately less time need be budgeted. Selecting the process, calibration sources can be conveniently placed optimum detector positions includes consideration of the in the equipment before its installation and the empirical
*Response tierms refer to neutron or gamma response, as                items which may attenuate the radiation emanating .from appropriate.                                                         the standard.
                                                                    5.23-9


5.23-7
the individual measurements. Further, the variability need to conveniently measure the line-of-sight background between these measurements can provide an indication of by moving the detector to one side without changing its the measurement uncertainty.


When neutron assay is employed or when the                                To use this relationship, the detector is first collection zone consists of a hollow box, pipe, or duct,                    positioned at point d and a reading is taken. Point d is attenuation is either relatively uniform or negligibly                      the center of the first subzone, selected to coincide with small. The calibration of each collection zone then                          the physical edge of the calibration' zone. The detector is becomes a matter of appropriately averaging the                              then moved a distance 2D along the traverse to the geometric response variations. The average response of                      center point of the second subzone, and the second the entire collection zone is assumed to properly                            measurement taken. The cycle*is repeated to include all represent that zone. -'If, however, it is known that                        of the larger collection zone. The value interpreted for plutonium accumulates in one particular location within                      calibration for each subzone *.corresponds to the a collection 'zone, the response of the standard is                          maximum of the traverse across each subzone because emphasized when located near the principal collection                        the response has been flattened. The content of the site.                                                                        entire collection zone is the sum of the contributions from the subzones.
orientation.


If the item to be assayed consists of a large                  5.   Estimation of the Holdup Error unit, assay performance may be enhanced by subdividing the unit into smaller contiguous measurement zones.                              The overall uncertainty associated with the The repeat dimensions of the subzones are determined                        measured plutonium holdup is due to (1) the by measuring the rcsponse while moving the standard                          uncertainty in. the observed response and (2) the along an axis perpendicular to the detector centerline.                     uncertainty in the interpretation of that response. The By studying the response curve, the distance D is                           random uncertainty components in this application are selected as the point beyond which sufficient activity is                  .frequently negligible in comparison with the geometric detected to flatten the response within the subzone.                         uncertainty and the uncertainty in the isotopic Each subzone will measure 2D across its face. An                            composition. In this assay application, it is appropriate example is illustrated 'in Figure B-3. As the response                      to estimate the assay error components by assuming the about the centerline is assumed to be symmetrical, only                      measured range -(Ri) of the ith fluctuation constitutes an half of the traverse is indicated. In Figure B-3, D is                      interval four standard deviations wide. The midpoint of selected such that the area under the curve to the right                    the range estimates the mean effect, and the' distance of D is approximately equal to the area above the curve                      from the "midpoint to each extreme comprises an to the leftof D (Area A 1 = Area A 2 ). Note: the distance                  estimated 95% confidence interval. 'The error from the collection zone to the detector or the distance                     attributable to this effect is then approximately from the crystal face to the end of the collimator, or                                        2 both, can be varied to divide the collection zone into an                              2=(R)
5.1.3 Gamma Ray Attenuation Corrections
                                                                                                                                      (6)
    5.1.2 Holdup Measurement Procedure To obtain useful assay results by detecting 375- to The measurement and analysis of gamma or neutron
integral number of subzones.
                                                                    450-keV gamma rays, it is necessary to correct each assay radiation from a collection zone may be carried out by for attenuation of the signal, either within the plutonium treating the material distribution as a point, line, or area holdup material or by structural materials. Without this source, as described in Section B.4.1, or as one of the critical correction, the assay is no more than a lower limit special cases that may have been measured, as mentioned in on the true holdup value. The attenuation correction may Section B.4.2. If the nature of the material distribution is be based on calculations of known attenuation in uniform uncertain for a particular detector position, a measurement materials, on earlier measurements of materials similar to of the detector counting-rate dependence on the source-to those found in the plant equipment, or on direct measure detector distance, r, may reveal the most appropriate ments of gamma ray transmission through the actual counting-rate geometry with which to interpret thc data.


AREA A1 If a.severe effect is~noted, the response can often be corrected for the variation in the corresponding
equipment. Details on establishing an appropriate attenua tion correction are given in Laboratory Exercise No. 4 of After the assay positions for the detector and shadow Reference 4. Additional treatment of gamma ray attenua shields are established for each collection zone, permanent          tion corrections is given in Reference 13.
    100,
                                                                            parameter by measuring the. value of that particular parameter at the time of the assay. Using a measured relationship between the response and the value of that parameter, the observed response is corrected.


5.1 Response Uncertainties S                            A  MEASUREMENT DATA POINTS
markings that indicate detector location (including height)
                                                                                        '5.1.1 Counting Statistics VISUAL FIT TO MEASUREMENT DATA
and orientation will ensure reproducibility of subsequent
                                                                                              The magnitude of the uncertainties attributable to variations in the geometric distribution A                                      and in the attenuation of the radiations are expected to dominate the total 'response uncertainty. 'The relative S                                    RA  .
                                                                            5.1.4 Gamma Ray Interferences measurements for these positions. Uniquely labeling each assay site will facilitate unambiguous reference to each Variability in the observed gamma ray response may measurement and its location in the assay log. Furthermore, arise as a result of the presence of extraneous gamma ray assay site labels and markings can indicate whether neutron          emitters or as a result of fluctuations in the background or gamma ray measurements are to be made. Alphabetic from the Compton scattering of higher energy gamma rays.
                                                                              standard deviation due to counting statistics can usually be made as small as desired through '(I) using more AREA A2 efficient detectors or (2) extending the counting period.


Having 1000 to't0,000 net counts is generally sufficient for most holdup assay applications.
labels (for example, "G" for gamma and "N" for neutron)
                                                                      The magnitude of this effect is generally small. It can be and color-coded tape markings of the sites would be useful.


5.1.2 Instrument Instabilities
monitored by observing the spectrum with a multichannel Protecting the markings (for example, with clear epoxy)
          0        25          5D            75          100
                                                                      analyzer, but, unless data on periodically recovered holdup will ensure their long-term durability.
        DISTANCE FROM DETECTOR CENTERLINE TO POINT SOURCE, CENTIMETERS
                                                                                      -      Fluctuations in ambient        temperature, FIGURE B-3    EQUIVALENT  DIAMETER SUSZONE      TO ACHtEVE A FLAT
          PLANAR RESPONSE. SELECT D SUCH THAT AREA A 1      = A2.            humidity,    electronic noise, and line      voltage (for
                                                                      5.23-8


non-battery-powered electronic units) generally affect                          5.2.1 Interfering Radiations the stability of electronic systems. The magnitude of this uncertainty can be estimated by monitoring the check                                5.2.1.1 Gamma Ray Assay standard response and determining the range of variability as described in Section B.5 of this guide,                                    An uncertainty in the observed gamma ray response may arise due to the presence of extraneous
accumulations are in error, this contribution can be ignored.
          5.1.3 Geometric Uncertainty                              gamma ray emitters or due to fluctuations in the background from the' Compmon scattering of The geometrical variation in the observed          higher-energy gamma rays. The shape of the background response is measured by moving the calibration source gamma ray spectrum may change in such cases to such within the bounds of each collection .zone. Two -cases are an extent that even with the energy windows stabilized.


described below.
After measuring the gamma or neutron radiation intensity              5.1.5 Matrix Effects on Neutron Assay at each detector position in a given collection zone, the line-of-sight background is measured by moving the detector A change in the neutron yield for a plutonium sample of and collimator to one side (still pointing in the same fixed isotopic content can be caused by a change in the direction as during the assay) and measuring the radiation concentration of high-(a,n)-yield impurities in the matrix.


the background correction is irregular and uncertain.
intensity from the surrounding materials. During the                If it is possible to estimate the range of permissible impurity background measurement, the vessel in which the holdup is           concentrations, the variation in a typical neutron yield being measured must not be in the field of view of the              can be calculated using the method given in the appendix to detector. Because uncertainties in geometry, attenua                this guide.


The magnitude of this effect is generally smal
tion, or sample matrix will usually dominate the total response variability, the counting period need not be long.              5.1.6 Effect of Isotopic Uncertainty Having 1000 to 10,000 net counts is generally sufficient for most holdup applications.


====l. It can be====
Gamma ray measurements of plutonium holdup provide  2 39 a direct determination of the fissile plutonium (i.e.,        Pu This procedure is repeated at all measurement posi and 241 Pu) holdup in the zone under consideration. On the tions and in all counting geometries designated for each              other hand, neutron techniques measure only the 240Pu collection zone. The final holdup value for the zone is effective content, and chemical techniques provide obtained from the average of the individual measurements elemental analysis without consideration of the isotopic (each one being corrected for the effects of attenuation and makeup. Thus, knowledge of the isotopic composition of any variation in geometry relative to the calibration measure the plutonium is necessary to correlate holdup measure ment).
                5.1.3.1 Isolated Collection Zones                  .monitored by observing the spectrum with a multichannel analyzer, but unless the data on When a single unit comprises a                periodically recovered 'holdup accumulations are in collection zone, the standard is moved .to all .sites within        error, this contribution can be ignored.
                                                                        ments with chemistry and accountability value


the zone at which an accumulation of plutonium might occur. With sufficient collimation, the response for. the collection zone under investigation is independent of its                          5.2.1.2 Neutron Assay neighbor zones. The average of the response, weighted to reflect 'prejudgments on the likelihood of accumulation                                  A change in the neutron yield for a sites, is then used as the calibration point. As shown in          plutonium sample of fixed isotopic content is primarily Section B.5, the range of values can be assumed to                 attributable to the fluctuation in the concentration of comprise an expectation interval four standard                      high (an) yield impurities.* Judgment can be used to deviations wide. The geometric error is then estimated              determine the range *of permissible impurity using Equation 6.                                                  concentrations. The variation in a typical neutron yield can then be predicted using the methods discussed in the
====s. Gamma====
                5.1.3.2 Overlapping Collection Zones                Appendix of this guide. Again, the range of permissible variations is assumed to constitute an acceptance interval When a collection zone is subdivided          from which the component error is computed using into overlapping subzones, the geometric uncertainty              Equation 6.
                                                                                                              2 39 ray assays must be divided by the         pu isotopic fraction, Whenever possible, the collection zone is assayed in a                                                            240
                                                                                                                            Pu effective and neutron assays must be converted from variety of ways. For example, one could measure an appara to total plutonium in order to express holdup in terms of tus up close and treat it as an area source; the measurement total plutonium.


due to the dimension perpendicular to the detector collection zone centerline is eliminated through the
could then be repeated at a large distance, treating the zone as a point source. It may be better to measure some zones If the process equipment is thoroughly cleaned each from several different directions-especially if complicated time the isotopic composition is changed, the holdup may attenuation corrections are called for in some of the count          consist primarily of the current material. In that case, the ing geometries. Several independent measurements of one              declared isotopic composition can be used. When mixing zone can provide an average holdup value that is better than
                                                                              5.2.2 Isotopic Uncertainties area-averaging calibration method described in Section
                                                                5.23-10
4.3.


If the process equipment is cleaned each The uncertainty in the depth                  time the isotopic composition of the plutonium feed is dimension in each subzone can be determined through                varied, the holdup will consist primarily of the current the procedure outlined 'for isolated collection zones.            material. New calibration standards can be prepared or Judgment can be used to weight the calibrationdata to              the previous yield data can be normalized using the emphasize principal accumulation sites.                            methods presented in tht Appendix to correct tor t*his effect. When mixing occurs, use of the stream-averaged
5.3 Estimation of Bias occurs, use of the stream-averaged isotopic composition is esti appropriate. Bounds on the isotopic composition are                                             When a single collection zone is cleared      out, it is desirable and      lowest                                                            1 to perform a holdup assay before, Hbefore, and after, mated by considering         the     batches    of  highest This
          5.1.4 Attenuation Uncertainty                            isotopic composition is appropriate. The uncertainty bounds are estimated by considering the highest .,id If the attenuation is not extreme, it can be        lowest fissile isotopic batches and computing the measured in situ, mocked up, or computed for the                   corresponding range.
.omposition and computing the corresponding range.                                         H fter, the cleanout if possible. By comparing the amount into    the measure of variability must then be incorporated                                            oplutonium removed, Pur, to the recovery amount pre making        direct dicted through the in situ holdup assays, Pua, the collection estimated holdup standard deviation                before comparisons with the chemical analyses. The variability in                                  zone calibration can be updated, and the calibration and stan isotopic composition can be expressed as an estimated range     and          assay standard deviations can be based on relevant data.


different conditions encountered. The worst and best cases can be assumed 'to determine the range of                          5.3 Holdup and Its Associated Error permissible effects. Using Equation 6, the magnitude of Ihis uncertainty component can then be estimated.                           The amount of Pu holdup can be ,measured Again, judgmaent is appropriate to weight the correction          through the systematic application of the program factor.                                                           developed in conjunction with the principles and pitfalls discussed herein. For each collection zone, measured
The amount of plutonium recovered, Pur, during the dard deviation defined as one-half the              observed deviation cleanout of a specific collection zone can be assayed then combined in quadrature              with  the   standard gamma given by Equation 1 in Section B.5.2. In general,                                          through sampling and chemical analysis, through calorim sensitive    to    isotopic etry, or through other applicable nondestructive assay ray measurements      of 239  pu    will  be less
      5.2 Interpretation Uncertainties                            holdup and its error can be determined.
                                                      240
                                                            Pu.                                                                                                      or variations than neutron measurement of                                                      methods (e.g., spontaneous fission coincidence detection gamma ray assay).
5.2 Assignment of Standard Error measure                  The assay value for the recovered amount is computed as The assignment of a standard error to a holdup                                       the difference in the holdup assays before and after the difficult    on  a rigid  statistical  basis.    This is ment is extremely                                                                          cleanout:
  because the only statistically predictable fluctuations (e.g.,
                                                                              negli counting statistics) in this application are frequently                                                                                                          (2)
                                    variability    due  to    counting        geo              Pua = Hbefore - Hafter gible in comparison with material    distribution),    gamma      ray    attenua metry (including                                                                                The percent difference, A, between the assay and neutron tion, gamma ray background and interferences,                                               recovery values for the plutonium holdup is then computed:
                                                              It  is  important matrix effects, and instrument instabilities.


Two factors are central to the issue here, assuming that the calibration standard material is similar        *Over a long period of time the a-particle production ratc to the held-up material.                                            increases due to the ingrowth of Am-24 1.
and guard to recognize that the variability can be large                                                  A = 100 (Pua - PUr)/Pur                                          (3)
                                    the   standard     deviation          of the against underestimating holdup  value  in  a   collection    zone.  Careful      measure overall                                                                                    A running tabulation of the quantities Pua, Pur, and A (as ments must be carried out during the calibration procedure                                well as their standard deviations, oa, ar, and OA) is kept in from to determine the range of detector responses resulting                                    the  assay log for each  collection  zone.


5.23-9
parameters.     A  useful    discussion variations in measurement of these ideas is presented in Reference 10.                                                    The average value, A, of the percent differences between holdup Pua and Pur will serve as an estimate of the bias in the assay  for that collection  zone  and  will  also provide  quantita of the A reasonable estimate of the standard deviation                                        tive justification for revision of the assay calibration for that holdup    for    a    given    collection      zone      may be                                            The  root-mean-square      deviations, measured values          zone to remove the bias.


5.3.1 Initial Operations                                  measurement method for this inventory component, it is necessary to consider -the -factors' in -the -following During the initial phase of operations, the           sections.
obtained by consideration of the range of holdup                                            aA, of the percent differences, Ai from their mean value, measurements        performed          on obtained from the variety of                                                                A, serve as a check  on the appropriateness    of the   size  of the as  suggested    in  the  previous        section.


error associated with the in situ assay of plutonium holdup is estimated by combining the component errors                Note: Care must be exercised during the fabrication and determined in the preceding sections of this guide (B3.5.1          use of check sources and-calibration. standards to ensure and B.5.2).                                                          their continued integrity and to prevent contamination.
that collection zone, average of             estimated standard deviation of the holdup measurements.


5.3.2 Routine Operations                                    4.  Delineation of Assay Collection Zones To ensure the validity of assay predictions                A plan of each plutonium processing facility should and to more realistically estimate the uncertainty in                 be examined.,to establish, independent collection zones.
The mean value for the holdup is defined as the                                            To the extent that the standard deviation of Pur is small on    the    collec the various (corrected) measurement results                                                compared with the uncertainty         in Pua  (usually  an  adequate a,  for that    mean      value    is                                                                        in size tion zone. The standard deviation,                                                          assumption), the quantity sA should be comparable the  range    of holdup    values      obtained      in                                              For  K  measurements      of the estimated as one-half                                                                      to the standard deviation of Pua.


those predictions, it is necessary to establish a program            Individual    glove  boxes      and  similar    containment to measure, the amount of plutonium recovered when a                  structures should be so-identified.. Using the layout and collection zone is cleaned out. By comparing the aniount              touring the facility, -an. assay. site(s) for. each collection of plutonium recovered to the recovery amount                        zone should be selected:
if a large the measurements. This estimate is conservative                                            percent differences, Ai, for     a  given    collection   zone,   the made.       For      a small number of measurements have been                                                            quantity sA is given by:
predicted, the collection zone calibration can be updated and the assay error can be based on relevant verification              1. Assay site(s) - should afford a clear, unobstructed tests.                                                                view ,9f-the collection zone with no other collection or
                                            actual  standard      deviation      can number of measurements, the the   range.   In  such    cases    the   actual be larger than one-half                                                                                  FK       -E2          1/2 calculated.
                                                                  ....storage. areas in_ the line- of sight of the .collimator The update data is computed as the                     assembly. Location of the. detector probe above or difference in the assays before and after cleanout:                  below, the -collection zone- should be considered if an unobstructed side , view is not possible. If an (PU)assay = Rbefore -     Rafter        ,        (7)       - unobstructed view is not, possible, shadow, shielding should be used to isolate the collection zone, for assay.


The difference.(A) in assay and recovery,
standard deviation of the holdup values must be                                                                      ) /(K - 1
                                                                      2. The assay site should be set back as far as possible A = (PII)assay  - (Pu)recovery                    (8)          from each collection zone to reach a compromise between interference from neighbor zones and efficient is then computed.                                                    counting..
                                                                                                                          2 s  A [i  E (Ai              1I                                (4)
              The standard deviation in the A values (s.)            3.' Gamma ray assay should ,be applied to measure the is computed separately for" each collection zone,                    plutonium- held up in all collection. zones containing less including no more than the twelve preceding                          than the neutron- detection limit- and for' single measurement tests:                                                    containment structures which do not contain irregularly shaped structural components capable of significantly
                                                                          counting In some cases, it may be unavoidable that the that    they  contribute      significantly        to statistics are so poor                                                                                                                0
                                                                -,*attenuating. the emerging gamma rays. Neutron assay should: be applied to measure the accumulation of sA                                  (K- 1                (9)          plutonium holdup in all structures not suitable, for gamma ray assay.      -        .
                                                                                                Equation 4 assumes that all the A's are equal. For a variability.     In such  an instance,      the   overall                                      weighted    sums,   see Reference 14.
              When a value of A is determined, it is used            4. Each collection zone should be uniquely numbered.


to update the estimate sb. The -standard, deviation                  (Neutron collection zones could be preceded by an "N",
the measurement                                                                            calculation of sA using as the square holdup standard deviation, CF(h-u)' is defined of  the  standard       deviation                                                                                    or root of the sum of the squares                                                                Note that, if the holdup measurements (ie., Hbefore and   the  standard    deviation      due to                                                                          can    still due to counting, O(stat),                                                                 Hafter) contain a constant bias, their difference Pur.
estimate s. can be used to estimate the~error in. the assay          gamma ray collection zones by a "G". Subzones should prediction for the collection zone for which it has been              be identified by an alphabetic suffix to the collection established.                                                      -   zone identification.)       .
              The amount of plutonium collected during            .5. Each assay site should be' marked with paint or the cleanout of a specific collection zone can be assayed            colored tape on the floor. (To be consistent, blue tape through sampling and chemical analysis, through                      should be used for neutron assay sites, orange for gamma calorimetry, or through other applicable nondestructive              ray sites.) The height setting for midpoint assay should assay methods (eg.,. spontaneous fission coincidence                be recorded in the measurement log corresponding to detection or gamma ray assay). Each of these topics is              each assay site....
the subject of a Regulatory Guide.


2.., Assay Instruments
measurement fluctuations,          O(meas);    that  is,                                  provide useful information in the comparison with between      Pua  and  Pur  does    not However, a small difference that  the  bias  associated    with    H  is small.
 
necessarily mean
        0
          (h-u) =    (stat) + a(meas)                                            (1)
                                                                                        5.23-11
 
This ambiguity is reduced in importance if the cleanout is                5. Areas may be denoted as problem areas so that such that Hafter is much smaller than Hbefore. In addition,          careful holdup measurements will be made in these areas the use of several holdup measurements from varying van              each time plant holdup is to be determined; or the area may tage points, as suggested earlier, will help to minimize the          be labeled as a spot-check zone, where accumulations are \
bias associated with incorrect geometrical or attenuation            known to be low and careful holdup assays are needed less corrections in one measurement configuration.                        frequently.


==C. REGULATORY POSITION==
==C. REGULATORY POSITION==
Neutron and gamma ray assay capability should be To develop a program for the periodic in situ assay            provided using separate or compatible' electronics with of plutonium residual holdup as an acceptable                        interchangeable detector probes. Compatible electronics
                                                            5.23-10


should provide for both He-3 or BF3 neutron detection                       2.1.3 Gamma Ray Check Source and Nal(TI) gamma ray detection. The electronics unit should have a temperature coefficient of less than 0.1%                          To ensure the continued normal operation per 'C. Battery-powered electronics should be provided            of each system, an encapsulated plutonium check source to expedite assays.                                                should be provided. The source should be small enough to be implanted in a section of shielding material so
===2. ASSAY INSTRUMENTS===
      2.1 Gamma Ray Assay                                          shaped as to close off the collimator opening. The check source should be positioned adjacent to the detector.
    To develop a program for the periodic in situ assay of                Neutron and gamma ray assay capability can be provided, plutonium residual holdup as a method acceptable to the              if desired, using separate or compatible electronics with NRC staff for measuring this inventory component, it is              interchangeable detector probes. Compatible electronics necessary to consider the factors in the following sections.          can provide for both 3He or BF 3 neutron detection and NaI(T1) gamma ray detection. The electronics unit should Care must be exercised during the fabrication and use of          have a temperature coefficient of less than 0.1 percent per check sources and calibration standards to ensure their              degree centigrade. Battery-powered electronics can expedite continued integrity and to prevent contamination. In                  assays.


Gamma ray assay should be 'based on the                 The source should contain an amount of plutonium activity observed in the energy range from 375 keV to              sufficient to provide a gross count rate of 1000 to
addition, the usual precautions for safeguarding plutonium should be taken.                                                      2.1 Gamma Ray Assay
440 keV, excluding the composite gamma ray complex                  10,000 counts per second.
1. DELINEATION OF COLLECTION ZONES AND                                    Gamma ray assay should be based on the activity observed ASSAY SITES                                                      in the energy range from 375 to 450 keV, excluding the composite gamma ray complex centered at 333 keV. Yield Preliminary radiation survey measurements of the                 data for appropriate gamma rays are presented in Sec plutonium processing facility should be used to budget the            tion B. 2.1 of this guide.


centered at 333 keV. Yield data for appropriate gamma rays are presented in Section B.2.1 of this guide.                          2.1.4 Gamma Ray Calibration Source To permit the calibration of gamma .ray
measurement time to emphasize high-holdup areas, to establish independent collection zones, and to determine                  2.1.1 Detector Selection detector positions within the zones.
          2.1.1 Detector Selection                                 assay collection zones, a calibration standard should be fabricated by encapsulating plutonium oxide in a disk.


Gamma ray detectors should have FWHM             The isotopic composition of the plutonium and the resolution equal to or better than 7.5% at 662 keV                 abundance of Am-241 should be measured and be (Cs- 137 gamma ray). NaI(TI) can meet such                        chosen to be nominally representative of the plutonium specifications and is suitable for this application. The           being processed. The total amount of plutonium crystal depth should be sufficient to detect a significant         encapsulated should be closely monitored. Attenuation percentage of 400-keV gamma rays. For NaI(TI), the                losses within the bed of PuO 2 and through the minimum depth should be one inch. A two-inch depth is             encapsulating material should be measured and the recommended.                                                      calibration standard response normalized to counts per gram incorporating these corrections.
Gamma ray detectors for holdup measurements should
    1. At each collection zone, detector positions (assay            have FWHM (full width at half maximum) resolution better sites) should be chosen so that the material holdup can be            than 10 percent at 662 keV ( 1 3 7 Cs gamma ray). NaI(Tl)
measured from several vantage points around the zone. At              detectors can exhibit resolutions as good as 7 percent and each assay site, the detector should have an exclusive                are suitable for this application. The crystal depth should view of the collection zone being assayed. If necessary,              be sufficient to detect a significant percentage of 400-keV
shadow shielding should be used to isolate the region being          gamma rays. For NaI(T1), the minimum depth should be assayed from other collection zones. Detector positions              1 inch (2.5 cm); a 2-inch (5-cm) depth is recommended.


The crystal should be stabilized with a suitable radioactive source. An'&#xfd; internal Cs] seed                     2.2 Neutron Assay containing Am-241 is recommended for this application.
should be chosen to minimize the measurement ambiguities, as described in Section B.5.1.1.                                          The crystal should be stabilized with a suitable radioac tive source. An internal seed containing 241Am is recom
    2. Each assay site should be permanently marked with              mended for this application. The electronics should be paint or colored tape on the floor to ensure reproducible            capable of stabilizing on the reference radiation emitted by assay positions. The markings should be protected (for                the seed. The crystal face (external to the cover) should be example, with clear epoxy) to ensure their long-term                  covered with 0.75 mm of cadmium and 1.5 mm of lead to durability. Detector height and orientation should be                filter low-energy radiations.


The electronics should be capable of stabilizing on the
clearly indicated in the assay log for each measurement site and, if possible, included in the site markings.                          Two single-channel analyzers should be provided with lock-set energy windows. One channel should be set to
                                                                              2.2.1 Neutron Detector Selection reference radiation emitted by the seed. The crystal face (external to the cover) should be covered with 0.075 to Neutron detectors should have high
    3. Each assay site should be uniquely labeled to facilitate      admit gamma rays from 375 to 450 keV. Unless equilibrium unambiguous reference to that site in the assay log. A                of the 2 3 7 U and 24 1 pu can be ensured, the 333-keV region labeling and color-coding convention should be established            of the gamma ray spectrum should be completely excluded.
0.150 cm cadmium sheet to filter low-energy radiations.


detection efficiency and be capable of operating in the presence of gamma radiation. He-3 and BF 3 neutron Two single-channel analyzers should be
to distinguish neutron assay sites from gamma ray assay              The second channel should be set above the first window to sites.                                                                provide a background subtraction for the assay window.
.provided with lock-set energy windows. One channel                detectors are recommended for this application. Multiple detector tubes with matdhed operating performance should be set to admit gamma rays from 390 keV to 440
                                                                  should be connected in parallel to a single preamplifier keV unless equilibrium of the U-237 and Pu-241 can be to increase the overall detection efficiency obtainable assured. The 333-keV region of the gamma ray spectrum from a single detector tube. Neutron detectors should be should be excluded. With Nal detectors, it is necessary surrounded by a layer of neutron moderator material to to exclude the 375 keV gamma ray to ensure that the enhance their detection efficiency. The neutron tail from the 333 keV complex is not added. The second moderator layer should be covered with a low-energy channel should be set above the first window to provide neutron absorber to filter out extraneous neutrons from a background correction for the assay window. This the desired signal. A recommended configuration is second window should be set from approximately 450
                                                                  diagrammed in Figure B-I.


keV to 600 keV.
This second window should be set from approximately 475
    4. Gamma ray assay should be used for collection zones            to 575 keV. The width and position of this window is a containing less plutonium than the neutron detection limit.          matter of personal preference in how the background Also, gamma ray assay should be used for all structures that          subtraction should be done. These analyzers should be do not contain irregularly shaped components capable of              packaged as one integral unit.


2.2.2 Neutron Collimator
significantly attenuating the emerging gamma rays. Neutron assay should be used for all structures not suitable for                  2.1.2 Gamma Ray Collimator gamma ray assay. There may be some large structures such as furnaces that can be measured only with small interior                A cylinder of shielding material such as lead should be probes or with thermoluminescent dosimeters.                          made concentric with the gamma ray detecto
          2.1.2 Gamma Ray Collimator A cylinder of shielding material such as                           A slab collimator or concentric cylinder lead should be made c(ncentric with the gamma ray                 collimator of a suitable neutron moderator material detector. The end of the cylinder opposite the crystal            (e.g., polyethylene) should be constructed to completely should be blocked with the shielding material. The                surround the detector with its associated moderator and thickness of the collimator should -be chosen to provide          filter assembly, 'leaving open orly the collimator sufficient directionality for the specific facility (1.5 cm        channel. A recommended 'configuration is shown in of lead thickness should be sufficient for most                    Figure B-1.


applications). The collimator sleeve should be extendible over the end of the crystal to reproducible settings to                            The moderator thickness should be vary the degree of collimation for different collection            selected to provide. the directionality required for each zones.                                                            facility. A directionality profile providing a 10:1
====r. The end of====
                                                          5.23-11
                                                                5.23-12


response ratio (six inches of polyethylene) should be               so that the response from the calibration standards will adequate for most applications; however, each situation            not be influenced by the in-process material.
the cylinder opposite the crystal should be blocked with                2.3 Service Cart the shielding material. The thickness of the collimator should be chosen to provide sufficient directionality for the              A cart carrying electronics and both detector probes specific facility (1.5 cm of lead thickness should be sufficient        should be provided. The capability to raise or lower the for most applications). The collimator sleeve should be                probes to reproducible settings should be included.


should be evaluated as discussed in Part B of this guide.
fixed over the end of the detector crystal at a reproducible setting identical to that used in the calibration measure             
 
===3. CALIBRATION===
ments.


3.1 Instrument Check
3.1 Instrument Check
            2.2.3 !NeutronCheck Source The stability of the neutron and gamma ray Any neutron source which emits                      detection systems should be tested prior to each approximately        100-10,000 neutrons/second is                inventory by comparing the observed counts obtained acceptable for this application. The source should be              from the check source, minus the counts with the small enough to be contained within a section of,                  shaped shield in place but without the check source, to neutron moderator material so shaped as to completely              the readings obtained prior to previous inventories. If fill the collimator channel of the detector assembly. The          the measurement is consistent with previous data (i.e., is source should be implanted ,directly adjacent to the                within plus or minus two single-measurement standard neutron detectors, outside the cadmium thermal neutron              deviations of the mean value of previous data), all filter. A recommended configuration for this assembly is            previously established calibrations using this detection diagrammed in Figure B.2.                                          system should be considered valid. If the measurement is not consistent, the operation of the ..unit should be
    2.1.3 Gamma Ray Calibrationand Check Sources The stability of the neutron and gamma ray detection Standard sources of 2 3 9 pu should be provided for                systems should be tested prior to each inventory by compar calibration of the measurement system for the basic measure            ing the observed counts obtained from the check source, ment geometries described in Section B.4. A small encapsu              minus the counts with the shaped shield in place but with lated plutonium sample can be used both as a calibration                out the check source, to the readings obtained prior to standard for the point source counting geometry and as a                previous inventories. If the measurement is consistent with check source for verification of instrument stability. For              previous data (i.e., is within + 2 single-measurement standard the line and area calibrations, large plutonium foils can be            deviations of the mean value of previous data), all previously used, or the calibrations can be derived from a series of              established calibrations using this detection system should measurements made with the point source. The gamma ray                  be considered valid. If the measurement is not consistent, self-attenuation correction should be clearly specified for            the operation of the unit should be checked against the all foils and samples.                                                  manufacturer's recommendations and repaired or recali brated, as required. These check source measurements should be supplemented with regular remeasurements of
            2.2.4 Neutron Assay Calibration Standard                checked against the manufacturer's recommendations and repaired or recalibrated, as required.
2.2 Neutron Assay                                                      instrument calibrations to ensure continued proper instru ment performance over the entire operating range.


To permit the, calibration of neutron assay
2.2.1 Neutron DetectorSelection
* collection zones, a calibration standard should be
                                                                        3.2 System Response Calibration Neutron detectors should have high detection efficiency and be capable of operating in the presence of gamma                      The response of the detection system should be deter radiation. BF 3 and 3He neutron detectors are recommended              mined with well-known quantities of plutonium in the basic for this application. Neutron detectors should be surrounded            measurement geometries described in Section B.4. If there by a layer of neutron moderator material to enhance their              are special counting geometries in the facility that are not detection efficiency. The neutron moderator layer should                readily represented by one of the basic configurations, be covered with a low-energy neutron absorber to filter out            these geometries should also be mocked up and measured extraneous neutrons from the desired signal.                            during the calibration procedure.
                                                                          3.2 Zone Calibration
-fabricated by encapsulating PuO 2 . The PuO 2 should be nominally representative of the plutonium being The geometric response profile for each processed in isotopic composition, in Am-241 content,
"and in the content of high (a,n) yield target materials.             collection zone should be determined by measuring the variation in the response as a calibration standard is The amount of plutonium to be encapsulated should be moved within the defined limits of the collection zone.


chosen to be representative of the amounts of plutonium The. response variation should then be averaged to estimated to be held up in typical neutron assay determine the response per gram of plutonium for that collection zones.
2.2.2 Neutron Collimator                                           


collection zone. The averaging should be weighted to reflect known local accumulation sites within each
===4. ASSAY PROCEDURES===
                'The' neutron yield of the calibration collection-zone. The response per gram should be used standard should be measured and also computed using to directly translate the observed response to grams oi the. method described in the Appendix. The observed plutonium, after the response is corrected for neutron. count rate should be normalized. 6 If the background.
    A slab collimator or concentric cylinder collimator of             4.1 Assay Log polyethylene should completely surround the detector, leaving open only a detection channel in one direction. The                 An assay log should be maintaine


,predicted response differs by more than 10%76, the response should be normalized as discussed in Section
====d. Each collection zone====
                                                                                3.2.1 Subzone Calibration B.2.2.4.
.moderator thickness should be selected to provide the                  should have a separate section in the assay log, with the directionality required for each facility. A directionality            corresponding calibration derived on the page facing the profile providing a 10:1 response ratio is desirable. However,          assay data sheet. Recording space should be provided for portable detectors a 3:1 ratio may be used.                        for the date of measurement, gross counts, corrected counts, and the corresponding grams of plutonium from the
    2.2.3 Neutron Calibrationand Check Source                          calibration in addition to position and instrument electronic setting verification. There should also be provision for A 50- to 100-gram sample of plutonium should be                     recording data from recovery operations and holdup assay adequate both as a point source calibration standard and as            comparisons, as described in Section B.5.3.


2.3 Service Cart                                                            When a collection zone is too large to be accurately measured in a single assay, the collection zone A cart carrying electronics and both detector            should be divided into overlapping subzones. The repeat probes should be provided. The capability to raise or               dimensions of each subzone perpendicular to the lower the probes to reproducible settings should be                 detector-to-collection-zone line should be determined so included.                                                            that the response variation across that distance is nulled.
a check source. The isotopic composition, 241Am content, and high-(c,n)-yield impurity composition should be                    4.2 Preassay Procedures representative of the plutonium being processed. The neutron yield of the standard should be independently                      Prior to inventory, the isotopic composition of the measured, if possible, and also computed using the method              plutonium processed during the current operational period described in the appendix of this guide. If the measured and           should be determined. Variations in the neutron and calculated yields differ by more than 20 percent, any future            gamma ray yield data from the calibration standard should yield calculations should be normalized to be consistent               be calculated. Either the calibration data or the predicted with this measurement.                                                  holdup should then be corrected to reflect this difference.


Using this procedure, the residual geometric uncertainty
5.23-13
      2.4 Notation of Operating Parameters                          should be determined by measuring the response as a calibration standard is moved along the depth When compatible electronics are used to                  coordinate. The calibrated response should then reflect facilitate neutron and gamma ray assay, a notation of              the average of the depth response, weighted to reflect athe respective settings should be affixed to the                    known accumulation sites.


electronics unit. To decrease the likelihood of incorrect settings, the neutron probe and the -appropriate                    4.  Asmy Procedures electronics settings should be color-coded blue; the gamma ray probe and :corresponding electronics settings                  4.1 Ammy LoA
Prior to each inventory, the operation of the neutron           should initial the measurement log to ensure compliance for and gamma ray assay detection systems should be checked,            each collection zone.
should be coded orange.


An assay log should be maintaine
as described in Regulatory Position 3.1.


====d.  Each====
When the preceding steps have been completed, the          t Prior to any assay measurements, feed into the process          measurement at each collection zone should be taken, line should be stopped. All in-process material should be            recorded, and converted to grams of plutonium. If each processed through to forms amenable to accurate account            value is within an expected or permissible range, the assayist ability. All process, scrap, and waste items containing            can proceed to the next collection zone. However, if the plutonium should be removed to approved storage areas to            collection zone contains an unexpectedly large amount of minimize background radiations.                                    plutonium, it should be cleaned to remove the accumulation for conversion to a more accurately accountable material
3.    Calibration                                                  collection zone or subzone should have a separate page in the amy log, with the corresponding calibration Each collection zone should be independently                  derived on the page facing the assay data sheet.
4.3 Measurements                                                    category. After the cleanout has been completed, the zone should be reassayed.


calibrated when all in-process material has been located           Recording space should be provided for the date of
Before beginning the holdup measurements, it is advis able to conduct a preliminary gamma survey of the collec tion zones to point up the zones where holdup accumula              5. ESTIMATION OF HOLDUP ERROR
                                                            5.23-112
tions are the highest (and therefore where the most careful measurements should be made). In zones where accumula                  During the initial implementation of the holdup measure tions are shown to be very low by the survey, spot-check           ment program, the holdup uncertainty for each collection measurements may be adequate, as pointed out earlier.              zone should be estimated from the range of values obtained in the various measurements on that zone, as described in Before assaying each collection zone, the operator              Section B.5.2. As a history of comparisons between holdup should verify the floor location, probe height, and probe          measurements and cleanout recovery data becomes avail orientation. The electronic settings should be verified every      able, these data should be used to adjust for bias and to
1 or 2 hours with the check source. During the actual assay        revise the magnitudes of the holdup uncertainties, as of the collection zones, the check source should be removed        described in Section B.5.3.


measurement, gross counts, corrected counts, and the            collection zone. The operator should initial the corresponding grams plutonium from the calibration in            measurement log to assure conmpliance for each addition to position and instrument electronic setting          collection zone.
or shielded so as not to interfere with the measurement.


verification.
Prior to taking a measurement, a visual check of the zone              During each physical inventory, the calibration in at and the line of sight of the detector probe should be made        least 10 percent of the collection zones should be updated to ensure that no obvious changes have been made to the            on the basis of the comparison between holdup and cleanout process area and that no unintended accumulations of              recovery measurements. In any case, all calibrations should plutonium remain within the collection zone. The operator          be updated at least once per year.


Having met all preceding requirements, the
I
    4.2 Preassay Procedures                                    measurement at each site should be taken, recorded, and converted to grams plutonium. If each value is within an Prior to inventory, the isotopic composition of      expected or permissible range, -the cart should be moved the plutonium processed during the current operational          to the next site and the cycle repeated. If a high period should be determined. Variations in the neutron          response is noted, the cause should be investigated. If and gamma ray yield data from the calibration standard          the collection zone contains an unexpectedly large should be calculated. Either the calibration data or the        content of plutonium, that collection zone should be predicted holdup should then be corrected to reflect this        cleaned to remove the accumulation for conversion to a difference.                                                      more accurately accountable material category. After the cleanout has been completed, the zone should be Prior to each inventory, the operation of the        reassayed and the recovered material quantity used to neutron and gamma ray assay detection systems should              test the validity of the zone calibration.
                                                            5.23-14


be checked.
REFERENCES
                                                                      8.      H. E. Preston and W. J. Symons, "The Determination in Gloveboxes by of the Gamma                  of Residual Plutonium Masses R. Gunnink et al., "A Re-evaluation Branching    Intensities of            Remote Measurements Using Solid Thermoluminescent Ray Energies and Absolute                                                                                          Energy    Author
                  8                    and Am-241:' Lawrence                Dosimeters," United Kingdom Atomic U-237, Pu-23 , -239,-240, -241,                                          ity, Winfrith, England,  AEEW-R13    59,  1980.


5.  Estimation of the Holdup Error Prior to any assay measurements, feed into the process line should be stopped. All in-process material                During the initial implementation of this program, should be processed through to forms amenable to                  the error quoted for the holdup. assay should be accurate accountability. All process, scrap, and waste            computed on the basis of estimating the error items containing plutonium should be removed from the            components, as described in SectionsB.5.1 and B.5.2.
1976.


process areas to approved storage areas to minimize                    Prior to the cleanout of any collection zone for background radiations.                                           whatever purpose, that zone should be prepared for assay and measured as described in:Section C.4 of this
Livermore Laboratory, UCRL-52139, of the L. D. Mclsaac, "Gamma                A. Ohno and S. Matsuura, "Measurement
    4.3 Measurements                                            guide. Following this assay, the collection zone should be cleaned out and the collected plutonium should then The assay cart should:be moved in sequence to        be assayed using an appropriately accurate assay the assay site(s) corresponding to each collection zone.         method. When the collection zone has been cleaned and Assaying all gamma ray sites before assaying neutron              the collected plutonium removed,, the collection zone sites (or Vice versa) is recommended.                             should be reassayed. The recovered plutonium should be used to update the calibration and,. from the. sixth test Before assaying each collection zone, the            on, should serve as the assay error estimate. Separate operator should verify the floor location, probe                  records should be maintained for each collection zone to selection, probe height, and electronics settings. All            estimate the error in assaying the plutonium holdup.
2.   J. E. Cline, R. J. Gehrke, and                                   9.                                               in  a  Spent  Fuel the Fissionable    Nuclides and Asso                Gamma Dose Rate Distribution Rays Emitted by                                                                                                            Detector,"
                                                      Idaho Falls,             Assembly with a Thermoluminescent ciated Isotopes," Aerojet Nuclear Co.,                                                                       485, 1980.


check and calibration sources should be sufficiently removed so as not to interfere with the measurement.                   To ensure that error predictions remain current, Prior to taking a measurement, a visual check of the              only data of the twelve preceding independent tests zone and the line of sight of the detector probe should          should be used to estimate the assay error. Collection be made to assure that no obvious changes have been              zones not cleaned for other purposes should be cleaned made to the process area and that no unintended                  for assay verification at intervals not to exceed two accumulations of plutonium remain within the                      months.
Nuclear Technology, Vol. 47, p.


REFERENCES
Idaho, ANCR-1069, July 1972.
1.   R. Gunnink and R. J. Morrow, "Gamma Ray                      4.    An example of a collimator for uranium gamma ray Energies and Absolute Branching Intensities for                   assay is found in R. B. Walton, et al, "Measurements
 
                    , 1Pu and 2 4 1 Am," UCRL,51087
and H. C. Keller, "A
    238 2 39 240 24
                                              Material Safeguards      10.    W. D. Reed, Jr., J. P. Andrews,
          ,     ,                                                       of UF 6 Cylinders with Portable Instruments," Nucl.
3.    L. A. Kull, "Catalogue of Nuclear                                                                      23 sU  with Limit of Error Laboratories, BNL                Method for Surveying for                                  5 Instruments," Battelle National                                                                        Management,    Vol. 2, p. 39 ,
                                                                                Analysis," Nclear aterials
      17165, August 1972.                                                      1973.
 
"Fundamentals of
4.    R. H. Augustson and T. D. Reilly,                                                                              "Self-Multiplication Fissionable Material,"              N. Ensslin, J. Stewart, and J. Sapir, Passive Nondestructive -Assay of                                 11.
 
Coincidence  Counting,"
                                              LA-5651-M, 1974;                  Correction Factors for Neutron Los Alamos Scientific Laboratory,                                                          Management,    Vol.  VIII, No. 2, p. 60,
                              al.,  "Fundamentals      of Passive            Nuclear Materials also T. D. Reilly et                                                    1979.
 
Assay of  Fissionable Material: Labora Nondestructive Laboratory, tory Workbook," Los Alamos Scientific                                                                                    for Nuclear LA-5651-M, Suppl., 1975.                                          12.    M. S. Zucker et al, "Holdup Measurements            Manage Plants,"  Nuclear  Materials Fuel Manufacturing of UF Cylinders                ment, Vol. X, p. 239,    1981.
 
5.    R. B. Walton et al., "Measurements Technology, with Portable Instruments," Nuclear Vol. 21, p. 133, 1974.                                                                                        "Bulk Sample Self
                                                                          13.    J. L. Parker and T. D. Reilly,                     Measure of Residual Pluto                Attenuation Correction by Transmission
  6.    C. H. Kindle, "In Situ Measurement                                                                the  ERDA    X-  and Gamma-Ray
                                                          5, No. 3,              ment," Proceedings of nium," Nuclear MaterialsManagement, Vol.                                                                            (Conf. 760639),
        p. 540, 1976.                                                            Symposium, Ann Arbor, Michigan p. 219, May 1976.
 
"Total Room
  7.    J. W. Tape, D. A. Close, and R. B. Walton,                                                                     and Error Analysis Holdup of Plutonium        Measured  with   a Large-Area        14.    P. R. Bevington, Data Reduction              1969.
 
Management,                for the Physical  Sciences,  McGraw-Hill, Neutron Detector,"     Nuclear    Materials Vol. 5, No. 3, p. 533, 1976.


(July 1971).                                                      Technol., 21, 133 (1974).
5.23-15
2.  J. E. Cline, R. J. Gehrke, and L. D. Mclsaac,
                                                                  5.  W. D. Reed, Jr., J. P. Andrews, and H. C. Keller, "A
    "Gamma Rays Emitted by the Fissionable Nuclides Method for Surveying for Uranium-235 with Limit and Associated Isotopes," ANCR-1069 (July 1972).
                                                                        of Error Analysis," Gulf-GA-A12641 (June 1973).
3.  L. A. Kull, "Catalogue          of Nuclear Material Safeguards Instruments,"        BNL-17165 (August
    1972).
                                                          5.23-13


APPENDIX
APPENDIX
                                        NEUTRON YIELD COMPUTATIONS
            A. NEUTRON YIELD COMPUTATIONS                                 The coefficients 2.50 and 1.70 are the spontaneous fission yields of 2 3 8 pu and 2 4 2 Pu relative to 24&deg;Pu. The concept \
    The following model for the calculation of the total             2.    (a,n) Neutrons spontaneous neutron yield from plutonium-bearing materials assumes that the plutonium is widely                             The maior contribution to the total neutron dispersed. With this condition, there will be no                       production from (ax) reactions will typically be due to significant neutron production created through induced                the 048 (an) Ne-21 reaction when the plutonium exists fission of Pu-239 or Pu-241. The total neutron yield per              as the oxide. The yield from this reaction per gram of gram of plutonium holdup will then be the sum of the                   plutonium can be calculated using the isotopic spontaneous fission and (an) contributions:                           weight fractions (Wj) and the Yi yield data given in Table 1.
    The following model for the calculation of the total                   of effective 2 4 0 pu mass reflects the fact that most of the spontaneous neutron yield from plutonium-bearing materials                 spontaneous fission yield is due to that isotope.
 
assumes that the plutonium is widely dispersed. With this condition, there will be no significant neutron production                 2. (cz,n) NEUTRONS
through induced fission of 39pu or 2 4 1 Pu. The total neutron yield of plutonium holdup will then be the sum of                    When the plutonium holdup is in the form of oxide, the the spontaneous fission and (ct,n) contributions:                        major contribution from (ct,n) reactions will be due to the
                                                                          0-18(a,n) 21Ne reaction. The additional neutron yield is Yn= YSF + Y(a,n)                                          (1)        typically 50 to 100 percent of the spontaneous fission yield. The (a,n) yield can be calculated from the yields per
1. SPONTANEOUS FISSION NEUTRONS                                          gram of each isotope of Pu(Yi) given in Table A-1:
    To determine the spontaneous neutron yield of pluto                      Y(ay,n) oxide = 1 MiYi                                    (5)
nium, the isotopic composition must be known. (The                                                1y contribution from 23SU spontaneous fission is usually negligible even if uranium is present in large quantities.)              The summation over Mi should also include          24 1 Am, which The yield from the plutonium isotopes is given by:                        is a strong alpha emitter.
 
YSFp M2 3 8Q 2 38 + M2 4 0 Q 2 4 0 + M2 4 2 Q 2 4 2        (2)            In addition to (c4n) production in the oxide itself, certain low-Z impurities in the oxide can contribute substan where Mi is the total mass of the ith plutonium isotope, and              tially. Values for the yields of neutrons obtained in bombard Qi is the spontaneous fission neutron yield per gram of the               ing thick targets of these elements with 5.2-MeV alpha ith isotope. Using the yield data from Table A-l, Equation 2              particles are given in Table A-2. Further research may can be rewritten as:                                                      change these values somewhat, but they are sufficient for computing the approximate effect of these elements if they YSF = (1030 n/sec-gram)M 24 0 (effective)                  (3)        exist as impurities in PuO . One method for doing this is to compute the impurity (x,n) yield relative to the oxide where                                                                    (ct,n) yield:
    M2 4 0 (effective) = 2.50M 2 38 + M240 + 1.70M 2 4 2      (4)            Y(an)impurity - Y(cn)oxide f (WiAoI )/(Pok          io)  (6)
                                                                    ble A-1 ALPHA PARTICLE AND SPONTANEOUS FISSION NEUTRON YIELDS
                                                                                                                        Y.
 
Qi                      PuO 2 U0 2 Half-Life          Alpha Activity                Spontaneous Fission              (a,n) Yield*
            Nuclide                (yr)              (a/sec-gram)                    (n/sec-gram)                  (n/sec-gram)
            238pu,              87.78              6.33 x 1011                  2.57 x 103                        1.4 x 104
            239pu
                                24,150            2.30 x 109                    2.22 x 10-2                      42.5
            2 4 0
                  pu
                                6,529              8.43 x 109                    1.03 x 103                        157
            2 4 1 pu
                                  14.35**          9.39 x 107                    5.00 x 10-2                        1.3
            2 4 2 pu
                                379,000            1.44 x 108                    1.75 x 103                        2.2
            24 1 Am            433.8              1.27 x 10"1                  6.05 x 10f'                      2957
            234U                2.47 x 105        2.29 x 108                    5.67 x 10-3                      4.65
                                                                                                                                3
            235U
                                7.1 x 108          7.93 x 104                    5.96 x 10-4                        1.37 x 10-
            238 U
                                4.51 x 109        1.23 x 104                    1.12 x 10-2                        1.93 x 10-4
 
* Oxygen yield from PuO 2form only.
 
**&branching
              *            ratio: 2.46 x i0T5 .
                                                                5.23-16
 
Table A-2                                                        Table A-3 (a,n) NEUTRON YIELDS FOR                            SAMPLE CALCULATION FOR 1 GRAM OF PLUTONIUM
                VARIOUS LIGHT ELEMENTS
                                                                                    Isotopic            YSF        Y(a, n)o ide P.                            Nuclide        Composition          (n/sec)        (n/sec)I
                                  Neutron Yield                    23 SPU            0.003              8            42 Element        per 106 Alphas                    2 3 pt            0.756              0            32
                                                                    24 PU              0.185            191            29 Be                  58                            S1 Pu              0.045              0              0
                B                    18                          242                0.011            19              0
                F                    6.4                          241  Am            0.003              0              9
                7 Li                  1.3 Na                  -1.5                          Total Yields                        218            112 NatMg                0.89 Al                  0.44 Natsi                0.077 C                    0.05 O                    0.050                            Using the isotopic composition given in Table A-3 and using Equation 3, the spontaneous fission neutron yield can
              *Most of these yields are based on:                  be found to be 218 n/sec for 1 gram of plutonium. Then J. K. Baki and J. Gomez del Campo,
                "Neutron Yields from Alpha-Particle                the neutron production in the oxide can be calculated Bombardment," Nuclear Science and                  using the masses Mi of each isotope and the yields Y- from Engineering, Vol. 71, p. 18, 1979.
 
the fifth column of Table A-1. The result of 112 n)sec is given in the last column of Table A-3. Note that the alpha In Equation 6, P. is the (a,n) neutron yield in the impurity      particle yield of ylutonium is nearly constant in time, but element, and P 0 is the yield in oxygen (0.050 neutrons/106          that, because 24 Am builds up in time, the total alpha alphas); A. is the atomic weight of the impurity element,            production increases at a rate of roughly 0.3 percent per and Ao that for oxygen (16); Ij is the concentration of the          month in typical reactor fuel impurity expressed in parts per million (by weight) of plutonium oxide, and Io is oxide (118,000 ppm). If the impurity concentration is expressed as ppm of plutonium,                The impurity (ct,n) yields are calculated in Table A-4.
 
it can be converted to ppm of plutonium oxide by multi              The calculation is based on impurities in PuO 2 only. The plying by the gravimetric dilution factor, 0.882.                    mixed oxides are assumed to consist of blended PuO 2 and UO2 particles approximately 40 pm in diameter where To summarize the calculation of (ct,n) neutron yields in        most alpha particles stop within the PuO 2 particles. If the oxide that also contains impurities, Y(a,n) from all sources        particle size were smaller or the mixed oxide were created is given by:                                                         through coprecipitation, the uranium impurity content would also contribute to the plutonium (a,n) yiel
 
====d. In the====
                                                          (7)        present example, it is sufficient to use the neutron yields P.
 
Y(        i MiYf + .0027EP_-I-/A-)
              Z,n)        "      I
                                                                    from Table A-2, the concentrations Ij from Table A-4, and Equation 6 or 7.
 
Elements other than those listed in Table A-2 yield no neutrons by (ct,n) reactions for the alpha energies obtained from plutonium and americium decay. Also note that the                The total neutron yield from 1 gram of plutonium in summation over i must include 241 Am and that the summa            PuO 2 is then 218 + 112 + 47 = 377 n/sec. Using the gravi tion over j includes only the oxygen that is not bound up as        metric dilution factor of 0.882, this is 333 n/sec for 1 gram plutonium oxide.                                                    of PuO 2 . If the PuO 2 is blended so that PuO 2 / (PuO 2 +
                                                                    UO ) = 0.03, the neutron yield from 1 gram of mixed
3.  SAMPLE CALCULATION FOR PuO 2 -UO 2                              oxide is 10 n/sec.
 
Consider the case of I gram of recycle plutonium                    The impurity (ct,n) yields, Pi, used in this example are blended to 3 percent by weight of PuO 2 in a UO 2 matrix            currently known to about 10 percent accuracy for most where the isotopic composition is as given in Table A-3. For        elements and 50 percent accuracy for the others. The oxide mixed oxides, the oxygen density is approximately the                              .J are known to 10 percent or better. Both (atn) yields, Y..,
same as in PuO 2 alone. Also, plutonium and uranium have            yield calculations must assume perfect mixing, however.
 
similar atomic numbers. For these reasons, it may be                For these reasons, neutron yield calculations are accurate to assumed that the oxygen (ca,n) yield in mixed oxide is              10 percent at best, and the neutron holdup measurement the yield in PuO 2 , further reduced by the blending ratio,          calibration should be based on representative standards PuO 2 /(PuO 2 + U0 2 ).                                            rather than calculation wherever possible.
 
5.23-17
 
Table A-4                                  Let f238, f239' f240' f241, and f2 4 2 represent the weight fractions of the respective plutonium isotopes in the IMPURITY (ca,n) YIELD                              unknown sample. The 2 4 0 pu effective weight fraction, f (effective), can be defined as:
                                                                      240
                      Arbitrary Concentration I            Impurity (cn)                                                                      (8)
                      in PuO 2            Yield (I12 n/sec)            f2 4 0 (effective) = M2 4 o(effective)/Mpu(total)
Impurity            (ppm by wgt)            (0.00 2 7 )Pjlj/Aj where Li                        9                        1 Be                        8                      16                    f2 4 0 (effective) = 2.50f 2 39 + f 24 0 + 1.70f 2 4 2  (9)
B                        10                        5 C                      200                        0                Generally, as previously mentioned in this guide, the F                      125                      13                relative measurement uncertainty of M2 4 0 (effective) in a
0 (moisture)          4600                        4                holdup measurement will be much larger than that of Na                      120                        8                f;40(effective), so the relative error in Mpu(total) is essen Total                                            47 (n/sec)        tially equal to that of M2 4 0 (effective).
                                                                        As an example calculation, the sample of isotopic composition given in Table A-3 has an effective fraction given by:
  B. CONVERSION OF MEASURED M D4(EFFECTIVE)                                                                + 0.185 + 1.70(0.011)
                TO TOTAL PLUTONId"                                                        = 2.50(0.003)
                                                                        f 24 0 (effective) = 0.21
                                    240
  To convert a measured effective        pu mass to actual          Thus, a holdup measurement of 35 + 10 grams 24&deg;pu total plutonium, one must use both the relationship between          effective corresponds to 166 + 47 grams total plutonium, these two quantities, as shown in Equation 4, and the                where the relative error in the total plutonium result was known isotopic composition of the samples being measured.            taken to be equal to that of the M2 4 0 (effective) result.
 
5.23-18
 
VALUE/IMPACT STATEMENT
                                                                      1.3.4 Public
 
===1. PROPOSED ACTION===
                                                                      No adverse impact on the public can be foreseen.
 
1.1 Description
                                                                    1.4 Decision on Proposed Action Licensees authorized to possess at any time more than
1 kilogram of plutonium are required by Part 70, "Domestic The regulatory guide should be revised to reflect improve Licensing of Special Nuclear Material," of Title 10 of the ments in measurement techniques and to bring the language Code of Federal Regulations to calculate a material balance        of the guide into conformity with current usage.
 
based on a measured physical inventory at intervals not to exceed 2 months. Further, these licensees are required to         
 
===2. TECHNICAL APPROACH===
conduct their nuclear material physical inventories in compliance with specific requirements set forth in Part 70.            Not applicable.
 
Inventory procedures acceptable to the NRC staff are detailed in Regulatory Guide 5.13, "Conduct of Nuclear
 
===3. PROCEDURAL APPROACH===
Material Physical Inventories."
                                                                        Of the procedural alternatives considered, revision of the Plutonium residual holdup is defined as the plutonium existing regulatory guide was selected as the most advanta inventory component remaining in and about process geous and cost effective.


Yn = YSF + Y(,t,n)                                  (1)
equipment and handling areas after those collection areas have been prepared for inventory. This 'regulatory guide
                                                                            Y(an) Oxy            WiYi                        (3)
                                                                    4. STATUTORY CONSIDERATIONS
1. Spontaneous Fission Neutrons To determine the spontaneous neutron yield per gram of plutonium held up within a collection zone, the                     The yield per gram of Put 2 is calculated by multiplying the yield per gram of plutonium by the isotopic composition of the plutonium and uranium (if gravimetric dilution factor (Pu/PuO 2 - 0.882).
describes procedures acceptable to the NRC staff for the in situ assay of the residual plutonium holdup.                       4.1 NRC Authority
present) must be known. The contribution from spontaneous fission can generally be calculated by neglecting the contribution from U-238:                                      The presence of certain impurities can contribute substantially to the total (atn) production rate.
1.2 Need for Proposed Action The authority for the proposed action is derived from the Atomic Energy Act of 1954, as amended, and the Regulatory Guide 5.23 was published in 1974. The Energy Reorganization Act of 1974, as amended, and is proposed action, a revision to this guide, is needed to bring implemented through the Commission's regulations, in the guide up to date with respect to advances in measure          particular 10 CFR Part 70.


YSF = W 2 3 8 Q 2 3 8 + W240Q240 + W242Q242                      Approximate values of (a,n) impurity yields for the highest yield (an) target materials are given in Table 2.
ment methods, as well as changes in terminology.


(2)        To compute the impurity (an) contribution, the total a particle production is determined. Production rates of a where                                                                  particles per gram of the principal nuclides of interest are shown in Table 1. This contribution to the total Wi = weight fraction of the ith plutonium                        neutron yield can be computed using the relationship:
4.2 Need for NEPA Assessment
      isotope. For reactor fuel applications, W2 3 8 + W239
1.3 Value/Impact of Proposed Action The proposed action is not a major action that may
      + W240*'W241 +W242* I
    1.3.1 NRC Operations significantly affect the quality of the human environment and does not require an environmental impact statement.
      Qi = spontaneous fission neutron yield per gram of                     Y(a,n) Impurity = Y 0 TPjlj i
                                                                                                                                (4)
      the ith plutonium isotope (see Table 1).
                                                              TABLE 1 a Particle and Spontaneous Fission Neutron Yields
                                                                                                    8 Half-life              Alpha Activity                  PuO 2 (mn) Yield          Spontaneous Fission Nuclide                      (yr)                (r/sec-gram)                      (n/sac-ram)                  (n/sec-gram)
Pu-238                  87.78                    6.33 x 1011                      1.71 x 104                  2.57 x    103 Pu-239                  24,150                  2.30 x 109                        54.5                        2.22 x    10-2 Pu-240                  6,529                    8.43 x 109                        202.1                        1.03,x  103 Pu-241                  14 . 3 5 b              9.39 x 10'                        2.03                        2.43 x    10-2 Pu-242                  379,000                  1.44 x 108                        3.13                        1.75 x  103 Am-241                  433.8                    1.27 x 10 11                      3.46 x 103                  6.05 x    10-1 U-234                    2.47 x 105              2.29 x 108                        4.65                        5.67 x    10-3 U-235                    7.1 x 108                7.93 x 104                        1.37 x 10-3                5.96 x    10-4 U-238                    4.51 x 109              1.23 x 104                        1.93 x 10--4                !.12 x  10-2 a - Oxygen yield from PuO2 form only.


b - &-branching ratio - 2.46 x 10-5
The regulatory positions will be brought up to date.
                                                                5.23-14


where                                                                          Pj    = (an) yield per ppm of the impurity j (see Table 2)
S. RELATIONSHIP TO OTHER EXISTING OR PROPOSED
      Y,,        = total a production Ii    = impurity j content, expressed in ppm (weight) of plutonium.
    1.3.2 Other Government Agencies                                    REGULATIONS OR POLICIES
    Not applicable.


=          WWiai    + WArnm'Am i                                                3.    Sample Calculation (PuO 2 -UO 2 )
The proposed action is one of a series of revisions of exist ing regulatory guides on nondestructive assay techniques.
      Wi          = Pu isotopic weight fractions                                Consider the case of recycle plutonium blended t6
                                                                          3 wt %Pu in a normal U0 2 matrix, Where the isotopic WAm = Am weight fraction = Am/Pu                                    composition is Pu-238 (.25%). Pu-239 (75.65%), Pu-240
                                                                          (18.48%), Pu-241 (4.5%), Pu-242 (1.13%), and Am-241 ai          = a yield per gram of nuclide i (see Table 1)          (.28% of Pu).
                                                                                For mixed oxides, the oxygen density is approximately the same for the case ofPuO.. This fact, TABLE 2                          together with the atomic similarity of uranium and (Q,n) Yield Rats of Low-Z Impurities in Pu02a                    plutonium, justifies the assumption that the oxygen (a,n) yield per gram of mixed oxide is the yield per gram of PuO 2 , further reduced by the blending ratio, P.            Pu/(Pu + U).
I mpurity                                            (n/a-ppm)
                                                                                Using the values given in Table I, the spontaneous Li .........................                  6.29 x 10-12            fission yield and total a production per gram of Be .........................                  2.00 x 10 -' 0          plutonium can be computed. Results are shown in Table
                                                                            3.


B ....        ......................          4.63 x 10-11 C ..........................                    2.77 x 10-13 The a particle yield of plutonium is constant in time
1.3.3 Industry
    0b .......              . .................      1.56 x b0-"13 for all intents. However, the Am-241 a production in- F ...........................                  2 .44 x 1O-Il          creases at a rate which results in approximately a 0.3%;.
                                                                    6. SUMMARY AND CONCLUSIONS
     Na .........................                    3.00 x 10-12          increase per month in the total a production, for the g...........................                  2.67 x 10-12          range of plutonium isotopic compositions intended for if    .........................                1.45 x 10-1 2          reactor fuel application.
     Since industry is already applying the methods and procedures discussed in the guide, updating these methods              Regulatory Guide 5.23 should be revised.


Si    .........................                3.25 x 10-13 In the present example, the impurity levels of the aAssumnes zero yield from all other impurities.                          principal (a.n) target materials are shown in Table 4. The bOxygen not contained in oxide.                                          neutron yields attributable to (an) interactions on those TABLE 3 Sample Calculation Spontaneous Fission                  Alpha Production            PuO 2 1a,n)a Nuclide                                    W              (nsec-g Pu)                        (cx/sec-g Pu)            (n/sec.- Pu)
and procedures should have no adverse impact.
Pu7238                                        .0025              6.4                              1.58  x  199                42.6 Pu-239                                        .7565              <.05                            1.74  x  109                41.3 Pu-240                                        .1848              189.4                          1.56  x  l09                37.3 Pu-241                                        .0450              <.05                            4.23    x  106                0. i Pu-242                                        .0113              19.8                            1.63  x  106                <0.05 Ain-241                                      .0028              <.05                            3.56    x  108                9.7 rotal Yields                                                    215.6                          5.26    x  109                131.0
-- oxygen yield only.


impurities are also shown in Table 4, calculated using the                              TABLE 4 a particle production rate of 5.3 x 109 a/sec-g Pu,                                Impurity (.,n) Yield computed above. In this example, the mixed oxides are composed of blended PuO 2 and U0 2 particles Arbitrary approximately 40 microns in diameter. If the particle size were smaller or the mixed oxide was created                                      Concentration      (a,n) Yield Impurity                  (ppm)            Wnisec-g. Pu)
5.23-19
through coprecipitation, the uranium impurity content would also contribute to~the plutonium(an) yield. This contribution can be ignored for large particles and              Li                          9              0.30
estimated by combining the impurities for small particles        Be                          8              8.42 and coprecipitatedoxides.                                        B                          10              2.44 C                          200              .30
    The total neutron yield in this example is 380              F                          125            16.0
n/sec-g Pu. In this example, the percentage of plutonium          OR              ...    4600              3.77 tq the total Pu + 0 is 0.8835. Using this gravimetric            Na                        120              1.90
dilutign factor, the neutron yield is 336 n/sec-g PuO2 . If the PuO 2 is blended with U0 2 to 3%, i.e., PuO 2 /PuO 2 +                                          Total    33.1 U0 2 = 0.03, the neutron yield. from the blend will be
10.1 n/sec-g MO.                                                aOxygen present in moisture, not as oxide.


5.23-16}}
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(Task SG 045-4), Revision 1, in Situ Assay of Plutonium Residual Holdup
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Revision 1*

February 1984 U.S. NUCLEAR REGULATORY COMMISSION

REGULATORY GUIDE

OFFICE OF NUCLEAR REGULATORY RESEARCH

REGULATORY GUIDE 5.23 (Task SG 0454)

IN SITU ASSAY OF PLUTONIUM RESIDUAL HOLDUP

information obtained in the holdup survey can be used to

A. INTRODUCTION

locate principal plutonium accumulations and to ensure that other areas of the process contain less than the Part 70, "Domestic Licensing of Special Nuclear detectable amount of plutonium. Once located, substantial Material," of Title 10 of the Code of Federal Regulations accumulations can be recovered, transforming the pluto requires licensees authorized to possess more than 1 kilo nium to a more accurately measurable inventory compo gram of plutonium to calculate a material balance based nent. Having reduced the amount of plutonium holdup, on a measured physical inventory at intervals not to the standard error on the remeasurement of the remaining exceed 2 months. Further, these licensees are required to holdup may be sufficiently reduced to be compatible with conduct their nuclear material physical inventories in overall SEID requirements.

compliance with specific requirements set forth in Part 70.

Inventory procedures acceptable to the NRC staff are Any guidance in this document related to information detailed in Regulatory Guide 5.13, "Conduct of Nuclear collection activities has been cleared under OMB Clearance Material Physical Inventories." No. 3150-0009.

Plutonium residual holdup is defined as the plutonium

B. DISCUSSION

inventory component remaining in and about process equipment and handling areas after these collection areas Plutonium accumulates in cracks, pores, and zones of have been prepared for inventory. Whenever possible, poor circulation within process equipment. The walls of process equipment should be designed' and operated so as process vessels and associated plumbing often become to minimize the amount of holdup. In this guide, proce coated with plutonium during solution processing. Surfaces dures acceptable to the NRC staff for the in situ assay of internal and adjacent to process equipment, especially the plutonium residual holdup are described. glovebox walls and floors, accumulate deposits of pluto nium that can become appreciable. Plutonium also accu Assay information may be used in one of two ways: mulates in air filters and associated ductwork. The absolute amounts of plutonium holdup must be small for efficient

1. When the standard error (estimator) of plutonium processing and proper hazards control. However, the total holdup is compatible2 with constraints on the overall holdup can be large relative to the plant inventory differ standard error of the inventory difference (SEID), the mate ence (ID) but have no significant impact on the ID if it rial balance can be computed using the measured contents remains reasonably constant. It is the change in the of plutonium holdup. Additional cleanout and recovery for holdup between beginning inventory and ending inventory accountability will then not be necessary. that may impact the ID.

2. When the standard error of plutonium holdup is not The measurement procedures described in this guide compatible with constraints on the overall SEID, the involve the detection of gamma rays and neutrons that are spontaneously emitted by the plutonium isotopes. Because are the gamma rays of interest are emitted by the major isotope, Design features to minimize holdup in process equipment

1 the subject of a series of regulatory guides (5.8, 5.25, and

5.42). 23gpU, gamma ray assay is the preferred method whenever 239 its acceptance criteria are satisfied. The amount of pu

2 Compatibility exists when the contribution of the standard to error of the holdup to the total plant SEID is not large enough cause the overall SEID to exceed allowed limit

s. If the plant SEID

exceeds allowed limits because of an excessive contribution from The substantial number of changes in this revision has made it the holdup standard error, compatibility does not exist and the impractical to indicate the changes with lines in the margin.

remedial steps of paragraph 2 needto be taken.

Comments should be sent to the Secretary of the Commission, USNRC REGULATORY GUIDES U.S. Nuclear Regulatory Commission, Washington, D.C. 20555, Attention: Docketing and Service Branch.

Regulatory Guides are issued to describe and make available to the public methods acceptable to the NRC staff of Implementing The guides are issued in the following ten broad divisions:

specific parts of the Commission's regulations, to delineate tech niques used by the staff in evaluating specific problems or postu 1. Power Reactors 6. Products lated accidents or to provide guidance to applicants. Regulatory 2. Research and Test Reactors 7. Transportation Guides are not substitutes for regulations, and compliance with 3. Fuels and Materials Facilities 8. Occupational Health them is not required. Methods and solutions different from those set 4. Environmental and Siting 9. Antitrust and Financial Review out in the guides will be acceptable if they provide a basis for the 5. Materials and Plant Protection 10. General findings requisite to the issuance or continuance of a permit or Government license by the Commission. Copies of issued guides may be purchased at the current guides in spe Printing Office price. A subscription service for future Office.

from Printing This guide was Issued after consideration of comments received cific divisions is available through the Government prices may the public. Comments and suggestions for improvements in these Information on the subscription service and current GPO Commission, guides are encouraged at all times, and guides will new Informaas be revised, be obtained by writing the U.S. Nuclear Regulatory appropriate, to accommodate comments and to reflect Washington, D.C. 20555, Attention: Publications Sales Manager.

tion or experience.

holdup in a piece of equipment is proportional to the relatively uniform cross section. When a collection zone measured intensity of the emitted gamma rays after suitable contains a complex item of equipment with significant corrections are made for attenuation by intervening mate self-shielding properties, the uncertainty in the holdup, rials, for self-attenuation by plutonium, for scattering, for measurement may be primarily due to attenuation of K

geometrical factors, and for background radiation. radiation in the internal structure. In such cases, neutron assay from the outside and thermoluminescent dosimeter If plutonium is held up in furnaces, grinders, or other assay from the inside may be applicable.

heavy equipment that is too dense to permit the escape of gamma rays, an assay based on spontaneous fission neutrons If delineation of collection zones is not possible, two from the even isotopes of plutonium may be possible. This alternatives are experiments with mockup geometries or technique requires knowledge of the isotopic composition complex numerical calculations.

of the plutonium, some knowledge of its chemical form, and knowledge of the presence of other radionuclide 2. APPLICABLE METHODS AND INSTRUMENTS

impurities.

Two considerations are critical to the selection of Thermoluminescent dosimetry is a third technique that methods and instruments. First, to perform an assay, one can be used to measure holdup from the inside of large must ensure that the plutonium radiations reach the detector pieces of equipment. This technique is also useful for and are detected. Second, the observed response must be carrying out measurements in an unobtrusive manner attributable to the collection zone being assayed. Therefore, outside normal plant operating hours. the assay scheme is developed around penetrating radiations, and the detector is collimated to provide for sufficient For all three techniques, the proportionality factors directionality in the response to resolve a collection zone between amount of holdup and detector response are best from its neighboring zones and from the background.

determined prior to the holdup measurement by assays of Finally, some effort may be necessary to employ external

"shadow shielding" to block radiation being produced in known quantities of plutonium distributed in well-defined and representative geometries, as discussed below. adjacent collection zones from the field of view of the collimated detector.

1. DELINEATION OF COLLECTION ZONES

2.1 Gamma Ray Assay Typical plutonium process facilities comprise a number of interconnected gloveboxes that contain work areas and Under closely controlled conditions, the measured most process equipment, in-process storage areas, and plutonium gamma ray spectrum can be interpreted in terms self-contained process equipment. Also, solution process of the abundance of each gamma ray emitter present in the ing requires tanks, plumbing, and pumping equipment, sample. Because of the large number of gamma rays (Refs. 1 which are often located in close proximity to the glovebox and 2) present, many regions of the observed spectrum are lines. Finally, storage areas for feed, scrap and waste, and characterized by overlapping lines. To accomplish the assay, final product are often located in close proximity to the it is necessary to select an appropriate spectral region and plutonium process area. provide a detection system with sufficient resolution to measure the activity from the isotopes of interest.

To accomplish the holdup measurements, it is essential to consider the facility in terms of a series of zones that can Gamma ray assay has an advantage over neutron assay in be independently assayed. Such zones are designated that the emissions are primarily from the principal isotopes

"collection zones." Each plutonium-processing facility can of interest. Because of the hiih emission rate of gamma be conceptually divided into a series of contiguous collection rays, a detection sensitivity of less than 1 gram is generally attainable.

zones on the basis of process activities and collection geometries. Individual machines, filters, pipes, tanks, The most useful portion of the spectrum for holdup gloveboxes, or surface areas that can be isolated from one another may be suitable discrete collection zones. Great assay is the 2 3 9 pu gamma ray complex in the 375- to

45 0-keV range. 3 The yields of these lines are given in care is needed to define all collection zones so that (1) the Table 1.

assay of the zone can be performed with a minimum of interference from nearby zones, (2) the detector can be positioned reproducibly and in such a way that the radiation being measured experiences a minimum, or easily predicted, attenuation in the apparatus being measured, and (3) the 3

1n typical Nal detectors (with energy resolution of 10 percent distribution of material in the zone can be represented by at 414 keV), the 414-keV photopeak will produce counts in the one of the distribution geometries used in the calibration approximate energy range of 373 to 455 keV. Thus, an energy window from 375 to 450 keV will include most of the 414-keV

procedure described below. full-energy counts for a variety of detector resolutions. Further more, suca, window setting will include a significant fraction of the

375-keV P9Pgamma rays (see Table 1), but will also exclude Gamma ray assay for plutonium holdup is practical 'mosorftihe otentially interfering 332-keV gamma rays from when the collection zone consists of a single structure of

5.23-2

Table 1 2 inches (5 cm) with a thickness of 2 inches is recommended.

For germanium detectors, a moderate-volume coaxial

23 9 detector is recommended.

PROMINENT GAMMA RAYS FROM pu IN 375- to 450-keV ENERGY RANGE

2.1.2 Collimatorsand Absorbers for Gamma Rays Intensity

2 39 Pu) A shaped shield constructed of any heavy-element Energy (y/sec-g material is appropriate for gamma ray collimation. For cost,

375.0 3.59 x 104 availability, and ease of fabrication, lead is recommended.

380.2 0.70 x 104 Less than 2 percent of all 400-keV gamma rays striking a

382.7 0.59 x 104 1.5-cm-thick sheet of lead will pass through without suffer

0.26 x 104 ing an energy loss.

392.5

393.1 1.01 x 104

413.7 3.43 x 104 The collimator will be most effective when it is con

422.6 0.27 x 104 centric about the crystal and photomultiplier and com pletely covers the photomultiplier bas

e. Extending the

9.85 x 104 collimator forward of the crystal at least a distance equal to Total half the diameter of the crystal, and preferably the full diameter, is recommended (Ref. 5). Making this distance variable to reproducible settings will permit adjustment over a range of collection zone sizes. However, it is highly

2.1.1 Gamma Ray Detection Instruments desirable to select collection zones and counting geometries so that one collimator setting will suffice for all measure Gamma ray detection systems consist of a scintillation ments. This will simplify the calibration procedures because or semiconductor gamma ray detector and appropriate the calibration constants depend strongly on the dimensions electronics (Refs. 3 and 4). Required electronics include at and placement of the collimator aperture.

least a single-channel analyzer and a timer-scaler unit. A

second single-channel analyzer viewing the same detector The collimator not only defines the effective field of pulses used to determine the background radiation correc view but also shields the detector from unwanted radiation.

tion is a timesaving feature. A number of portable battery To effectively accomplish this latter purpose, the collimator powered systems are commercially available for this applica material must also cover the rear of the detector. This is tion.

usually easy to achieve with portable Nal detectors but requires more effort when germanium detectors are used.

The detection efficiency andresolution (10 to 15 percent) Intensive 50- to 100-keV X-ray radiation and 60-keV

of NaI(Tl) is generally adequate for holdup measurements. 241 Am gamma ray radiation are often emitted by process CdTe, Ge(Li), and high-purity germanium (also known as equipment, and this radiation can tie up the detector intrinsic germanium) detectors have better resolution than electronics unnecessarily. A 1.5-mm-thick layer of lead (on NaI(Tl) but are more costly and more difficult to operate. the outside) and a 0.75-mm-thick layer of cadmium (on the For more information on Ge(Li) and intrinsic germanium inside) may be placed against the front face of the detector detectors, see Regulatory Guide 5.9, "Guidelines for to alleviate this problem. This graded energy shield will Germanium Spectroscopy Systems for Measurement of absorb most of the low-energy photons incident on the Special Nuclear Material," and the references cited therein. detector without substantially reducing the number of gamma rays detected in the 375- to 450-keV range.

237 The 332.3-keV gamma ray from U, a short-lived

24 1 the principal inter 2.1.3 Check Source for Gamma Ray Assay

(6.75 days) daughter of pu, is usually ference for 2 39 Pu assay by Nal detection of the 375- to

7 It is important to check the operation of the detection

450-keV complex. If the 2 3 U is in equilibrium with

24 1 pu, the intensity of this gamma ray is 1.15 x 106 system each time the instrumentation is moved or otherwise disturbed (e.g., power outage) during the course of each y/sec-g 2 pu. Since this gamma ray is also emitted in the

4 1 inventory sequence. Either recalibrating one or more decay of 241 Am, the interference from this decay branch collection zones and comparing the results to previous may also be important in case of preferential americium analyses or testing the instrument with a check source is holdups. To avoid this interference when using Nal detectors, appropriate. When the response remains within the expected the assay energy window is adjusted to span the range from value, the previous calibration data are assumed to be valid.

390 to 450 keV for plutonium holdup with high americium If not, the energy window may have shifted, or the unit content.

may be in need of repair and recalibration.

An appropriate check source enables the stability of the Detector crystal dimensions are selected to provide a assay instrument to be tested at any location. Such a source high probability of detecting gamma rays from the 375- to can be prepared by implanting a small encapsulated plutonium

450-keV complex and a low probability of detecting sample (containing '.,0.5 gram of plutonium) in the face of high-energy radiation. For NaI, a crystal diameter of

5.23-3

a plug of shielding material. The plug is shaped to fit and low gamma ray sensitivity in the detectors are important.

close the collimator channel, and the source is positioned to Gas-filled proportional counters containing He or BF 3 are be adjacent to the crystal when the plug is in place. suitable for this purpose. Typical fill pressures are 1 to 4 atmospheres. One advantage of 3 He for in-plant applications The check source is fabricated in a manner that will is that the operating voltage of 3He counters is about ensure its internal stability. Other than radiations increasing 75 percent of that required for BF 3 counters.

from the ingrowth of 2 4 1 Am, the emission rate of the check source should remain constant. The efficiency of 3 He and BF 3 counters increases as the energy of the neutrons decreases. Embedding gas-filled

2.2 Neutron Assay counters in polyethylene to moderate the incoming neutrons to thermal or epithermal energies will improve their effi Neutrons are emitted in the spontaneous fission of 238Pu, ciency. A nearly optimum design can be obtained by center

24°Pu, and 242Pu and through the interaction of emitted ing the counters in 10 cm of polyethylene with 2 to 3 cm alpha particles with certain light nucleL These neutrons of polyethylene between adjacent counters.

suffer little attenuation in passing through uranium or plutonium or through most structural and containment To shield the detector from low-energy neutrons that materials. Glovebox windows may reduce the energy of may produce a complicated response pattern, the modera emerging neutrons, but, because of their regular and con tor material is covered with a thermal neutron absorber.

stant shape, their effect can generally be factored into the Cadmium sheeting approximately 0.075 cm thick may be assay calibration. used for this application.

To be useful for the assay of plutonium holdup, the 2.2.2 Collimatorsfor Neutron Detectors neutron production rate per gram of plutonium must be known. The spontaneous fission contribution to the total To assay a specific collection zone in the presence of neutron production can be computed from basic nuclear other distributed sources of plutonium, it is necessary to data after the isotopic composition of the contained collimate the detector. This is accomplished by stopping plutonium has been determined. Computing the (ca,n) neutrons coming to the detector from all directions except contribution requires a knowledge of the chemical form of the desired one. The cadmium surrounding the detector will the plutonium and the amount and distribution of certain stop essentially all neutrons striking the detector with high-(ca,n)-yield target materials. energies below 0.4 eV. By adding moderator material around the outside of the detector in all directions except The background count rate from neutron detectors may for the collimator channel, neutrons coming from be a substantial part of the observed activity, often corre unwanted directions will lose energy in this shield and will sponding to as much as 20 grams of plutonium in typical be absorbed in the cadmium cover. For each 6 inches holdup assays. Thus, neutron assay is primarily applicable (15 cm) of polyethylene added, the collimator assembly to the measurement of significant accumulations of pluto provides a factor of approximately 10 in the directionality nium. of the response.

The measured neutron yield from prepared calibration An example of a collimated neutron detector assembly standards is used to calibrate each neutron assay collection for plutonium holdup assay is shown in Figure 1. This zone. In the appendix to this guide, a method is given to assembly has a polyethylene shield thickness of 6 inches calculate the anticipated neutron yield. This method (15 cm) and a directionality of 10 to 1. The combined provides the ability to calculate the neutron yield when the weight of the detector and collimator exceeds the require isotopic or impurity composition of the plutonium holdup ments for a hand-held probe. For this reason and to provide is different from that of the calibration standards. The for reproducible positioning at each assay, a sturdy cart method can be used to calculate a ratio of the neutron housing such a detector and its associated electronics is production rate of the unknown material to the neutron recommended. In order to assay items at different heights, production rate of the standard material. The yield from the capability to raise and lower the assembly to reproduc the holdup material is then determined by multiplying the ible settings is needed.

measured "known" material yield by the computed ratio.

An example of a small commercially available hand-held

2.2.1 Neutron Detection Instruments detector is given in References 5 and 6. This Shielded Neutron Assay Probe (SNAP) is 12 inches (30.5 cm) high To effectively employ the spontaneous neutron yield as and 10 inches (25.4 cm) in diameter and contains two 3He a measure of plutonium holdup, it is necessary to detect the detectors. It includes a 2-inch-thick (5 cm) polyethylene neutrons in the presence of a more intense gamma ray shield that provides a directionality of 3 to

1. The SNAP

background and to collimate the detector so that neu has been used to measure plutonium holdup, UO2 F2 trons emanating from the collection zone under assay are holdup, and UF 6 enrichment. It is recommended for the preferentially detected. assay of well-defined concentrations of plutonium in pumps, grinders, pipe elbows, or other items of equipment Holdup assay is performed under in-plant conditions where portability and accessibility are more important than where ruggedness, portability, high neutron efficiency, and directionality.

5.23-4

DETECTOR CABLE ACCESS CHANNEL

(TOP SECTION ONLY) r-.

117

1 I5.1cm 15.1cm 3" IT

4.5cm FRONT VIEW -- ,c + TOP VIEW

+

N DETECTOR

"HANNELS

"I 2.6cmnDIAl METER (TYP)

I I I I

IIFRONT I

I II9cI

VIEW

6 3c I I I IIl I

m W9mITH0,7I CDIUMSI E

4 DETECTOR TUBE SUBASSEMBLY

4 Assembly includes three BF 3 or 3He tubes (2.54 cm diameter). Unit can be modified to increase or decrease the number of tubes. Moderator thickness of 15 cm provides u 10:1 directionality. Addi tional polyethylene can be added to improve directionality (e.g., 30 cm polyethylene provides *, 100:1 directionality). Components are bolted or strapped to remain in a fixed configuration.

Figure 1. Collimated Neutron Detector Assembly for Plutonium Holdup Assay

5.23-5

3. ISOLATION OF COLLECTION ZONES

A third example of holdup measurement by neutron detection is given in Reference 7. In this case, a completely

3 To ensure that each collection zone is independently uncollimated polyethylene slab containing a row of He assayed, it is necessary to shield the detector from all detectors was suspended in midair in some of the processing radiations except those radiations emanating from the rooms of an industrial plutonium facility. The response of collection zone being assayed. This is principally accom the detector was found to be proportional to the total plished through the use of the collimators described in room holdup if the plutonium was reasonably uniformly Sections B.2.1.2 and B.2.2.2. Two additional means exist to distributed and if the room was isolated from external isolate a collection zone, detector positioning and shadow sources. The calibration procedure for the use of. this shielding.

detector will not be described here. However, it is recom mended as a means for quickly verifying total room holdup 3.1 Detector Positioning when measurements of the holdup in individual items or equipment are not needed.

An unobstructed side view of a collection zone is pre

+/-erred. When plutonium is located behind the zone under

2.2.3 Check Source for Neutron Assay assay in another collection zone or a storage facility, an additional background assay may be performed with the To ensure the proper operation of the neutron assay detector above or below the collection zone and pointing at system prior to making an assay, it is necessary to test the the material behind the zone under assay. It is important to response of the instrument. An appropriate neutron assay prevent, or account for, moving objects within the field of check source can be measured, or the detector response for view. If this is not done, variations in shielding and scatter one or more collection zones can be recalibrated and ing can affect the measurement.

compared to the results of previous calibrations.

3.2 Shadow Shielding An appropriate neutron assay check source can be prepared by implanting a small encapsulated plutonium It may not be possible to avoid interfering radiations by source (containing about 5 grams of plutonium) into collimator design or by choosing the detector position for the face of a plug of neutron moderating material (see assay. In such cases, it may be possible to move a shield Figure 2). The plug is fabricated to fit and close the colli panel between the source of interfering radiations and the mator channel. When the response from the check source collimator zone under assay. If the shield panel is very thick remains within the expected value, the previous calibration and its dimensions match or exceed the back side of the data are assumed to be valid. collection zone under assay, no interfering radiations will penetrate through the shadow shield to the detector. While such characteristics are desirable, the size of such a shield

2.3 Thermoluminescent Dosimeter (TLD) Assay would limit its transportability. A rectangular panel mounted on wheels as an upright panel and containing %5 cm of neu Crystals of LiF, CaF 2 , CaSe 4 , or other compounds can tron moderator (e.g., benelex, WEP, or polyethylehe) and store energy at manganese or dysprosium impurity centers ,0.5 cm of lead sheet is recommended. To use such a when they are struck by gamma or neutron radiation. At panel, it is necessary to measure the response of the collec some later time, the crystals can be heated rapidly to tion zone with and without the shield in place. Also, the several hundred degrees centigrade to induce thermo gamma and neutron transmission factors of the shield itself luminescence. The light output at this time is proportional must be measured beforehand with a representative pluto to the amount of radiation received. Thermoluminescent nium sample. From these measurements, the assay of the detectors that are primarily gamma sensitive, graded X-ray collection zone can be corrected for background radiation shields, read-out instrumentation, and other accessories are transmitted through the shield.

commercially available.

4. CALIBRATION FOR HOLDUP MEASUREMENTS

TLDs have been used to measure the holdup in glove boxes by placing them at regular intervals on the outside 4.1 Basic Counting Geometries surfaces. The TLDs are left in place overnight in order to accumulate a measurable dose. Accuracies of +20 percent There are three fundamental counting geometries that relative to cleanout values are reported for plutonium of can be used to represent most collection zones. These known isotopic composition. TLDs have also been used to geometries are distinguished by the spatial distribution of measure the holdup in the interior of large furnaces that are the source material and the resulting dependence of the not accessible by other means. For both of these examples, detector counting rate on the source-to-detector distance, r.

calibration requires either careful dose and geometry calculations or mockups of the actual collection zone. 4.1.1 Point Source Because their use is relatively new and only a few published references exist (Refs. 8 and 9), TLDs will not be discussed If the material being assayed is distributed over an area further in this guide. However, they could be useful for with dimensions that are small compared with the source- special applications.

5.23-6

COLLIMATOR

TOP VIEW

-CHECK SOURCE

COVER

CHECK SOURCE

FRONT VIEW

Figure 2. Neutron Collimator Channel Plug and Check Source

5.23-7

to-detector distance and if the material resides entirely sources although neutron assay is usually restricted to within the detector field of view, the zone can be treated as dense, isolated items of equipment that can be represented a point source. The detector count rate for a point source as point sources. For both neutron and gamma measure varies inversely as the square of the source-to-detector ments, corrections to the point and line source calibrations distance (count rate is proportional to l/r 2 ). Any equip for different detector distances are made using the 1/r 2 or ment measured at great distances or any small pieces of 1 /r count-rate dependence, respectivel

y. For further detailed

4 equipment or equipment parts fall in this category. discussion of the measurement of detector responses for these basic geometries, see Reference 10.

4.1.2 Line Source For gamma ray assay, the calibration of the point source If the material being assayed is distributed along a linear response can be accomplished with a well-characterized path so that only a segment of that distribution length is encapsulated standard plutonium foil smaller in size than contained in the detector field of view, the zone can be the detector collimator opening. This foil can also serve as treated as a line source. The detector count rate for a line the check source for verification of the continued stability source varies inversely as the source-to-detector distance of the instrument settings in the field. It is important that (count rate is proportional to I/r). Examples of this type of care be taken in the preparation of this calibration standard holdup geometry include isolated sections of piping and to ensure that the amount of encapsulated 2 3 9 pu is well long, narrow ducts or columns. known. It is also important to measure the gamma ray attenuation through the encapsulating material and the

4.1.3 Area Source self-attenuation of the plutonium foil and to correct the calibration standard response to compensate for these If the material being assayed is spread over an area so effects. Enough 2 3 9 Pu needs to be encapsulated in this large that it covers the full field of view of the detector for standard to provide count rates that will ensure good a range of source-to-detector distances, the zone can be statistical precision of the calibration in a reasonable period assayed as an area source. As long as the material being of time.

viewed is uniformly distributed, the detector count rate will be independent of the source-to-detector distance.

However, for holdup applications, uniform material distribu For neutron assay, it is probably necessary to encapsulate tion is rare; so the source-to-detector distance can affect the a larger amount of material in the calibration standard instrument response and needs to be specified. Furthermore, because the spontaneous neutron production rate is signifi when there are several measurement locations covering a cantly less than the 375- to 450-keV gamma ray production large area (such as a floor), it is important to maintain the rate. A quantity of 50 to 100 grams of plutonium is ade same source-to-detector distance (even if material distribu quate for most applications. Again, it is important to know tion is uniform within a given measurement area) so that the exact quantity and isotopic composition of the pluto the number of measurement areas needed to cover the nium. Also, the neutron calibration standard may generate entire area remains constant. Examples of this type of assay more neutrons than directly attributable to the spontaneous geometry include floors, walls, glovebox floors, and large fission and (a ,n) reactions. Because a relatively large quantity rectangular ducting. of PuO 2 Ls encapsulated in the neutron assay calibration standard, some spontaneous fission or (a,n) neutrons may

4.2 Calibration of Detector Response be absorbed in 239pu or 24 1pu nuclei, producing additional neutrons through the induced fission reactio

n. The amount

4.2.1 Mockup of Known Material Distributions of multiplication depends in a complex manner on the amount and distribution of PuO 2 and on the surrounding When a gamma ray assay is used and a collimator setting medium (Ref. 11). For 50 grams distributed in the bottom has been selected, the detector responses for the three basic of a 4-inch-diameter (10 cm) can, a self-multiplication of source distribution geometries listed above need to be 0.5 percent of the total neutron output would be typical.

determined. For the point source, the response is expressed At 100 grams, 1 to 2 percent may be expected. Thus, this as (counts per minute)/gram of 2 3Pu at a specified source effect is typically smaller than other errors associated with to-detector distance. For the line source, the response is holdup measurements and can be neglected if the standard expressed as (counts per minute)/(gram of 2 3 9 pu per unit contains 100 grams or less of well-distributed material. The length) at a specified source-to-detector distance. For the chemical and isotopic composition of the plutonium will area source1 the response is expressed as (counts per minute)/ have a larger effect, as described in the appendix to this (gram of 39pu per unit area) at a specified source-to guide.

detector distance. When neutron assay is used, the response for a point source is expressed as (counts per minute)/gram of 24°Pu effective at a specified source-to-detector distance. The measurement of the line source response is best Calculation of 2 4 °pu effective from the plutonium isotopic accomplished by constructing a cylindrical surface distribu composition is described in the appendix to this guide. tion of plutonium with the aid of large foils. It is also Analogous expressions can be given for line and area possible to establish the line source response using a point

4Caution: small deposits of plutonium could exhibit very large source, as described in Reference 4. The line source geom gamma ray self-attenuation and could therefore require great care in etry is closest to that of the pipes and ducts likely to be analysis or could require neutron assay. encountered in actual measurements.

5.23-8

measurements of the detector responses can be made. This The area source response can be measured with the same procedure would be a valuable supplement to calibration plutonium foils laid flat to simulate the expected distribu data obtained from mockups of standard counting geom tion on surfaces such as walls and floors. The area response etries and comparisons with cleanout recovery data.

can also be established using a point source. The point source is measured at different radial distances from the

5. HOLDUP MEASUREMENTS AND STANDARD ERROR

center of the field of view of the collimated detector. The response at each radial distance is weighted by the area of a The measurement of holdup in a complex plant environ concentric ring at that radius. From these weighted re ment can involve a very large number of measurements. In a sponses, it is then possible to calculate the area of a circular stable plant environment where the process behavior is well region of uniform plutonium deposition that would yield this known and well characterized, it may be possible to arrange the same total response as the point source. From of 23 9 Pu per the holdup measurement program so that:

equivalent area, the expected response/(gram unit area) can be derived. Further useful details on this a. Careful and extensive holdup measurements are made procedure may be found in Reference 12. For both line and infrequently (e.g., annually) and area calibrations, the self-attenuation of the foils or point sources also needs to be taken into account.

b. At more frequent intervals (e.g., at inventory times),

careful measurements are made in known problem There may be special material distribution geometries areas, and "spot check" measurements are made in in the facility that are not readily represented by one of the the other, less used, zones where accumulations are three basic configurations described above. These special known to be low.

geometries may be mocked up as carefully as possible with large plutonium foils and point sources to produce a usable Such management of measurement resources can result in a detector response calibration for these special cases. Examples very effective holdup measurement program at minimum of special cases might be concave or convex equipment costs surfaces or the internal volume of a rectangular cavity (see Ref. 10). Because material particle sizes (or material deposit 5.1 Holdup Measurements thicknesses) have a significant effect on the self-attenuation of the gamma ray signals, it is important to use (whenever In performing the holdup measurements, one must be practical) well-characterized process material for preparing aware of the large variability in holdup assays arising calibration standards and to duplicate to the extent possible primarily from variability in the measurement conditions process holdup distribution relative to particle size or thick (e.g., background, geometry, gamma ray or neutron attenua ness. Furthermore, holdup in floors is often deposited at tion, material distribution). Accordingly, it is important various depths into the floor, rather than on the surface.

to perform the assays from as many vantage points as Thus, calibration standards for such measurements need to possible for each collection zone. If this is impractical on a incorporate the appropriate geometry and matrix effects.

routine basis because of time or space constraints, one Core samples of a floor may be needed to establish typical might consider multiple measurements initially on a collec concentrations at various floor depths.

tion zone, followed by fewer routine measurements at repre sentative assay sites. Careful thought in the selection of Calibration of the holdup measurement system using this measurement points and measurement strategy will mini procedure is recommended until a history of comparisons mize ambiguities in the interpretation of the data.

between predicted and recovered holdup quantities is developed. If it is possible to take holdup measurements

5.1.1 Selection of Collection Zones and Detector before and after the cleanout of a piece of shut-down Positions process equipment, they can be used to establish this comparison history and improve the accuracy of the Location and configuration of collection zones are calibration for each collection zone.

established on the basis of a detailed physical examination and a radiation survey of the physical layout of the facility.

4.2.2 Measurement of CalibrationSources in Actual Preliminary measurements are needed to determine the ProcessEquipment optimum detector positions for the holdup assays. If nonuniform distribution of material in a collection zone is One method for calibrating detector response to holdup suspected or if the process apparatus is sufficiently compli radiation in process equipment is to place a known calibra cated to require extensive attenuation corrections for tion source in various positions in that equipment and certain counting geometries, multiple measurements are record the detector responses. In this way, the overall advisable for the collection zone. More than one detector detector response (including all corrections for attenuation position may be necessary. In the cases where radiation and geometry) is determined empirically. Unfortunately, surveys have pointed out zones of high holdup collection, this procedure is impractical, if not impossible, in process extra care will be necessary in the holdup measurements for equipment already in operation. However, if those respon those zones to minimize their contribution to the overall sible for holdup assays are made aware of occasions when holdup variability. Where radiation surveys show little hold new equipment is brought into the plant for installation in up, proportionately less time need be budgeted. Selecting the process, calibration sources can be conveniently placed optimum detector positions includes consideration of the in the equipment before its installation and the empirical

5.23-9

the individual measurements. Further, the variability need to conveniently measure the line-of-sight background between these measurements can provide an indication of by moving the detector to one side without changing its the measurement uncertainty.

orientation.

5.1.3 Gamma Ray Attenuation Corrections

5.1.2 Holdup Measurement Procedure To obtain useful assay results by detecting 375- to The measurement and analysis of gamma or neutron

450-keV gamma rays, it is necessary to correct each assay radiation from a collection zone may be carried out by for attenuation of the signal, either within the plutonium treating the material distribution as a point, line, or area holdup material or by structural materials. Without this source, as described in Section B.4.1, or as one of the critical correction, the assay is no more than a lower limit special cases that may have been measured, as mentioned in on the true holdup value. The attenuation correction may Section B.4.2. If the nature of the material distribution is be based on calculations of known attenuation in uniform uncertain for a particular detector position, a measurement materials, on earlier measurements of materials similar to of the detector counting-rate dependence on the source-to those found in the plant equipment, or on direct measure detector distance, r, may reveal the most appropriate ments of gamma ray transmission through the actual counting-rate geometry with which to interpret thc data.

equipment. Details on establishing an appropriate attenua tion correction are given in Laboratory Exercise No. 4 of After the assay positions for the detector and shadow Reference 4. Additional treatment of gamma ray attenua shields are established for each collection zone, permanent tion corrections is given in Reference 13.

markings that indicate detector location (including height)

and orientation will ensure reproducibility of subsequent

5.1.4 Gamma Ray Interferences measurements for these positions. Uniquely labeling each assay site will facilitate unambiguous reference to each Variability in the observed gamma ray response may measurement and its location in the assay log. Furthermore, arise as a result of the presence of extraneous gamma ray assay site labels and markings can indicate whether neutron emitters or as a result of fluctuations in the background or gamma ray measurements are to be made. Alphabetic from the Compton scattering of higher energy gamma rays.

labels (for example, "G" for gamma and "N" for neutron)

The magnitude of this effect is generally small. It can be and color-coded tape markings of the sites would be useful.

monitored by observing the spectrum with a multichannel Protecting the markings (for example, with clear epoxy)

analyzer, but, unless data on periodically recovered holdup will ensure their long-term durability.

accumulations are in error, this contribution can be ignored.

After measuring the gamma or neutron radiation intensity 5.1.5 Matrix Effects on Neutron Assay at each detector position in a given collection zone, the line-of-sight background is measured by moving the detector A change in the neutron yield for a plutonium sample of and collimator to one side (still pointing in the same fixed isotopic content can be caused by a change in the direction as during the assay) and measuring the radiation concentration of high-(a,n)-yield impurities in the matrix.

intensity from the surrounding materials. During the If it is possible to estimate the range of permissible impurity background measurement, the vessel in which the holdup is concentrations, the variation in a typical neutron yield being measured must not be in the field of view of the can be calculated using the method given in the appendix to detector. Because uncertainties in geometry, attenua this guide.

tion, or sample matrix will usually dominate the total response variability, the counting period need not be long. 5.1.6 Effect of Isotopic Uncertainty Having 1000 to 10,000 net counts is generally sufficient for most holdup applications.

Gamma ray measurements of plutonium holdup provide 2 39 a direct determination of the fissile plutonium (i.e., Pu This procedure is repeated at all measurement posi and 241 Pu) holdup in the zone under consideration. On the tions and in all counting geometries designated for each other hand, neutron techniques measure only the 240Pu collection zone. The final holdup value for the zone is effective content, and chemical techniques provide obtained from the average of the individual measurements elemental analysis without consideration of the isotopic (each one being corrected for the effects of attenuation and makeup. Thus, knowledge of the isotopic composition of any variation in geometry relative to the calibration measure the plutonium is necessary to correlate holdup measure ment).

ments with chemistry and accountability value

s. Gamma

2 39 ray assays must be divided by the pu isotopic fraction, Whenever possible, the collection zone is assayed in a 240

Pu effective and neutron assays must be converted from variety of ways. For example, one could measure an appara to total plutonium in order to express holdup in terms of tus up close and treat it as an area source; the measurement total plutonium.

could then be repeated at a large distance, treating the zone as a point source. It may be better to measure some zones If the process equipment is thoroughly cleaned each from several different directions-especially if complicated time the isotopic composition is changed, the holdup may attenuation corrections are called for in some of the count consist primarily of the current material. In that case, the ing geometries. Several independent measurements of one declared isotopic composition can be used. When mixing zone can provide an average holdup value that is better than

5.23-10

5.3 Estimation of Bias occurs, use of the stream-averaged isotopic composition is esti appropriate. Bounds on the isotopic composition are When a single collection zone is cleared out, it is desirable and lowest 1 to perform a holdup assay before, Hbefore, and after, mated by considering the batches of highest This

.omposition and computing the corresponding range. H fter, the cleanout if possible. By comparing the amount into the measure of variability must then be incorporated oplutonium removed, Pur, to the recovery amount pre making direct dicted through the in situ holdup assays, Pua, the collection estimated holdup standard deviation before comparisons with the chemical analyses. The variability in zone calibration can be updated, and the calibration and stan isotopic composition can be expressed as an estimated range and assay standard deviations can be based on relevant data.

The amount of plutonium recovered, Pur, during the dard deviation defined as one-half the observed deviation cleanout of a specific collection zone can be assayed then combined in quadrature with the standard gamma given by Equation 1 in Section B.5.2. In general, through sampling and chemical analysis, through calorim sensitive to isotopic etry, or through other applicable nondestructive assay ray measurements of 239 pu will be less

240

Pu. or variations than neutron measurement of methods (e.g., spontaneous fission coincidence detection gamma ray assay).

5.2 Assignment of Standard Error measure The assay value for the recovered amount is computed as The assignment of a standard error to a holdup the difference in the holdup assays before and after the difficult on a rigid statistical basis. This is ment is extremely cleanout:

because the only statistically predictable fluctuations (e.g.,

negli counting statistics) in this application are frequently (2)

variability due to counting geo Pua = Hbefore - Hafter gible in comparison with material distribution), gamma ray attenua metry (including The percent difference, A, between the assay and neutron tion, gamma ray background and interferences, recovery values for the plutonium holdup is then computed:

It is important matrix effects, and instrument instabilities.

and guard to recognize that the variability can be large A = 100 (Pua - PUr)/Pur (3)

the standard deviation of the against underestimating holdup value in a collection zone. Careful measure overall A running tabulation of the quantities Pua, Pur, and A (as ments must be carried out during the calibration procedure well as their standard deviations, oa, ar, and OA) is kept in from to determine the range of detector responses resulting the assay log for each collection zone.

parameters. A useful discussion variations in measurement of these ideas is presented in Reference 10. The average value, A, of the percent differences between holdup Pua and Pur will serve as an estimate of the bias in the assay for that collection zone and will also provide quantita of the A reasonable estimate of the standard deviation tive justification for revision of the assay calibration for that holdup for a given collection zone may be The root-mean-square deviations, measured values zone to remove the bias.

obtained by consideration of the range of holdup aA, of the percent differences, Ai from their mean value, measurements performed on obtained from the variety of A, serve as a check on the appropriateness of the size of the as suggested in the previous section.

that collection zone, average of estimated standard deviation of the holdup measurements.

The mean value for the holdup is defined as the To the extent that the standard deviation of Pur is small on the collec the various (corrected) measurement results compared with the uncertainty in Pua (usually an adequate a, for that mean value is in size tion zone. The standard deviation, assumption), the quantity sA should be comparable the range of holdup values obtained in For K measurements of the estimated as one-half to the standard deviation of Pua.

if a large the measurements. This estimate is conservative percent differences, Ai, for a given collection zone, the made. For a small number of measurements have been quantity sA is given by:

actual standard deviation can number of measurements, the the range. In such cases the actual be larger than one-half FK -E2 1/2 calculated.

standard deviation of the holdup values must be ) /(K - 1

2 s A [i E (Ai 1I (4)

counting In some cases, it may be unavoidable that the that they contribute significantly to statistics are so poor 0

Equation 4 assumes that all the A's are equal. For a variability. In such an instance, the overall weighted sums, see Reference 14.

the measurement calculation of sA using as the square holdup standard deviation, CF(h-u)' is defined of the standard deviation or root of the sum of the squares Note that, if the holdup measurements (ie., Hbefore and the standard deviation due to can still due to counting, O(stat), Hafter) contain a constant bias, their difference Pur.

measurement fluctuations, O(meas); that is, provide useful information in the comparison with between Pua and Pur does not However, a small difference that the bias associated with H is small.

necessarily mean

0

(h-u) = (stat) + a(meas) (1)

5.23-11

This ambiguity is reduced in importance if the cleanout is 5. Areas may be denoted as problem areas so that such that Hafter is much smaller than Hbefore. In addition, careful holdup measurements will be made in these areas the use of several holdup measurements from varying van each time plant holdup is to be determined; or the area may tage points, as suggested earlier, will help to minimize the be labeled as a spot-check zone, where accumulations are \

bias associated with incorrect geometrical or attenuation known to be low and careful holdup assays are needed less corrections in one measurement configuration. frequently.

C. REGULATORY POSITION

2. ASSAY INSTRUMENTS

To develop a program for the periodic in situ assay of Neutron and gamma ray assay capability can be provided, plutonium residual holdup as a method acceptable to the if desired, using separate or compatible electronics with NRC staff for measuring this inventory component, it is interchangeable detector probes. Compatible electronics necessary to consider the factors in the following sections. can provide for both 3He or BF 3 neutron detection and NaI(T1) gamma ray detection. The electronics unit should Care must be exercised during the fabrication and use of have a temperature coefficient of less than 0.1 percent per check sources and calibration standards to ensure their degree centigrade. Battery-powered electronics can expedite continued integrity and to prevent contamination. In assays.

addition, the usual precautions for safeguarding plutonium should be taken. 2.1 Gamma Ray Assay

1. DELINEATION OF COLLECTION ZONES AND Gamma ray assay should be based on the activity observed ASSAY SITES in the energy range from 375 to 450 keV, excluding the composite gamma ray complex centered at 333 keV. Yield Preliminary radiation survey measurements of the data for appropriate gamma rays are presented in Sec plutonium processing facility should be used to budget the tion B. 2.1 of this guide.

measurement time to emphasize high-holdup areas, to establish independent collection zones, and to determine 2.1.1 Detector Selection detector positions within the zones.

Gamma ray detectors for holdup measurements should

1. At each collection zone, detector positions (assay have FWHM (full width at half maximum) resolution better sites) should be chosen so that the material holdup can be than 10 percent at 662 keV ( 1 3 7 Cs gamma ray). NaI(Tl)

measured from several vantage points around the zone. At detectors can exhibit resolutions as good as 7 percent and each assay site, the detector should have an exclusive are suitable for this application. The crystal depth should view of the collection zone being assayed. If necessary, be sufficient to detect a significant percentage of 400-keV

shadow shielding should be used to isolate the region being gamma rays. For NaI(T1), the minimum depth should be assayed from other collection zones. Detector positions 1 inch (2.5 cm); a 2-inch (5-cm) depth is recommended.

should be chosen to minimize the measurement ambiguities, as described in Section B.5.1.1. The crystal should be stabilized with a suitable radioac tive source. An internal seed containing 241Am is recom

2. Each assay site should be permanently marked with mended for this application. The electronics should be paint or colored tape on the floor to ensure reproducible capable of stabilizing on the reference radiation emitted by assay positions. The markings should be protected (for the seed. The crystal face (external to the cover) should be example, with clear epoxy) to ensure their long-term covered with 0.75 mm of cadmium and 1.5 mm of lead to durability. Detector height and orientation should be filter low-energy radiations.

clearly indicated in the assay log for each measurement site and, if possible, included in the site markings. Two single-channel analyzers should be provided with lock-set energy windows. One channel should be set to

3. Each assay site should be uniquely labeled to facilitate admit gamma rays from 375 to 450 keV. Unless equilibrium unambiguous reference to that site in the assay log. A of the 2 3 7 U and 24 1 pu can be ensured, the 333-keV region labeling and color-coding convention should be established of the gamma ray spectrum should be completely excluded.

to distinguish neutron assay sites from gamma ray assay The second channel should be set above the first window to sites. provide a background subtraction for the assay window.

This second window should be set from approximately 475

4. Gamma ray assay should be used for collection zones to 575 keV. The width and position of this window is a containing less plutonium than the neutron detection limit. matter of personal preference in how the background Also, gamma ray assay should be used for all structures that subtraction should be done. These analyzers should be do not contain irregularly shaped components capable of packaged as one integral unit.

significantly attenuating the emerging gamma rays. Neutron assay should be used for all structures not suitable for 2.1.2 Gamma Ray Collimator gamma ray assay. There may be some large structures such as furnaces that can be measured only with small interior A cylinder of shielding material such as lead should be probes or with thermoluminescent dosimeters. made concentric with the gamma ray detecto

r. The end of

5.23-12

the cylinder opposite the crystal should be blocked with 2.3 Service Cart the shielding material. The thickness of the collimator should be chosen to provide sufficient directionality for the A cart carrying electronics and both detector probes specific facility (1.5 cm of lead thickness should be sufficient should be provided. The capability to raise or lower the for most applications). The collimator sleeve should be probes to reproducible settings should be included.

fixed over the end of the detector crystal at a reproducible setting identical to that used in the calibration measure

3. CALIBRATION

ments.

3.1 Instrument Check

2.1.3 Gamma Ray Calibrationand Check Sources The stability of the neutron and gamma ray detection Standard sources of 2 3 9 pu should be provided for systems should be tested prior to each inventory by compar calibration of the measurement system for the basic measure ing the observed counts obtained from the check source, ment geometries described in Section B.4. A small encapsu minus the counts with the shaped shield in place but with lated plutonium sample can be used both as a calibration out the check source, to the readings obtained prior to standard for the point source counting geometry and as a previous inventories. If the measurement is consistent with check source for verification of instrument stability. For previous data (i.e., is within + 2 single-measurement standard the line and area calibrations, large plutonium foils can be deviations of the mean value of previous data), all previously used, or the calibrations can be derived from a series of established calibrations using this detection system should measurements made with the point source. The gamma ray be considered valid. If the measurement is not consistent, self-attenuation correction should be clearly specified for the operation of the unit should be checked against the all foils and samples. manufacturer's recommendations and repaired or recali brated, as required. These check source measurements should be supplemented with regular remeasurements of

2.2 Neutron Assay instrument calibrations to ensure continued proper instru ment performance over the entire operating range.

2.2.1 Neutron DetectorSelection

3.2 System Response Calibration Neutron detectors should have high detection efficiency and be capable of operating in the presence of gamma The response of the detection system should be deter radiation. BF 3 and 3He neutron detectors are recommended mined with well-known quantities of plutonium in the basic for this application. Neutron detectors should be surrounded measurement geometries described in Section B.4. If there by a layer of neutron moderator material to enhance their are special counting geometries in the facility that are not detection efficiency. The neutron moderator layer should readily represented by one of the basic configurations, be covered with a low-energy neutron absorber to filter out these geometries should also be mocked up and measured extraneous neutrons from the desired signal. during the calibration procedure.

2.2.2 Neutron Collimator

4. ASSAY PROCEDURES

A slab collimator or concentric cylinder collimator of 4.1 Assay Log polyethylene should completely surround the detector, leaving open only a detection channel in one direction. The An assay log should be maintaine

d. Each collection zone

.moderator thickness should be selected to provide the should have a separate section in the assay log, with the directionality required for each facility. A directionality corresponding calibration derived on the page facing the profile providing a 10:1 response ratio is desirable. However, assay data sheet. Recording space should be provided for portable detectors a 3:1 ratio may be used. for the date of measurement, gross counts, corrected counts, and the corresponding grams of plutonium from the

2.2.3 Neutron Calibrationand Check Source calibration in addition to position and instrument electronic setting verification. There should also be provision for A 50- to 100-gram sample of plutonium should be recording data from recovery operations and holdup assay adequate both as a point source calibration standard and as comparisons, as described in Section B.5.3.

a check source. The isotopic composition, 241Am content, and high-(c,n)-yield impurity composition should be 4.2 Preassay Procedures representative of the plutonium being processed. The neutron yield of the standard should be independently Prior to inventory, the isotopic composition of the measured, if possible, and also computed using the method plutonium processed during the current operational period described in the appendix of this guide. If the measured and should be determined. Variations in the neutron and calculated yields differ by more than 20 percent, any future gamma ray yield data from the calibration standard should yield calculations should be normalized to be consistent be calculated. Either the calibration data or the predicted with this measurement. holdup should then be corrected to reflect this difference.

5.23-13

Prior to each inventory, the operation of the neutron should initial the measurement log to ensure compliance for and gamma ray assay detection systems should be checked, each collection zone.

as described in Regulatory Position 3.1.

When the preceding steps have been completed, the t Prior to any assay measurements, feed into the process measurement at each collection zone should be taken, line should be stopped. All in-process material should be recorded, and converted to grams of plutonium. If each processed through to forms amenable to accurate account value is within an expected or permissible range, the assayist ability. All process, scrap, and waste items containing can proceed to the next collection zone. However, if the plutonium should be removed to approved storage areas to collection zone contains an unexpectedly large amount of minimize background radiations. plutonium, it should be cleaned to remove the accumulation for conversion to a more accurately accountable material

4.3 Measurements category. After the cleanout has been completed, the zone should be reassayed.

Before beginning the holdup measurements, it is advis able to conduct a preliminary gamma survey of the collec tion zones to point up the zones where holdup accumula 5. ESTIMATION OF HOLDUP ERROR

tions are the highest (and therefore where the most careful measurements should be made). In zones where accumula During the initial implementation of the holdup measure tions are shown to be very low by the survey, spot-check ment program, the holdup uncertainty for each collection measurements may be adequate, as pointed out earlier. zone should be estimated from the range of values obtained in the various measurements on that zone, as described in Before assaying each collection zone, the operator Section B.5.2. As a history of comparisons between holdup should verify the floor location, probe height, and probe measurements and cleanout recovery data becomes avail orientation. The electronic settings should be verified every able, these data should be used to adjust for bias and to

1 or 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> with the check source. During the actual assay revise the magnitudes of the holdup uncertainties, as of the collection zones, the check source should be removed described in Section B.5.3.

or shielded so as not to interfere with the measurement.

Prior to taking a measurement, a visual check of the zone During each physical inventory, the calibration in at and the line of sight of the detector probe should be made least 10 percent of the collection zones should be updated to ensure that no obvious changes have been made to the on the basis of the comparison between holdup and cleanout process area and that no unintended accumulations of recovery measurements. In any case, all calibrations should plutonium remain within the collection zone. The operator be updated at least once per year.

I

5.23-14

REFERENCES

8. H. E. Preston and W. J. Symons, "The Determination in Gloveboxes by of the Gamma of Residual Plutonium Masses R. Gunnink et al., "A Re-evaluation Branching Intensities of Remote Measurements Using Solid Thermoluminescent Ray Energies and Absolute Energy Author

8 and Am-241:' Lawrence Dosimeters," United Kingdom Atomic U-237, Pu-23 , -239,-240, -241, ity, Winfrith, England, AEEW-R13 59, 1980.

1976.

Livermore Laboratory, UCRL-52139, of the L. D. Mclsaac, "Gamma A. Ohno and S. Matsuura, "Measurement

2. J. E. Cline, R. J. Gehrke, and 9. in a Spent Fuel the Fissionable Nuclides and Asso Gamma Dose Rate Distribution Rays Emitted by Detector,"

Idaho Falls, Assembly with a Thermoluminescent ciated Isotopes," Aerojet Nuclear Co., 485, 1980.

Nuclear Technology, Vol. 47, p.

Idaho, ANCR-1069, July 1972.

and H. C. Keller, "A

Material Safeguards 10. W. D. Reed, Jr., J. P. Andrews,

3. L. A. Kull, "Catalogue of Nuclear 23 sU with Limit of Error Laboratories, BNL Method for Surveying for 5 Instruments," Battelle National Management, Vol. 2, p. 39 ,

Analysis," Nclear aterials

17165, August 1972. 1973.

"Fundamentals of

4. R. H. Augustson and T. D. Reilly, "Self-Multiplication Fissionable Material," N. Ensslin, J. Stewart, and J. Sapir, Passive Nondestructive -Assay of 11.

Coincidence Counting,"

LA-5651-M, 1974; Correction Factors for Neutron Los Alamos Scientific Laboratory, Management, Vol. VIII, No. 2, p. 60,

al., "Fundamentals of Passive Nuclear Materials also T. D. Reilly et 1979.

Assay of Fissionable Material: Labora Nondestructive Laboratory, tory Workbook," Los Alamos Scientific for Nuclear LA-5651-M, Suppl., 1975. 12. M. S. Zucker et al, "Holdup Measurements Manage Plants," Nuclear Materials Fuel Manufacturing of UF Cylinders ment, Vol. X, p. 239, 1981.

5. R. B. Walton et al., "Measurements Technology, with Portable Instruments," Nuclear Vol. 21, p. 133, 1974. "Bulk Sample Self

13. J. L. Parker and T. D. Reilly, Measure of Residual Pluto Attenuation Correction by Transmission

6. C. H. Kindle, "In Situ Measurement the ERDA X- and Gamma-Ray

5, No. 3, ment," Proceedings of nium," Nuclear MaterialsManagement, Vol. (Conf. 760639),

p. 540, 1976. Symposium, Ann Arbor, Michigan p. 219, May 1976.

"Total Room

7. J. W. Tape, D. A. Close, and R. B. Walton, and Error Analysis Holdup of Plutonium Measured with a Large-Area 14. P. R. Bevington, Data Reduction 1969.

Management, for the Physical Sciences, McGraw-Hill, Neutron Detector," Nuclear Materials Vol. 5, No. 3, p. 533, 1976.

5.23-15

APPENDIX

A. NEUTRON YIELD COMPUTATIONS The coefficients 2.50 and 1.70 are the spontaneous fission yields of 2 3 8 pu and 2 4 2 Pu relative to 24°Pu. The concept \

The following model for the calculation of the total of effective 2 4 0 pu mass reflects the fact that most of the spontaneous neutron yield from plutonium-bearing materials spontaneous fission yield is due to that isotope.

assumes that the plutonium is widely dispersed. With this condition, there will be no significant neutron production 2. (cz,n) NEUTRONS

through induced fission of 39pu or 2 4 1 Pu. The total neutron yield of plutonium holdup will then be the sum of When the plutonium holdup is in the form of oxide, the the spontaneous fission and (ct,n) contributions: major contribution from (ct,n) reactions will be due to the

0-18(a,n) 21Ne reaction. The additional neutron yield is Yn= YSF + Y(a,n) (1) typically 50 to 100 percent of the spontaneous fission yield. The (a,n) yield can be calculated from the yields per

1. SPONTANEOUS FISSION NEUTRONS gram of each isotope of Pu(Yi) given in Table A-1:

To determine the spontaneous neutron yield of pluto Y(ay,n) oxide = 1 MiYi (5)

nium, the isotopic composition must be known. (The 1y contribution from 23SU spontaneous fission is usually negligible even if uranium is present in large quantities.) The summation over Mi should also include 24 1 Am, which The yield from the plutonium isotopes is given by: is a strong alpha emitter.

YSFp M2 3 8Q 2 38 + M2 4 0 Q 2 4 0 + M2 4 2 Q 2 4 2 (2) In addition to (c4n) production in the oxide itself, certain low-Z impurities in the oxide can contribute substan where Mi is the total mass of the ith plutonium isotope, and tially. Values for the yields of neutrons obtained in bombard Qi is the spontaneous fission neutron yield per gram of the ing thick targets of these elements with 5.2-MeV alpha ith isotope. Using the yield data from Table A-l, Equation 2 particles are given in Table A-2. Further research may can be rewritten as: change these values somewhat, but they are sufficient for computing the approximate effect of these elements if they YSF = (1030 n/sec-gram)M 24 0 (effective) (3) exist as impurities in PuO . One method for doing this is to compute the impurity (x,n) yield relative to the oxide where (ct,n) yield:

M2 4 0 (effective) = 2.50M 2 38 + M240 + 1.70M 2 4 2 (4) Y(an)impurity - Y(cn)oxide f (WiAoI )/(Pok io) (6)

ble A-1 ALPHA PARTICLE AND SPONTANEOUS FISSION NEUTRON YIELDS

Y.

Qi PuO 2 U0 2 Half-Life Alpha Activity Spontaneous Fission (a,n) Yield*

Nuclide (yr) (a/sec-gram) (n/sec-gram) (n/sec-gram)

238pu, 87.78 6.33 x 1011 2.57 x 103 1.4 x 104

239pu

24,150 2.30 x 109 2.22 x 10-2 42.5

2 4 0

pu

6,529 8.43 x 109 1.03 x 103 157

2 4 1 pu

14.35** 9.39 x 107 5.00 x 10-2 1.3

2 4 2 pu

379,000 1.44 x 108 1.75 x 103 2.2

24 1 Am 433.8 1.27 x 10"1 6.05 x 10f' 2957

234U 2.47 x 105 2.29 x 108 5.67 x 10-3 4.65

3

235U

7.1 x 108 7.93 x 104 5.96 x 10-4 1.37 x 10-

238 U

4.51 x 109 1.23 x 104 1.12 x 10-2 1.93 x 10-4

  • Oxygen yield from PuO 2form only.
    • &branching
  • ratio: 2.46 x i0T5 .

5.23-16

Table A-2 Table A-3 (a,n) NEUTRON YIELDS FOR SAMPLE CALCULATION FOR 1 GRAM OF PLUTONIUM

VARIOUS LIGHT ELEMENTS

Isotopic YSF Y(a, n)o ide P. Nuclide Composition (n/sec) (n/sec)I

Neutron Yield 23 SPU 0.003 8 42 Element per 106 Alphas 2 3 pt 0.756 0 32

24 PU 0.185 191 29 Be 58 S1 Pu 0.045 0 0

B 18 242 0.011 19 0

F 6.4 241 Am 0.003 0 9

7 Li 1.3 Na -1.5 Total Yields 218 112 NatMg 0.89 Al 0.44 Natsi 0.077 C 0.05 O 0.050 Using the isotopic composition given in Table A-3 and using Equation 3, the spontaneous fission neutron yield can

  • Most of these yields are based on: be found to be 218 n/sec for 1 gram of plutonium. Then J. K. Baki and J. Gomez del Campo,

"Neutron Yields from Alpha-Particle the neutron production in the oxide can be calculated Bombardment," Nuclear Science and using the masses Mi of each isotope and the yields Y- from Engineering, Vol. 71, p. 18, 1979.

the fifth column of Table A-1. The result of 112 n)sec is given in the last column of Table A-3. Note that the alpha In Equation 6, P. is the (a,n) neutron yield in the impurity particle yield of ylutonium is nearly constant in time, but element, and P 0 is the yield in oxygen (0.050 neutrons/106 that, because 24 Am builds up in time, the total alpha alphas); A. is the atomic weight of the impurity element, production increases at a rate of roughly 0.3 percent per and Ao that for oxygen (16); Ij is the concentration of the month in typical reactor fuel impurity expressed in parts per million (by weight) of plutonium oxide, and Io is oxide (118,000 ppm). If the impurity concentration is expressed as ppm of plutonium, The impurity (ct,n) yields are calculated in Table A-4.

it can be converted to ppm of plutonium oxide by multi The calculation is based on impurities in PuO 2 only. The plying by the gravimetric dilution factor, 0.882. mixed oxides are assumed to consist of blended PuO 2 and UO2 particles approximately 40 pm in diameter where To summarize the calculation of (ct,n) neutron yields in most alpha particles stop within the PuO 2 particles. If the oxide that also contains impurities, Y(a,n) from all sources particle size were smaller or the mixed oxide were created is given by: through coprecipitation, the uranium impurity content would also contribute to the plutonium (a,n) yiel

d. In the

(7) present example, it is sufficient to use the neutron yields P.

Y( i MiYf + .0027EP_-I-/A-)

Z,n) " I

from Table A-2, the concentrations Ij from Table A-4, and Equation 6 or 7.

Elements other than those listed in Table A-2 yield no neutrons by (ct,n) reactions for the alpha energies obtained from plutonium and americium decay. Also note that the The total neutron yield from 1 gram of plutonium in summation over i must include 241 Am and that the summa PuO 2 is then 218 + 112 + 47 = 377 n/sec. Using the gravi tion over j includes only the oxygen that is not bound up as metric dilution factor of 0.882, this is 333 n/sec for 1 gram plutonium oxide. of PuO 2 . If the PuO 2 is blended so that PuO 2 / (PuO 2 +

UO ) = 0.03, the neutron yield from 1 gram of mixed

3. SAMPLE CALCULATION FOR PuO 2 -UO 2 oxide is 10 n/sec.

Consider the case of I gram of recycle plutonium The impurity (ct,n) yields, Pi, used in this example are blended to 3 percent by weight of PuO 2 in a UO 2 matrix currently known to about 10 percent accuracy for most where the isotopic composition is as given in Table A-3. For elements and 50 percent accuracy for the others. The oxide mixed oxides, the oxygen density is approximately the .J are known to 10 percent or better. Both (atn) yields, Y..,

same as in PuO 2 alone. Also, plutonium and uranium have yield calculations must assume perfect mixing, however.

similar atomic numbers. For these reasons, it may be For these reasons, neutron yield calculations are accurate to assumed that the oxygen (ca,n) yield in mixed oxide is 10 percent at best, and the neutron holdup measurement the yield in PuO 2 , further reduced by the blending ratio, calibration should be based on representative standards PuO 2 /(PuO 2 + U0 2 ). rather than calculation wherever possible.

5.23-17

Table A-4 Let f238, f239' f240' f241, and f2 4 2 represent the weight fractions of the respective plutonium isotopes in the IMPURITY (ca,n) YIELD unknown sample. The 2 4 0 pu effective weight fraction, f (effective), can be defined as:

240

Arbitrary Concentration I Impurity (cn) (8)

in PuO 2 Yield (I12 n/sec) f2 4 0 (effective) = M2 4 o(effective)/Mpu(total)

Impurity (ppm by wgt) (0.00 2 7 )Pjlj/Aj where Li 9 1 Be 8 16 f2 4 0 (effective) = 2.50f 2 39 + f 24 0 + 1.70f 2 4 2 (9)

B 10 5 C 200 0 Generally, as previously mentioned in this guide, the F 125 13 relative measurement uncertainty of M2 4 0 (effective) in a

0 (moisture) 4600 4 holdup measurement will be much larger than that of Na 120 8 f;40(effective), so the relative error in Mpu(total) is essen Total 47 (n/sec) tially equal to that of M2 4 0 (effective).

As an example calculation, the sample of isotopic composition given in Table A-3 has an effective fraction given by:

B. CONVERSION OF MEASURED M D4(EFFECTIVE) + 0.185 + 1.70(0.011)

TO TOTAL PLUTONId" = 2.50(0.003)

f 24 0 (effective) = 0.21

240

To convert a measured effective pu mass to actual Thus, a holdup measurement of 35 + 10 grams 24°pu total plutonium, one must use both the relationship between effective corresponds to 166 + 47 grams total plutonium, these two quantities, as shown in Equation 4, and the where the relative error in the total plutonium result was known isotopic composition of the samples being measured. taken to be equal to that of the M2 4 0 (effective) result.

5.23-18

VALUE/IMPACT STATEMENT

1.3.4 Public

1. PROPOSED ACTION

No adverse impact on the public can be foreseen.

1.1 Description

1.4 Decision on Proposed Action Licensees authorized to possess at any time more than

1 kilogram of plutonium are required by Part 70, "Domestic The regulatory guide should be revised to reflect improve Licensing of Special Nuclear Material," of Title 10 of the ments in measurement techniques and to bring the language Code of Federal Regulations to calculate a material balance of the guide into conformity with current usage.

based on a measured physical inventory at intervals not to exceed 2 months. Further, these licensees are required to

2. TECHNICAL APPROACH

conduct their nuclear material physical inventories in compliance with specific requirements set forth in Part 70. Not applicable.

Inventory procedures acceptable to the NRC staff are detailed in Regulatory Guide 5.13, "Conduct of Nuclear

3. PROCEDURAL APPROACH

Material Physical Inventories."

Of the procedural alternatives considered, revision of the Plutonium residual holdup is defined as the plutonium existing regulatory guide was selected as the most advanta inventory component remaining in and about process geous and cost effective.

equipment and handling areas after those collection areas have been prepared for inventory. This 'regulatory guide

4. STATUTORY CONSIDERATIONS

describes procedures acceptable to the NRC staff for the in situ assay of the residual plutonium holdup. 4.1 NRC Authority

1.2 Need for Proposed Action The authority for the proposed action is derived from the Atomic Energy Act of 1954, as amended, and the Regulatory Guide 5.23 was published in 1974. The Energy Reorganization Act of 1974, as amended, and is proposed action, a revision to this guide, is needed to bring implemented through the Commission's regulations, in the guide up to date with respect to advances in measure particular 10 CFR Part 70.

ment methods, as well as changes in terminology.

4.2 Need for NEPA Assessment

1.3 Value/Impact of Proposed Action The proposed action is not a major action that may

1.3.1 NRC Operations significantly affect the quality of the human environment and does not require an environmental impact statement.

The regulatory positions will be brought up to date.

S. RELATIONSHIP TO OTHER EXISTING OR PROPOSED

1.3.2 Other Government Agencies REGULATIONS OR POLICIES

Not applicable.

The proposed action is one of a series of revisions of exist ing regulatory guides on nondestructive assay techniques.

1.3.3 Industry

6. SUMMARY AND CONCLUSIONS

Since industry is already applying the methods and procedures discussed in the guide, updating these methods Regulatory Guide 5.23 should be revised.

and procedures should have no adverse impact.

5.23-19

FIRST CLASS MAIL

UNITED STATES POSTAGE & FEES PAID

USNAC

NUCLEAR REGULATORY COMMISSION WASH0 C

PERMII No SiIL

WASHINGTON, D.C. 20555 OFFICIAL BUSINESS

PENALTY FOR PRIVATE USE, $300


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