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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


==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.
Part 70,  
"Domestic Licensing of Special Nuclear Material," of Title 10 of the Code of Federal Regulations requires licensees authorized to possess more than 1 kilo gram of plutonium to calculate a material balance based on a measured physical inventory at intervals not to exceed 2 months. Further, these licensees are required to conduct their nuclear material physical inventories in compliance with specific requirements set forth in Part 70.


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
Inventory procedures acceptable to the NRC staff are detailed in Regulatory Guide 5.13, "Conduct of Nuclear Material Physical Inventories."
    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.
Plutonium residual holdup is defined as the plutonium inventory component remaining in and about process equipment and handling areas after these collection areas have been prepared for inventory. Whenever possible, process equipment should be designed' and operated so as to minimize the amount of holdup. In this guide, proce dures acceptable to the NRC staff for the in situ assay of the plutonium residual holdup are described.


Nuclear Material Physical Inventories."
Assay information may be used in one of two ways:
1. When the standard error (estimator) of plutonium holdup is compatible2 with constraints on the overall standard error of the inventory difference (SEID), the mate rial balance can be computed using the measured contents of plutonium holdup. Additional cleanout and recovery for accountability will then not be necessary.
 
2. When the standard error of plutonium holdup is not compatible with constraints on the overall SEID, the
1 Design features to minimize holdup in process equipment are the subject of a series of regulatory guides (5.8, 5.25, and 5.42). 
2 Compatibility exists when the contribution of the standard error of the holdup to the total plant SEID is not large enough to cause the overall SEID to exceed allowed limit
 
====s. If the plant SEID ====
exceeds allowed limits because of an excessive contribution from the holdup standard error, compatibility does not exist and the remedial steps of paragraph 2 needto be taken.
 
USNRC REGULATORY GUIDES
Regulatory Guides are issued to describe and make available to the public methods acceptable to the NRC staff of Implementing specific parts of the Commission's regulations, to delineate tech niques used by the staff in evaluating specific problems or postu lated accidents or to provide guidance to applicants. Regulatory Guides are not substitutes for regulations, and compliance with them is not required. Methods and solutions different from those set out in the guides will be acceptable if they provide a basis for the findings requisite to the issuance or continuance of a permit or license by the Commission.
 
This guide was Issued after consideration of comments received from the public. Comments and suggestions for improvements in these guides are encouraged at all times, and guides will be revised, as appropriate, to accommodate comments and to reflect new Informa tion or experience.
 
information obtained in the holdup survey can be used to locate principal plutonium accumulations and to ensure that other areas of the process contain less than the detectable amount of plutonium. Once located, substantial accumulations can be recovered, transforming the pluto nium to a more accurately measurable inventory compo nent. Having reduced the amount of plutonium holdup, the standard error on the remeasurement of the remaining holdup may be sufficiently reduced to be compatible with overall SEID requirements.
 
Any guidance in this document related to information collection activities has been cleared under OMB Clearance No. 3150-0009.


==B. DISCUSSION==
==B. DISCUSSION==
Plutonium residual holdup is defined as the
Plutonium accumulates in cracks, pores, and zones of poor circulation within process equipment. The walls of process vessels and associated plumbing often become coated with plutonium during solution processing. Surfaces internal and adjacent to process equipment, especially glovebox walls and floors, accumulate deposits of pluto nium that can become appreciable. Plutonium also accu mulates in air filters and associated ductwork. The absolute amounts of plutonium holdup must be small for efficient processing and proper hazards control. However, the total holdup can be large relative to the plant inventory differ ence (ID) but have no significant impact on the ID if it remains reasonably constant. It is the change in the holdup between beginning inventory and ending inventory that may impact the ID.
,*  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.
 
The measurement procedures described in this guide involve the detection of gamma rays and neutrons that are spontaneously emitted by the plutonium isotopes. Because the gamma rays of interest are emitted by the major isotope,
23gpU, gamma ray assay is the preferred method whenever its acceptance criteria are satisfied. The amount of 2 3 9 pu The substantial number of changes in this revision has made it impractical to indicate the changes with lines in the margin.
 
Comments should be sent to the Secretary of the Commission, U.S. Nuclear Regulatory Commission, Washington, D.C. 20555, Attention: Docketing and Service Branch.
 
The guides are issued in the following ten broad divisions:
 
===1. Power Reactors ===
 
===6. Products ===
2. Research and Test Reactors
 
===7. Transportation ===
3. Fuels and Materials Facilities
 
===8. Occupational Health ===
4. Environmental and Siting
9. Antitrust and Financial Review
5. Materials and Plant Protection 10. General Copies of issued guides may be purchased at the current Government Printing Office price. A subscription service for future guides in spe cific divisions is available through the Government Printing Office.
 
Information on the subscription service and current GPO prices may be obtained by writing the U.S. Nuclear Regulatory Commission, Washington, D.C. 20555, Attention: Publications Sales Manager.
 
holdup in a piece of equipment is proportional to the measured intensity of the emitted gamma rays after suitable corrections are made for attenuation by intervening mate rials, for self-attenuation by plutonium, for scattering, for geometrical factors, and for background radiation.
 
If plutonium is held up in furnaces, grinders, or other heavy equipment that is too dense to permit the escape of gamma rays, an assay based on spontaneous fission neutrons from the even isotopes of plutonium may be possible. This technique requires knowledge of the isotopic composition of the plutonium, some knowledge of its chemical form, and knowledge of the presence of other radionuclide impurities.
 
Thermoluminescent dosimetry is a third technique that can be used to measure holdup from the inside of large pieces of equipment. This technique is also useful for carrying out measurements in an unobtrusive manner outside normal plant operating hours.


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.
For all three techniques, the proportionality factors between amount of holdup and detector response are best determined prior to the holdup measurement by assays of known quantities of plutonium distributed in well-defined and representative geometries, as discussed below.


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.
1. DELINEATION OF COLLECTION ZONES
Typical plutonium process facilities comprise a number of interconnected gloveboxes that contain work areas and most process equipment, in-process storage areas, and self-contained process equipment. Also, solution process ing requires tanks, plumbing, and pumping equipment, which are often located in close proximity to the glovebox lines. Finally, storage areas for feed, scrap and waste, and final product are often located in close proximity to the plutonium process area.


be computed using the measured contents of Pu holdup.
To accomplish the holdup measurements, it is essential to consider the facility in terms of a series of zones that can be independently assayed.


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
Such zones are designated
    "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'.
"collection zones." Each plutonium-processing facility can be conceptually divided into a series of contiguous collection zones on the basis of process activities and collection geometries.
                                                                                        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.
Individual machines, filters, pipes, tanks, gloveboxes, or surface areas that can be isolated from one another may be suitable discrete collection zones. Great care is needed to define all collection zones so that (1) the 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 distribution of material in the zone can be represented by one of the distribution geometries used in the calibration procedure described below.


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:
Gamma ray assay for plutonium holdup is practical when the collection zone consists of a single structure of relatively uniform cross section. When a collection zone contains a complex item of equipment with significant self-shielding properties, the uncertainty in the holdup, measurement may be primarily due to attenuation of radiation in the internal structure. In such cases, neutron assay from the outside and thermoluminescent dosimeter assay from the inside may be applicable.
    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
                                                                                          1. Power eactors the issuanc or continuance of a pearmil or licemniby the Comnission.                  2. Resmrch and Test Reactors             


===7. Transportation===
If delineation of collection zones is not possible, two alternatives are experiments with mockup geometries or 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.
2. APPLICABLE METHODS AND INSTRUMENTS
Two considerations are critical to the selection of methods and instruments. First, to perform an assay, one must ensure that the plutonium radiations reach the detector and are detected. Second, the observed response must be attributable to the collection zone being assayed. Therefore, the assay scheme is developed around penetrating radiations, and the detector is collimated to provide for sufficient directionality in the response to resolve a collection zone from its neighboring zones and from the background.


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.
Finally, some effort may be necessary to employ external
"shadow shielding" to block radiation being produced in adjacent collection zones from the field of view of the collimated detector.


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.
2.1 Gamma Ray Assay Under closely controlled conditions, the measured plutonium gamma ray spectrum can be interpreted in terms of the abundance of each gamma ray emitter present in the sample. Because of the large number of gamma rays (Refs. 1 and 2) present, many regions of the observed spectrum are characterized by overlapping lines. To accomplish the assay, it is necessary to select an appropriate spectral region and provide a detection system with sufficient resolution to measure the activity from the isotopes of interest.


neutron assay in that the emissions are primarily from Such zones are designated as "collection zones."
Gamma ray assay has an advantage over neutron assay in that the emissions are primarily from the principal isotopes of interest. Because of the hiih emission rate of gamma rays, a detection sensitivity of less than 1 gram is generally attainable.
                                                                      the principal isotopes qf linterest. -Because of the high emission rate of gammna rays, a detection sensitivity of
  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
The most useful portion of the spectrum for holdup assay is the 2 3 9pu gamma ray complex in the 375- to  
often located in close proximity to the plutonium process area.                                                              Energy                                    Intensity  (- /sec-g Pu-239)
45 0-keV range. 3 The yields of these lines are given in Table 1.
      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.
31n typical Nal detectors (with energy resolution of 10 percent at 414 keV), the 414-keV photopeak will produce counts in the approximate energy range of 373 to 455 keV. Thus, an energy window from 375 to 450 keV will include most of the 414-keV
full-energy counts for a variety of detector resolutions. Further more, suca, window setting will include a significant fraction of the
375-keV
P9P gamma rays (see Table 1), but will also exclude
'mos orftihe otentially interfering 332-keV gamma rays from
5.23-2 K


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
Table 1 PROMINENT GAMMA RAYS FROM 2 3 9 pu IN 375- to 450-keV ENERGY RANGE
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.
Intensity Energy (y/sec-g 2 3 9Pu)
375.0
380.2
382.7
392.5
393.1
413.7
422.6
3.59 x 104
0.70 x 104
0.59 x 104
0.26 x 104
1.01 x 104
3.43 x 104
0.27 x 104 Total
9.85 x 104
2.1.1 Gamma Ray Detection Instruments Gamma ray detection systems consist of a scintillation or semiconductor gamma ray detector and appropriate electronics (Refs. 3 and 4). Required electronics include at least a single-channel analyzer and a timer-scaler unit. A  
second single-channel analyzer viewing the same detector pulses used to determine the background radiation correc tion is a timesaving feature. A number of portable battery powered systems are commercially available for this applica tion.


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.
The detection efficiency andresolution (10 to 15 percent)
of NaI(Tl) is generally adequate for holdup measurements.


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
CdTe, Ge(Li), and high-purity germanium (also known as intrinsic germanium) detectors have better resolution than NaI(Tl) but are more costly and more difficult to operate.
      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.


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.
For more information on Ge(Li) and intrinsic germanium detectors, see Regulatory Guide 5.9,  
"Guidelines for Germanium Spectroscopy Systems for Measurement of Special Nuclear Material," and the references cited therein.


5.23-2
The 332.3-keV gamma ray from 2 3 7 U, a short-lived
(6.75 days) daughter of 2 4 1pu, is usually the principal inter ference for 2 3 9Pu assay by Nal detection of the 375- to
450-keV complex. If the 2 3 7 U is in equilibrium with
2 4 1pu, the intensity of this gamma ray is 1.15 x 106 y/sec-g 2 4 1pu. Since this gamma ray is also emitted in the decay of 241 Am, the interference from this decay branch may also be important in case of preferential americium holdups. To avoid this interference when using Nal detectors, the assay energy window is adjusted to span the range from
390 to 450 keV for plutonium holdup with high americium content.


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.
Detector crystal dimensions are selected to provide a high probability of detecting gamma rays from the 375- to
450-keV complex and a low probability of detecting high-energy radiation. For NaI, a crystal diameter of
2 inches (5 cm) with a thickness of 2 inches is recommended.


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
For germanium detectors, a moderate-volume coaxial detector is recommended.
            2.1.2 Collimators for Gamma Rays                      encapsulating material and correct the calibration standard response to compensate for that attenuation.


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.
2.1.2 Collimators and Absorbers for Gamma Rays A shaped shield constructed of any heavy-element material is appropriate for gamma ray collimation. For cost, availability, and ease of fabrication, lead is recommended.


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.
Less than 2 percent of all 400-keV gamma rays striking a  
1.5-cm-thick sheet of lead will pass through without suffer ing an energy loss.


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.
The collimator will be most effective when it is con centric about the crystal and photomultiplier and com pletely covers the photomultiplier base. Extending the collimator forward of the crystal at least a distance equal to 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 desirable to select collection zones and counting geometries so that one collimator setting will suffice for all measure ments. This will simplify the calibration procedures because the calibration constants depend strongly on the dimensions and placement of the collimator aperture.


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 collimator not only defines the effective field of view but also shields the detector from unwanted radiation.


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.
To effectively accomplish this latter purpose, the collimator material must also cover the rear of the detector. This is usually easy to achieve with portable Nal detectors but requires more effort when germanium detectors are used.


shifted, or the unit may be in need of repair and recalibration.
Intensive 50- to 100-keV X-ray radiation and 60-keV
241 Am gamma ray radiation are often emitted by process equipment, and this radiation can tie up the detector electronics unnecessarily. A 1.5-mm-thick layer of lead (on the outside) and a 0.75-mm-thick layer of cadmium (on the inside) may be placed against the front face of the detector to alleviate this problem. This graded energy shield will absorb most of the low-energy photons incident on the detector without substantially reducing the number of gamma rays detected in the 375- to 450-keV range.


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.
2.1.3 Check Source for Gamma Ray Assay It is important to check the operation of the detection system each time the instrumentation is moved or otherwise disturbed (e.g., power outage) during the course of each inventory sequence. Either recalibrating one or more collection zones and comparing the results to previous analyses or testing the instrument with a check source is appropriate. When the response remains within the expected value, the previous calibration data are assumed to be valid.


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
If not, the energy window may have shifted, or the unit may be in need of repair and recalibration.
                                                          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
An appropriate check source enables the stability of the assay instrument to be tested at any location. Such a source can be prepared by implanting a small encapsulated plutonium sample (containing '.,0.5 gram of plutonium) in the face of
          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.
5.23-3


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
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 plug is in place.
              _,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.
The check source is fabricated in a manner that will ensure its internal stability. Other than radiations increasing from the ingrowth of 2 4 1 Am, the emission rate of the check source should remain constant.


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.
2.2 Neutron Assay Neutrons are emitted in the spontaneous fission of 238Pu,
24&deg;Pu, and 242Pu and through the interaction of emitted alpha particles with certain light nucleL These neutrons suffer little attenuation in passing through uranium or plutonium or through most structural and containment materials. Glovebox windows may reduce the energy of emerging neutrons, but, because of their regular and con stant shape, their effect can generally be factored into the assay calibration.


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.
To be useful for the assay of plutonium holdup, the neutron production rate per gram of plutonium must be known. The spontaneous fission contribution to the total neutron production can be computed from basic nuclear data after the isotopic composition of the contained plutonium has been determined. Computing the (ca,n)  
contribution requires a knowledge of the chemical form of the plutonium and the amount and distribution of certain high-(ca,n)-yield target materials.


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%
The background count rate from neutron detectors may be a substantial part of the observed activity, often corre sponding to as much as 20 grams of plutonium in typical holdup assays. Thus, neutron assay is primarily applicable to the measurement of significant accumulations of pluto nium.
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 standards is used to calibrate each neutron assay collection zone. In the appendix to this guide, a method is given to calculate the anticipated neutron yield. This method provides the ability to calculate the neutron yield when the isotopic or impurity composition of the plutonium holdup is different from that of the calibration standards. The method can be used to calculate a ratio of the neutron production rate of the unknown material to the neutron production rate of the standard material. The yield from the holdup material is then determined by multiplying the measured "known" material yield by the computed ratio.


The spontaneous neutron production rate
2.2.1 Neutron Detection Instruments To effectively employ the spontaneous neutron yield as a measure of plutonium holdup, it is necessary to detect the neutrons in the presence of a more intense gamma ray background and to collimate the detector so that neu trons emanating from the collection zone under assay are preferentially detected.
          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.


5.23-4
Holdup assay is performed under in-plant conditions where ruggedness, portability, high neutron efficiency, and low gamma ray sensitivity in the detectors are important.
 
Gas-filled proportional counters containing He or BF 3 are suitable for this purpose. Typical fill pressures are 1 to 4 atmospheres. One advantage of 3 He for in-plant applications is that the operating voltage of 3He counters is about
75 percent of that required for BF 3 counters.
 
The efficiency of 3 He and BF 3 counters increases as the energy of the neutrons decreases. Embedding gas-filled counters in polyethylene to moderate the incoming neutrons to thermal or epithermal energies will improve their effi ciency. A nearly optimum design can be obtained by center ing the counters in 10 cm of polyethylene with 2 to 3 cm of polyethylene between adjacent counters.


COLLIMATED NEUTRON DETECTOR ASSEMBLY FOR PLUTONIUM HOLDUP ASSAY
To shield the detector from low-energy neutrons that may produce a complicated response pattern, the modera tor material is covered with a thermal neutron absorber.
          DETECTOR CABLE ACCESS CHANNEL
                (TOP SECTION ONLY)  --.-
                                                                                                                                -T-
                                                                                                                                17.5cm
                                                          - 5-1cm            1cmI              "  I
                                                                                                                                I
                                                                                                              TOP VIEW
                          FRONT VIEW
                                                                                                      4.5cm
                                                                  ..          ..              .      \NEUTRO N DETECTOR
                                                                                                          TUBE C HANNELS
                                                                              I      I                2.6cm DIA METER&#xfd; ITYP)
                                                        I                II                I I            " ,
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I      II              I        I
Cadmium sheeting approximately 0.075 cm thick may be used for this application.
                                                                                      I .1                FRONT VIEW
                                            &#xa2;r    39cm                              I          I
                                        68                                I I        II
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.1I    I      I I            I.
2.2.2 Collimators for Neutron Detectors To assay a specific collection zone in the presence of other distributed sources of plutonium, it is necessary to collimate the detector. This is accomplished by stopping neutrons coming to the detector from all directions except the desired one. The cadmium surrounding the detector will stop essentially all neutrons striking the detector with energies below 0.4 eV. By adding moderator material around the outside of the detector in all directions except for the collimator channel, neutrons coming from unwanted directions will lose energy in this shield and will be absorbed in the cadmium cover. For each 6 inches
(15 cm) of polyethylene added, the collimator assembly provides a factor of approximately 10 in the directionality of the response.


I I I I                I
An example of a collimated neutron detector assembly for plutonium holdup assay is shown in Figure 1. This assembly has a polyethylene shield thickness of 6 inches
                                                                I                II        -I
(15 cm) and a directionality of 10 to 1. The combined weight of the detector and collimator exceeds the require ments for a hand-held probe. For this reason and to provide for reproducible positioning at each assay, a sturdy cart housing such a detector and its associated electronics is recommended. In order to assay items at different heights, the capability to raise and lower the assembly to reproduc ible settings is needed.
                                                                        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
An example of a small commercially available hand-held detector is given in References 5 and 6. This Shielded Neutron Assay Probe (SNAP) is 12 inches (30.5 cm) high and 10 inches (25.4 cm) in diameter and contains two 3He detectors. It includes a 2-inch-thick (5 cm) polyethylene shield that provides a directionality of 3 to
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
===1. The SNAP ===
has been used to measure plutonium holdup, UO2 F2 holdup, and UF 6 enrichment. It is recommended for the assay of well-defined concentrations of plutonium in pumps, grinders, pipe elbows, or other items of equipment where portability and accessibility are more important than directionality.


MODERATOR\              NEUTRON COLLIMATOR                to the same geometry as found in the neutron assay
5.23-4
                                /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
                          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
DETECTOR CABLE ACCESS CHANNEL
                                                                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.
(TOP SECTION ONLY) r-.
6
117
1 I5.1cm 15.1cm
3" I T
4.5cm
+
TOP VIEW
-- ,c
3c
"I
2.6cmnDIAl I I
I
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I
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I
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I
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N DETECTOR
"HANNELS
METER
(TYP)
VIEW
DETECTOR TUBE SUBASSEMBLY
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.


adequate for most applications.                                Two additional means exist to further isolate a collection zone.
Figure 1. Collimated Neutron Detector Assembly for Plutonium Holdup Assay
5.23-5 FRONT VIEW
+
4
4


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.
A third example of holdup measurement by neutron detection is given in Reference 7. In this case, a completely uncollimated polyethylene slab containing a row of 3He detectors was suspended in midair in some of the processing rooms of an industrial plutonium facility. The response of the detector was found to be proportional to the total room holdup if the plutonium was reasonably uniformly distributed and if the room was isolated from external sources. The calibration procedure for the use of. this detector will not be described here. However, it is recom mended as a means for quickly verifying total room holdup when measurements of the holdup in individual items or equipment are not needed.


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.
2.2.3 Check Source for Neutron Assay To ensure the proper operation of the neutron assay system prior to making an assay, it is necessary to test the response of the instrument. An appropriate neutron assay check source can be measured, or the detector response for one or more collection zones can be recalibrated and compared to the results of previous calibrations.


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
An appropriate neutron assay check source can be prepared by implanting a small encapsulated plutonium source (containing about 5 grams of plutonium) into the face of a plug of neutron moderating material (see Figure 2). The plug is fabricated to fit and close the colli mator channel. When the response from the check source remains within the expected value, the previous calibration data are assumed to be valid.


====y. A rectangular====
2.3 Thermoluminescent Dosimeter (TLD) Assay Crystals of LiF, CaF2 , CaSe 4 , or other compounds can store energy at manganese or dysprosium impurity centers when they are struck by gamma or neutron radiation. At some later time, the crystals can be heated rapidly to several hundred degrees centigrade to induce thermo luminescence. The light output at this time is proportional to the amount of radiation received. Thermoluminescent detectors that are primarily gamma sensitive, graded X-ray shields, read-out instrumentation, and other accessories are commercially available.
                                                          5.23-6


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.
TLDs have been used to measure the holdup in glove boxes by placing them at regular intervals on the outside surfaces. The TLDs are left in place overnight in order to accumulate a measurable dose. Accuracies of +20 percent relative to cleanout values are reported for plutonium of known isotopic composition. TLDs have also been used to measure the holdup in the interior of large furnaces that are not accessible by other means. For both of these examples, calibration requires either careful dose and geometry calculations or mockups of the actual collection zone.


required.*                          --
Because their use is relatively new and only a few published references exist (Refs. 8 and 9), TLDs will not be discussed further in this guide. However, they could be useful for special applications.
                                                                              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.
3. ISOLATION OF COLLECTION ZONES
To ensure that each collection zone is independently assayed, it is necessary to shield the detector from all radiations except those radiations emanating from the collection zone being assayed. This is principally accom plished through the use of the collimators described in Sections B.2.1.2 and B.2.2.2. Two additional means exist to isolate a collection zone, detector positioning and shadow shielding.


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.
3.1 Detector Positioning An unobstructed side view of a collection zone is pre
+/-erred. When plutonium is located behind the zone under assay in another collection zone or a storage facility, an additional background assay may be performed with the detector above or below the collection zone and pointing at the material behind the zone under assay. It is important to prevent, or account for, moving objects within the field of view. If this is not done, variations in shielding and scatter ing can affect the measurement.


and solve for 'Rlnterference:
3.2 Shadow Shielding It may not be possible to avoid interfering radiations by collimator design or by choosing the detector position for assay. In such cases, it may be possible to move a shield panel between the source of interfering radiations and the collimator zone under assay. If the shield panel is very thick and its dimensions match or exceed the back side of the 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 would limit its transportability. A rectangular panel mounted on wheels as an upright panel and containing %5 cm of neu tron moderator (e.g., benelex, WEP, or polyethylehe) and  
                              (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:
,0.5 cm of lead sheet is recommended. To use such a panel, it is necessary to measure the response of the collec tion zone with and without the shield in place. Also, the gamma and neutron transmission factors of the shield itself must be measured beforehand with a representative pluto nium sample. From these measurements, the assay of the collection zone can be corrected for background radiation transmitted through the shield.
                                                                        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
4. CALIBRATION FOR HOLDUP MEASUREMENTS
4.1 Basic Counting Geometries There are three fundamental counting geometries that can be used to represent most collection zones. These geometries are distinguished by the spatial distribution of the source material and the resulting dependence of the detector counting rate on the source-to-detector distance, r.


====e. avoiding internal====
4.1.1 Point Source If the material being assayed is distributed over an area with dimensions that are small compared with the source-
*Response tierms refer to neutron or gamma response, as                items which may attenuate the radiation emanating .from appropriate.                                                          the standard.
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
5.23-7


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.
to-detector distance and if the material resides entirely within the detector field of view, the zone can be treated as a point source. The detector count rate for a point source varies inversely as the square of the source-to-detector distance (count rate is proportional to l/r 2 ). Any equip ment measured at great distances or any small pieces of equipment or equipment parts fall in this category. 4
4.1.2 Line Source If the material being assayed is distributed along a linear path so that only a segment of that distribution length is contained in the detector field of view, the zone can be treated as a line source. The detector count rate for a line source varies inversely as the source-to-detector distance (count rate is proportional to I/r). Examples of this type of holdup geometry include isolated sections of piping and long, narrow ducts or columns.
 
4.1.3 Area Source If the material being assayed is spread over an area so large that it covers the full field of view of the detector for a range of source-to-detector distances, the zone can be assayed as an area source. As long as the material being viewed is uniformly distributed, the detector count rate will be independent of the source-to-detector distance.
 
However, for holdup applications, uniform material distribu tion is rare; so the source-to-detector distance can affect the instrument response and needs to be specified. Furthermore, when there are several measurement locations covering a large area (such as a floor), it is important to maintain the same source-to-detector distance (even if material distribu tion is uniform within a given measurement area) so that the number of measurement areas needed to cover the entire area remains constant. Examples of this type of assay geometry include floors, walls, glovebox floors, and large rectangular ducting.
 
4.2 Calibration of Detector Response
4.2.1 Mockup of Known Material Distributions When a gamma ray assay is used and a collimator setting has been selected, the detector responses for the three basic source distribution geometries listed above need to be determined. For the point source, the response is expressed as (counts per minute)/gram of 2 3Pu at a specified source to-detector distance. For the line source, the response is expressed as (counts per minute)/(gram of 2 3 9 pu per unit length) at a specified source-to-detector distance. For the area source1 the response is expressed as (counts per minute)/
(gram of  
39pu per unit area) at a specified source-to 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.


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)
Calculation of 2 4 &deg;pu effective from the plutonium isotopic composition is described in the appendix to this guide.
                                                                                                                                      (6)
integral number of subzones.


AREA A1 If a.severe effect is~noted, the response can often be corrected for the variation in the corresponding
Analogous expressions can be given for line and area
    100,
4Caution: small deposits of plutonium could exhibit very large gamma ray self-attenuation and could therefore require great care in analysis or could require neutron assay.
                                                                            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
sources although neutron assay is usually restricted to dense, isolated items of equipment that can be represented as point sources. For both neutron and gamma measure ments, corrections to the point and line source calibrations for different detector distances are made using the 1/r2 or  
                                                                                        '5.1.1 Counting Statistics VISUAL FIT TO MEASUREMENT DATA
1 /r count-rate dependence, respectively. For further detailed discussion of the measurement of detector responses for these basic geometries, see Reference 10.
                                                                                              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  .
                                                                              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.
For gamma ray assay, the calibration of the point source response can be accomplished with a well-characterized encapsulated standard plutonium foil smaller in size than the detector collimator opening. This foil can also serve as the check source for verification of the continued stability of the instrument settings in the field. It is important that care be taken in the preparation of this calibration standard to ensure that the amount of encapsulated 2 3 9 pu is well known. It is also important to measure the gamma ray attenuation through the encapsulating material and the self-attenuation of the plutonium foil and to correct the calibration standard response to compensate for these effects. Enough 2 3 9Pu needs to be encapsulated in this standard to provide count rates that will ensure good statistical precision of the calibration in a reasonable period of time.


5.1.2 Instrument Instabilities
For neutron assay, it is probably necessary to encapsulate a larger amount of material in the calibration standard because the spontaneous neutron production rate is signifi cantly less than the 375- to 450-keV gamma ray production rate. A quantity of 50 to 100 grams of plutonium is ade quate for most applications. Again, it is important to know the exact quantity and isotopic composition of the pluto nium. Also, the neutron calibration standard may generate more neutrons than directly attributable to the spontaneous fission and (a ,n) reactions. Because a relatively large quantity of PuO2 Ls encapsulated in the neutron assay calibration standard, some spontaneous fission or (a,n) neutrons may be absorbed in 239pu or 24 1pu nuclei, producing additional neutrons through the induced fission reaction. The amount of multiplication depends in a complex manner on the amount and distribution of PuO2 and on the surrounding medium (Ref. 11). For 50 grams distributed in the bottom of a 4-inch-diameter (10 cm) can, a self-multiplication of
          0        25          5D            75          100
0.5 percent of the total neutron output would be typical.
        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
At 100 grams, 1 to 2 percent may be expected. Thus, this effect is typically smaller than other errors associated with holdup measurements and can be neglected if the standard contains 100 grams or less of well-distributed material. The chemical and isotopic composition of the plutonium will have a larger effect, as described in the appendix to this guide.
          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.
The measurement of the line source response is best accomplished by constructing a cylindrical surface distribu tion of plutonium with the aid of large foils. It is also possible to establish the line source response using a point source, as described in Reference 4. The line source geom etry is closest to that of the pipes and ducts likely to be encountered in actual measurements.


the background correction is irregular and uncertain.
5.23-8


The magnitude of this effect is generally smal
The area source response can be measured with the same plutonium foils laid flat to simulate the expected distribu tion on surfaces such as walls and floors. The area response can also be established using a point source. The point source is measured at different radial distances from the center of the field of view of the collimated detector. The response at each radial distance is weighted by the area of a concentric ring at that radius. From these weighted re sponses, it is then possible to calculate the area of a circular region of uniform plutonium deposition that would yield the same total response as the point source. From this equivalent area, the expected response/(gram of 2 3 9Pu per unit area) can be derived. Further useful details on this procedure may be found in Reference 12. For both line and area calibrations, the self-attenuation of the foils or point sources also needs to be taken into account.


====l. It can be====
There may be special material distribution geometries in the facility that are not readily represented by one of the three basic configurations described above. These special geometries may be mocked up as carefully as possible with large plutonium foils and point sources to produce a usable detector response calibration for these special cases. Examples of special cases might be concave or convex equipment surfaces or the internal volume of a rectangular cavity (see Ref. 10). Because material particle sizes (or material deposit thicknesses) have a significant effect on the self-attenuation of the gamma ray signals, it is important to use (whenever practical) well-characterized process material for preparing calibration standards and to duplicate to the extent possible process holdup distribution relative to particle size or thick ness. Furthermore, holdup in floors is often deposited at various depths into the floor, rather than on the surface.
                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.


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
Thus, calibration standards for such measurements need to incorporate the appropriate geometry and matrix effects.
                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.


due to the dimension perpendicular to the detector collection zone centerline is eliminated through the
Core samples of a floor may be needed to establish typical concentrations at various floor depths.
                                                                              5.2.2 Isotopic Uncertainties area-averaging calibration method described in Section
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
Calibration of the holdup measurement system using this procedure is recommended until a history of comparisons between predicted and recovered holdup quantities is developed. If it is possible to take holdup measurements before and after the cleanout of a piece of shut-down process equipment, they can be used to establish this comparison history and improve the accuracy of the calibration for each collection zone.
          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.


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
4.2.2 Measurement of Calibration Sources in Actual Process Equipment One method for calibrating detector response to holdup radiation in process equipment is to place a known calibra tion source in various positions in that equipment and record the detector responses. In this way, the overall detector response (including all corrections for attenuation and geometry) is determined empirically. Unfortunately, this procedure is impractical, if not impossible, in process equipment already in operation. However, if those respon sible for holdup assays are made aware of occasions when new equipment is brought into the plant for installation in the process, calibration sources can be conveniently placed in the equipment before its installation and the empirical measurements of the detector responses can be made. This procedure would be a valuable supplement to calibration data obtained from mockups of standard counting geom etries and comparisons with cleanout recovery data.
      5.2 Interpretation Uncertainties                            holdup and its error can be determined.


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.
5. HOLDUP MEASUREMENTS AND STANDARD ERROR
The measurement of holdup in a complex plant environ ment can involve a very large number of measurements. In a stable plant environment where the process behavior is well known and well characterized, it may be possible to arrange the holdup measurement program so that:
a. Careful and extensive holdup measurements are made infrequently (e.g., annually) and b. At more frequent intervals (e.g., at inventory times),
careful measurements are made in known problem areas, and "spot check" measurements are made in the other, less used, zones where accumulations are known to be low.


Such management of measurement resources can result in a very effective holdup measurement program at minimum costs
5.1 Holdup Measurements In performing the holdup measurements, one must be aware of the large variability in holdup assays arising primarily from variability in the measurement conditions (e.g., background, geometry, gamma ray or neutron attenua tion, material distribution). Accordingly, it is important to perform the assays from as many vantage points as possible for each collection zone. If this is impractical on a routine basis because of time or space constraints, one might consider multiple measurements initially on a collec tion zone, followed by fewer routine measurements at repre sentative assay sites. Careful thought in the selection of measurement points and measurement strategy will mini mize ambiguities in the interpretation of the data.
5.1.1 Selection of Collection Zones and Detector Positions Location and configuration of collection zones are established on the basis of a detailed physical examination and a radiation survey of the physical layout of the facility.
Preliminary measurements are needed to determine the optimum detector positions for the holdup assays. If nonuniform distribution of material in a collection zone is suspected or if the process apparatus is sufficiently compli cated to require extensive attenuation corrections for certain counting geometries, multiple measurements are advisable for the collection zone. More than one detector position may be necessary. In the cases where radiation surveys have pointed out zones of high holdup collection, extra care will be necessary in the holdup measurements for those zones to minimize their contribution to the overall holdup variability. Where radiation surveys show little hold up, proportionately less time need be budgeted. Selecting optimum detector positions includes consideration of the
5.23-9
5.23-9


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.
need to conveniently measure the line-of-sight background by moving the detector to one side without changing its orientation.


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.
5.1.2 Holdup Measurement Procedure The measurement and analysis of gamma or neutron radiation from a collection zone may be carried out by treating the material distribution as a point, line, or area source, as described in Section B.4.1, or as one of the special cases that may have been measured, as mentioned in Section B.4.2. If the nature of the material distribution is uncertain for a particular detector position, a measurement of the detector counting-rate dependence on the source-to detector distance, r, may reveal the most appropriate counting-rate geometry with which to interpret thc data.


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.
After the assay positions for the detector and shadow shields are established for each collection zone, permanent markings that indicate detector location (including height)
and orientation will ensure reproducibility of subsequent measurements for these positions. Uniquely labeling each assay site will facilitate unambiguous reference to each measurement and its location in the assay log. Furthermore, assay site labels and markings can indicate whether neutron or gamma ray measurements are to be made. Alphabetic labels (for example, "G" for gamma and "N" for neutron)
and color-coded tape markings of the sites would be useful.


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:
Protecting the markings (for example, with clear epoxy)  
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
will ensure their long-term durability.
                                                                  ....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,
After measuring the gamma or neutron radiation intensity at each detector position in a given collection zone, the line-of-sight background is measured by moving the detector and collimator to one side (still pointing in the same direction as during the assay) and measuring the radiation intensity from the surrounding materials. During the background measurement, the vessel in which the holdup is being measured must not be in the field of view of the detector.
                                                                      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..
              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
                                                                -,*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.     -        .
              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",
Because uncertainties in geometry, attenua tion, or sample matrix will usually dominate the total response variability, the counting period need not be long.
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
Having 1000 to 10,000 net counts is generally sufficient for most holdup applications.
 
This procedure is repeated at all measurement posi tions and in all counting geometries designated for each collection zone. The final holdup value for the zone is obtained from the average of the individual measurements (each one being corrected for the effects of attenuation and any variation in geometry relative to the calibration measure ment). 
Whenever possible, the collection zone is assayed in a variety of ways. For example, one could measure an appara tus up close and treat it as an area source; the measurement could then be repeated at a large distance, treating the zone as a point source. It may be better to measure some zones from several different directions-especially if complicated attenuation corrections are called for in some of the count ing geometries. Several independent measurements of one zone can provide an average holdup value that is better than the individual measurements.
 
Further, the variability between these measurements can provide an indication of the measurement uncertainty.
 
5.1.3 Gamma Ray Attenuation Corrections To obtain useful assay results by detecting 375- to
450-keV gamma rays, it is necessary to correct each assay for attenuation of the signal, either within the plutonium holdup material or by structural materials. Without this critical correction, the assay is no more than a lower limit on the true holdup value. The attenuation correction may be based on calculations of known attenuation in uniform materials, on earlier measurements of materials similar to those found in the plant equipment, or on direct measure ments of gamma ray transmission through the actual equipment. Details on establishing an appropriate attenua tion correction are given in Laboratory Exercise No. 4 of Reference 4. Additional treatment of gamma ray attenua tion corrections is given in Reference 13.
 
5.1.4 Gamma Ray Interferences Variability in the observed gamma ray response may arise as a result of the presence of extraneous gamma ray emitters or as a result of fluctuations in the background from the Compton scattering of higher energy gamma rays.
 
The magnitude of this effect is generally small. It can be monitored by observing the spectrum with a multichannel analyzer, but, unless data on periodically recovered holdup accumulations are in error, this contribution can be ignored.
 
5.1.5 Matrix Effects on Neutron Assay A change in the neutron yield for a plutonium sample of fixed isotopic content can be caused by a change in the concentration of high-(a,n)-yield impurities in the matrix.
 
If it is possible to estimate the range of permissible impurity concentrations, the variation in a typical neutron yield can be calculated using the method given in the appendix to this guide.
 
5.1.6 Effect of Isotopic Uncertainty Gamma ray measurements of plutonium holdup provide a direct determination of the fissile plutonium (i.e., 2 3 9Pu and 241 Pu) holdup in the zone under consideration. On the other hand, neutron techniques measure only the 240Pu effective content, and chemical techniques provide elemental analysis without consideration of the isotopic makeup. Thus, knowledge of the isotopic composition of the plutonium is necessary to correlate holdup measure ments with chemistry and accountability values. Gamma ray assays must be divided by the 2 3 9 pu isotopic fraction, and neutron assays must be converted from 2 4 0 Pu effective to total plutonium in order to express holdup in terms of total plutonium.
 
If the process equipment is thoroughly cleaned each time the isotopic composition is changed, the holdup may consist primarily of the current material. In that case, the declared isotopic composition can be use
 
====d. When mixing====
5.23-10
 
occurs, use of the stream-averaged isotopic composition is appropriate. Bounds on the isotopic composition are esti mated by considering the batches of highest and lowest
.omposition and computing the corresponding range. This measure of variability must then be incorporated into the estimated holdup standard deviation before making direct comparisons with the chemical analyses. The variability in isotopic composition can be expressed as an estimated stan dard deviation defined as one-half the observed range and then combined in quadrature with the standard deviation given by Equation 1 in Section B.5.2. In general, gamma ray measurements of 239 pu will be less sensitive to isotopic variations than neutron measurement of 2 4 0 Pu.
 
5.2 Assignment of Standard Error The assignment of a standard error to a holdup measure ment is extremely difficult on a rigid statistical basis. This is because the only statistically predictable fluctuations (e.g.,
counting statistics) in this application are frequently negli gible in comparison with variability due to counting geo metry (including material distribution), gamma ray attenua tion, gamma ray background and interferences, neutron matrix effects, and instrument instabilities. It is important to recognize that the variability can be large and guard against underestimating the standard deviation of the overall holdup value in a collection zone. Careful measure ments must be carried out during the calibration procedure to determine the range of detector responses resulting from variations in measurement parameters. A useful discussion of these ideas is presented in Reference 10.
 
A reasonable estimate of the standard deviation of the measured holdup for a given collection zone may be obtained by consideration of the range of holdup values obtained from the variety of measurements performed on that collection zone, as suggested in the previous section.
 
The mean value for the holdup is defined as the average of the various (corrected) measurement results on the collec tion zone. The standard deviation, a, for that mean value is estimated as one-half the range of holdup values obtained in the measurements. This estimate is conservative if a large number of measurements have been made. For a small number of measurements, the actual standard deviation can be larger than one-half the range. In such cases the actual standard deviation of the holdup values must be calculated.
 
In some cases, it may be unavoidable that the counting statistics are so poor that they contribute significantly to the measurement variability. In such an instance, the overall holdup standard deviation, CF(h-u)' is defined as the square root of the sum of the squares of the standard deviation due to counting, O(stat), and the standard deviation due to measurement fluctuations, O(meas); that is,
0 (h-u) =
(stat) + a(meas)
(1)
5.3 Estimation of Bias When a single collection zone is cleared out, it is desirable to perform a holdup assay before, H
1 before, and after, H fter, the cleanout if possible. By comparing the amount oplutonium removed, Pur, to the recovery amount pre dicted through the in situ holdup assays, Pua, the collection zone calibration can be updated, and the calibration and assay standard deviations can be based on relevant data.
 
The amount of plutonium recovered, Pur, during the cleanout of a specific collection zone can be assayed through sampling and chemical analysis, through calorim etry, or through other applicable nondestructive assay methods (e.g., spontaneous fission coincidence detection or gamma ray assay). 
The assay value for the recovered amount is computed as the difference in the holdup assays before and after the cleanout:
Pua = Hbefore - Hafter The percent difference, A, between the assay and recovery values for the plutonium holdup is then computed:
A = 100 (Pua - PUr)/Pur A running tabulation of the quantities Pua, Pur, and A (as well as their standard deviations, oa, ar, and OA) is kept in the assay log for each collection zone.
 
The average value, A, of the percent differences between Pu a and Pur will serve as an estimate of the bias in the holdup assay for that collection zone and will also provide quantita tive justification for revision of the assay calibration for that zone to remove the bias. The root-mean-square deviations, aA, of the percent differences, Ai from their mean value, A, serve as a check on the appropriateness of the size of the estimated standard deviation of the holdup measurements.
 
To the extent that the standard deviation of Pur is small compared with the uncertainty in Pua (usually an adequate assumption), the quantity sA should be comparable in size to the standard deviation of Pua. For K measurements of the percent differences, Ai, for a given collection zone, the quantity sA is given by:
FK
-E2
1/2 s
E (Ai
)2/(K - 1 A [i
1I
(4)
Equation 4 assumes that all the
0A's are equal. For a calculation of sA using weighted sums, see Reference 14.
 
Note that, if the holdup measurements (ie., Hbefore or Hafter) contain a constant bias, their difference can still provide useful information in the comparison with Pur.
 
However, a small difference between Pua and Pur does not necessarily mean that the bias associated with H is small.
 
5.23-11
(2)
(3)
 
This ambiguity is reduced in importance if the cleanout is such that Hafter is much smaller than Hbefore. In addition, the use of several holdup measurements from varying van tage points, as suggested earlier, will help to minimize the bias associated with incorrect geometrical or attenuation corrections in one measurement configuration.


==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
To develop a program for the periodic in situ assay of plutonium residual holdup as a method acceptable to the NRC staff for measuring this inventory component, it is necessary to consider the factors in the following sections.
                                                            5.23-10
 
Care must be exercised during the fabrication and use of check sources and calibration standards to ensure their continued integrity and to prevent contamination. In addition, the usual precautions for safeguarding plutonium should be taken.
 
1. DELINEATION OF COLLECTION ZONES AND
ASSAY SITES
Preliminary radiation survey measurements of the plutonium processing facility should be used to budget the measurement time to emphasize high-holdup areas, to establish independent collection zones, and to determine detector positions within the zones.
 
1. At each collection zone, detector positions (assay sites) should be chosen so that the material holdup can be measured from several vantage points around the zone. At each assay site, the detector should have an exclusive view of the collection zone being assayed. If necessary, shadow shielding should be used to isolate the region being assayed from other collection zones. Detector positions should be chosen to minimize the measurement ambiguities, as described in Section B.5.1.1.
 
2. Each assay site should be permanently marked with paint or colored tape on the floor to ensure reproducible assay positions. The markings should be protected (for example, with clear epoxy) to ensure their long-term durability. Detector height and orientation should be clearly indicated in the assay log for each measurement site and, if possible, included in the site markings.
 
3. Each assay site should be uniquely labeled to facilitate unambiguous reference to that site in the assay log. A
labeling and color-coding convention should be established to distinguish neutron assay sites from gamma ray assay sites.
 
4. Gamma ray assay should be used for collection zones containing less plutonium than the neutron detection limit.
 
Also, gamma ray assay should be used for all structures that do not contain irregularly shaped components capable of significantly attenuating the emerging gamma rays. Neutron assay should be used for all structures not suitable for gamma ray assay. There may be some large structures such as furnaces that can be measured only with small interior probes or with thermoluminescent dosimeters.
 
5. Areas may be denoted as problem areas so that careful holdup measurements will be made in these areas each time plant holdup is to be determined; or the area may be labeled as a spot-check zone, where accumulations are \\
known to be low and careful holdup assays are needed less frequently.


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.
Neutron and gamma ray assay capability can be provided, if desired, using separate or compatible electronics with interchangeable detector probes. Compatible electronics can provide for both 3He or BF 3 neutron detection and NaI(T1) gamma ray detection. The electronics unit should have a temperature coefficient of less than 0.1 percent per degree centigrade. Battery-powered electronics can expedite 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
2.1 Gamma Ray Assay Gamma ray assay should be based on the activity observed in the energy range from 375 to 450 keV, excluding the composite gamma ray complex centered at 333 keV. Yield data for appropriate gamma rays are presented in Sec tion B. 2.1 of this guide.
440 keV, excluding the composite gamma ray complex                 10,000 counts per second.


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
2.1.1 Detector Selection Gamma ray detectors for holdup measurements should have FWHM (full width at half maximum) resolution better than 10 percent at 662 keV (1 3 7 Cs gamma ray). NaI(Tl)
          2.1.1 Detector Selection                                assay collection zones, a calibration standard should be fabricated by encapsulating plutonium oxide in a disk.
detectors can exhibit resolutions as good as 7 percent and are suitable for this application. The crystal depth should be sufficient to detect a significant percentage of 400-keV
gamma rays. For NaI(T1), the minimum depth should be  
1 inch (2.5 cm); a 2-inch (5-cm) depth is recommended.


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.
The crystal should be stabilized with a suitable radioac tive source. An internal seed containing 241Am is recom mended for this application. The electronics should be capable of stabilizing on the reference radiation emitted by the seed. The crystal face (external to the cover) should be covered with 0.75 mm of cadmium and 1.5 mm of lead to filter low-energy radiations.


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.
Two single-channel analyzers should be provided with lock-set energy windows. One channel should be set to admit gamma rays from 375 to 450 keV. Unless equilibrium of the 2 3 7 U and 24 1 pu can be ensured, the 333-keV region of the gamma ray spectrum should be completely excluded.


The electronics should be capable of stabilizing on the
The second channel should be set above the first window to provide a background subtraction for the assay window.
                                                                              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
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
This second window should be set from approximately 475 to 575 keV. The width and position of this window is a matter of personal preference in how the background subtraction should be done. These analyzers should be packaged as one integral unit.
.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.
2.1.2 Gamma Ray Collimator A cylinder of shielding material such as lead should be made concentric with the gamma ray detecto


2.2.2 Neutron Collimator
====r. The end of====
          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.
5.23-12


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
the cylinder opposite the crystal should be blocked with the shielding material. The thickness of the collimator should be chosen to provide sufficient directionality for the specific facility (1.5 cm of lead thickness should be sufficient for most applications). The collimator sleeve should be fixed over the end of the detector crystal at a reproducible setting identical to that used in the calibration measure ments.
                                                          5.23-11


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.
2.1.3 Gamma Ray Calibration and Check Sources Standard sources of 2 3 9 pu should be provided for calibration of the measurement system for the basic measure ment geometries described in Section B.4. A small encapsu lated plutonium sample can be used both as a calibration standard for the point source counting geometry and as a check source for verification of instrument stability. For the line and area calibrations, large plutonium foils can be used, or the calibrations can be derived from a series of measurements made with the point source. The gamma ray self-attenuation correction should be clearly specified for all foils and samples.


should be evaluated as discussed in Part B of this guide.
2.2 Neutron Assay
2.2.1 Neutron Detector Selection Neutron detectors should have high detection efficiency and be capable of operating in the presence of gamma radiation. BF 3 and 3He neutron detectors are recommended for this application. Neutron detectors should be surrounded by a layer of neutron moderator material to enhance their detection efficiency. The neutron moderator layer should be covered with a low-energy neutron absorber to filter out extraneous neutrons from the desired signal.


3.1 Instrument Check
2.2.2 Neutron Collimator A slab collimator or concentric cylinder collimator of polyethylene should completely surround the detector, leaving open only a detection channel in one directio
            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
            2.2.4 Neutron Assay Calibration Standard                checked against the manufacturer's recommendations and repaired or recalibrated, as required.


To permit the, calibration of neutron assay
====n. The ====
* collection zones, a calibration standard should be
.moderator thickness should be selected to provide the directionality required for each facility. A directionality profile providing a 10:1 response ratio is desirable. However, for portable detectors a 3:1 ratio may be used.
                                                                          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.
2.2.3 Neutron Calibration and Check Source A 50- to 100-gram sample of plutonium should be adequate both as a point source calibration standard and as a check source. The isotopic composition, 241Am content, and high-(c,n)-yield impurity composition should be representative of the plutonium being processed. The neutron yield of the standard should be independently measured, if possible, and also computed using the method described in the appendix of this guide. If the measured and calculated yields differ by more than 20 percent, any future yield calculations should be normalized to be consistent with this measurement.


collection zone. The averaging should be weighted to reflect known local accumulation sites within each
2.3 Service Cart A cart carrying electronics and both detector probes should be provided. The capability to raise or lower the probes to reproducible settings should be included.
                '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.


,predicted response differs by more than 10%76, the response should be normalized as discussed in Section
===3. CALIBRATION ===
                                                                                3.2.1 Subzone Calibration B.2.2.4.
3.1 Instrument Check The stability of the neutron and gamma ray detection systems should be tested prior to each inventory by compar ing the observed counts obtained from the check source, minus the counts with the shaped shield in place but with out the check source, to the readings obtained prior to previous inventories. If the measurement is consistent with previous data (i.e., is within + 2 single-measurement standard deviations of the mean value of previous data), all previously established calibrations using this detection system should be considered valid. If the measurement is not consistent, the operation of the unit should be checked against the manufacturer's recommendations and repaired or recali brated, as required. These check source measurements should be supplemented with regular remeasurements of instrument calibrations to ensure continued proper instru ment performance over the entire operating range.


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.
3.2 System Response Calibration The response of the detection system should be deter mined with well-known quantities of plutonium in the basic measurement geometries described in Section B.4. If there are special counting geometries in the facility that are not readily represented by one of the basic configurations, these geometries should also be mocked up and measured during the calibration procedure.


Using this procedure, the residual geometric uncertainty
===4. ASSAY PROCEDURES ===
      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.
4.1 Assay Log An assay log should be maintained. Each collection zone should have a separate section in the assay log, with the corresponding calibration derived on the page facing the assay data sheet. Recording space should be provided for the date of measurement, gross counts, corrected counts, and the corresponding grams of plutonium from the calibration in addition to position and instrument electronic setting verification. There should also be provision for recording data from recovery operations and holdup assay comparisons, as described in Section B.5.3.


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
4.2 Preassay Procedures Prior to inventory, the isotopic composition of the plutonium processed during the current operational period should be determined. Variations in the neutron and gamma ray yield data from the calibration standard should be calculated. Either the calibration data or the predicted holdup should then be corrected to reflect this difference.
should be coded orange.


An assay log should be maintaine
5.23-13


====d.  Each====
Prior to each inventory, the operation of the neutron and gamma ray assay detection systems should be checked, as described in Regulatory Position 3.1.
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.


calibrated when all in-process material has been located            Recording space should be provided for the date of
Prior to any assay measurements, feed into the process line should be stopped. All in-process material should be processed through to forms amenable to accurate account ability. All process, scrap, and waste items containing plutonium should be removed to approved storage areas to minimize background radiations.
                                                            5.23-112


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.
4.3 Measurements 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 tions are the highest (and therefore where the most careful measurements should be made). In zones where accumula tions are shown to be very low by the survey, spot-check measurements may be adequate, as pointed out earlier.


verification.
Before assaying each collection zone, the operator should verify the floor location, probe height, and probe orientation. The electronic settings should be verified every
1 or 2 hours with the check source. During the actual assay of the collection zones, the check source should be removed or shielded so as not to interfere with the measurement.


Having met all preceding requirements, the
Prior to taking a measurement, a visual check of the zone and the line of sight of the detector probe should be made to ensure that no obvious changes have been made to the process area and that no unintended accumulations of plutonium remain within the collection zone. The operator should initial the measurement log to ensure compliance for each collection zone.
    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.


be checked.
When the preceding steps have been completed, the measurement at each collection zone should be taken, recorded, and converted to grams of plutonium. If each value is within an expected or permissible range, the assayist can proceed to the next collection zone. However, if the collection zone contains an unexpectedly large amount of plutonium, it should be cleaned to remove the accumulation for conversion to a more accurately accountable material category. After the cleanout has been completed, the zone should be reassayed.


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.
5. ESTIMATION OF HOLDUP ERROR
During the initial implementation of the holdup measure ment program, the holdup uncertainty for each collection zone should be estimated from the range of values obtained in the various measurements on that zone, as described in Section B.5.2. As a history of comparisons between holdup measurements and cleanout recovery data becomes avail able, these data should be used to adjust for bias and to revise the magnitudes of the holdup uncertainties, as described in Section B.5.3.


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
During each physical inventory, the calibration in at least 10 percent of the collection zones should be updated on the basis of the comparison between holdup and cleanout recovery measurements. In any case, all calibrations should be updated at least once per year.
    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.


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.
5.23-14 t
I


REFERENCES
REFERENCES
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
R. Gunnink et al., "A Re-evaluation of the Gamma Ray Energies and Absolute Branching Intensities of U-237, Pu-238 , -239,-240, -241, and Am-241:' Lawrence Livermore Laboratory, UCRL-52139, 1976.
                    , 1Pu and 2 4 1 Am," UCRL,51087
 
    238 2 39 240 24
2.
          ,     ,                                                       of UF 6 Cylinders with Portable Instruments," Nucl.
 
J. E. Cline, R. J. Gehrke, and L. D. Mclsaac, "Gamma Rays Emitted by the Fissionable Nuclides and Asso ciated Isotopes," Aerojet Nuclear Co., Idaho Falls, Idaho, ANCR-1069, July 1972.
 
3.
 
L. A. Kull, "Catalogue of Nuclear Material Safeguards Instruments," Battelle National Laboratories, BNL
17165, August 1972.
 
4.
 
R. H. Augustson and T. D. Reilly, "Fundamentals of Passive Nondestructive -Assay of Fissionable Material,"
Los Alamos Scientific Laboratory, LA-5651-M, 1974;
also T. D. Reilly et al., "Fundamentals of Passive Nondestructive Assay of Fissionable Material: Labora tory Workbook," Los Alamos Scientific Laboratory, LA-5651-M, Suppl., 1975.
 
5.
 
R. B. Walton et al., "Measurements of UF Cylinders with Portable Instruments," Nuclear Technology, Vol. 21, p. 133, 1974.
 
6.
 
C. H. Kindle, "In Situ Measurement of Residual Pluto nium," Nuclear Materials Management, Vol. 5, No. 3, p. 540, 1976.
 
7.
 
J. W. Tape, D. A. Close, and R. B. Walton, "Total Room Holdup of Plutonium Measured with a Large-Area Neutron Detector," Nuclear Materials Management, Vol. 5, No. 3, p. 533, 1976.
 
8.
 
H. E. Preston and W. J. Symons, "The Determination of Residual Plutonium Masses in Gloveboxes by Remote Measurements Using Solid Thermoluminescent Dosimeters," United Kingdom Atomic Energy Author ity, Winfrith, England, AEEW-R13 59, 1980.
 
9.
 
A. Ohno and S. Matsuura, "Measurement of the Gamma Dose Rate Distribution in a Spent Fuel Assembly with a Thermoluminescent Detector,"
Nuclear Technology, Vol. 47, p. 485, 1980.
 
10.
 
W. D. Reed, Jr., J. P. Andrews, and H. C. Keller, "A
Method for Surveying for 2 3 sU with Limit of Error Analysis," Nclear aterials Management, Vol. 2, p. 395 ,
1973.
 
11.
 
N. Ensslin, J. Stewart, and J. Sapir, "Self-Multiplication Correction Factors for Neutron Coincidence Counting,"
Nuclear Materials Management, Vol. VIII, No. 2, p. 60,
1979.
 
12.
 
M. S. Zucker et al, "Holdup Measurements for Nuclear Fuel Manufacturing Plants," Nuclear Materials Manage ment, Vol. X, p. 239, 1981.
 
13.


(July 1971).                                                      Technol., 21, 133 (1974).
J. L. Parker and T. D. Reilly, "Bulk Sample Self Attenuation Correction by Transmission Measure ment," Proceedings of the ERDA X- and Gamma-Ray Symposium, Ann Arbor, Michigan (Conf. 760639),
2.  J. E. Cline, R. J. Gehrke, and L. D. Mclsaac,
p. 219, May 1976.
                                                                  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).
14.
                                                                        of Error Analysis," Gulf-GA-A12641 (June 1973).
 
3.   L. A. Kull, "Catalogue          of Nuclear Material Safeguards Instruments,"        BNL-17165 (August
P. R. Bevington, Data Reduction and Error Analysis for the Physical Sciences, McGraw-Hill, 1969.
    1972).
 
                                                          5.23-13
5.23-15


APPENDIX
APPENDIX
                                        NEUTRON YIELD COMPUTATIONS
A. NEUTRON YIELD COMPUTATIONS  
    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 spontaneous neutron yield from plutonium-bearing materials assumes that the plutonium is widely dispersed. With this condition, there will be no significant neutron production through induced fission of
39pu or 2 4 1 Pu. The total neutron yield of plutonium holdup will then be the sum of the spontaneous fission and (ct,n) contributions:
Yn= YSF + Y(a,n)
(1)
1. SPONTANEOUS FISSION NEUTRONS
To determine the spontaneous neutron yield of pluto nium, the isotopic composition must be known. (The contribution from 23SU spontaneous fission is usually negligible even if uranium is present in large quantities.)
The yield from the plutonium isotopes is given by:
YSFp M2 3 8Q2 38 + M2 4 0 Q2 4 0 + M2 4 2 Q2 4 2 
(2)
where Mi is the total mass of the ith plutonium isotope, and Qi is the spontaneous fission neutron yield per gram of the ith isotope. Using the yield data from Table A-l, Equation 2 can be rewritten as:
YSF = (1030 n/sec-gram)M 24 0 (effective)
(3)
where M2 4 0 (effective) = 2.50M2 38 + M240 + 1.70M2 4 2
(4)
The coefficients 2.50 and 1.70 are the spontaneous fission yields of 2 3 8pu and 2 4 2Pu relative to 24&deg;Pu. The concept \\
of effective 2 4 0 pu mass reflects the fact that most of the spontaneous fission yield is due to that isotope.
 
2. (cz,n) NEUTRONS
When the plutonium holdup is in the form of oxide, the major contribution from (ct,n) reactions will be due to the
0-18(a,n) 21Ne reaction. The additional neutron yield is typically 50 to 100 percent of the spontaneous fission yield. The (a,n) yield can be calculated from the yields per gram of each isotope of Pu(Yi) given in Table A-1:
Y(ay,n) oxide = 1 MiYi
1y
(5)
The summation over Mi should also include 24 1 Am, which is a strong alpha emitter.
 
In addition to (c4n) production in the oxide itself, certain low-Z impurities in the oxide can contribute substan tially. Values for the yields of neutrons obtained in bombard ing thick targets of these elements with 5.2-MeV alpha particles are given in Table A-2. Further research may change these values somewhat, but they are sufficient for computing the approximate effect of these elements if they exist as impurities in PuO . One method for doing this is to compute the impurity (x,n) yield relative to the oxide (ct,n) yield:
Y(an)impurity - Y(cn)oxide f (WiAoI )/(Pok io)
(6)
ble A-1 ALPHA PARTICLE AND SPONTANEOUS FISSION NEUTRON YIELDS
Half-Life (yr)
87.78
24,150
6,529
14.35**
379,000
433.8
2.47 x 105
7.1 x 108
4.51 x 109 Alpha Activity (a/sec-gram)
6.33 x 1011
2.30 x 109
8.43 x 109
9.39 x 107
1.44 x 108
1.27 x 10"1
2.29 x 108
7.93 x 104
1.23 x 104 Qi Spontaneous Fission (n/sec-gram)
2.57 x 103
2.22 x 10-2
1.03 x 103
5.00 x 10-2
1.75 x 103
6.05 x 10f'
5.67 x 10-3
5.96 x 10-4
1.12 x 10-2
* Oxygen yield from PuO 2form only.
 
*
**&branching ratio: 2.46 x i0T5 .
5.23-16 Nuclide
238pu,
239pu
2 4 0 pu
2 4 1 pu
2 4 2 pu
2 4 1 Am
234U
235U
238 U
Y.
 
PuO2 U0 2 (a,n) Yield*
(n/sec-gram)
1.4 x 104
42.5
157
1.3
2.2
2957
4.65
1.37 x 10- 3
1.93 x 10-4
 
Table A-2 (a,n) NEUTRON YIELDS FOR
VARIOUS LIGHT ELEMENTS
P.
 
Neutron Yield Element per 106 Alphas Be
58 B
18 F
6.4
7 Li
1.3 Na
-1.5 NatMg
0.89 Al
0.44 Natsi
0.077 C
0.05 O
0.050
*Most of these yields are based on:
J. K. Baki and J. Gomez del Campo,
"Neutron Yields from Alpha-Particle Bombardment,"
Nuclear Science and Engineering, Vol. 71, p.
 
18,
1979.
 
In Equation 6, P. is the (a,n) neutron yield in the impurity element, and P0 is the yield in oxygen (0.050 neutrons/106 alphas); A. is the atomic weight of the impurity element, and Ao that for oxygen (16); Ij is the concentration of the 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, it can be converted to ppm of plutonium oxide by multi plying by the gravimetric dilution factor, 0.882.
 
To summarize the calculation of (ct,n) neutron yields in oxide that also contains impurities, Y(a,n) from all sources is given by:
Y(
Z,n)
MiYf + .0027EP_-I-/A-)
(7)
i
"
I
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 summation over i must include 241 Am and that the summa tion over j includes only the oxygen that is not bound up as plutonium oxide.
 
3. SAMPLE CALCULATION FOR PuO 2 -UO2 Consider the case of I gram of recycle plutonium blended to 3 percent by weight of PuO 2 in a UO 2 matrix where the isotopic composition is as given in Table A-3. For mixed oxides, the oxygen density is approximately the same as in PuO2 alone. Also, plutonium and uranium have similar atomic numbers. For these reasons, it may be assumed that the oxygen (ca,n) yield in mixed oxide is the yield in PuO 2, further reduced by the blending ratio, PuO2 /(PuO 2 + U0 2 ).
SAMPLE CALCULATION FOR 1 GRAM OF PLUTONIUM
Isotopic Nuclide Composition YSF
(n/sec)
Y(a, n)o ide (n/sec)I
23 SPU
0.003
8
42
2 3 pt
0.756
0
32
24 PU
0.185
191
29 S1 Pu
0.045
0
0
242
0.011
19
0
241 Am
0.003
0
9 Total Yields
218
112 Using the isotopic composition given in Table A-3 and using Equation 3, the spontaneous fission neutron yield can be found to be 218 n/sec for 1 gram of plutonium. Then the neutron production in the oxide can be calculated using the masses Mi of each isotope and the yields Y- from 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 particle yield of ylutonium is nearly constant in time, but that, because 24 Am builds up in time, the total alpha production increases at a rate of roughly 0.3 percent per month in typical reactor fuel The impurity (ct,n) yields are calculated in Table A-4.
 
The calculation is based on impurities in PuO 2 only. The mixed oxides are assumed to consist of blended PuO 2 and UO 2 particles approximately 40 pm in diameter where most alpha particles stop within the PuO2 particles. If the particle size were smaller or the mixed oxide were created through coprecipitation, the uranium impurity content would also contribute to the plutonium (a,n) yield. In the present example, it is sufficient to use the neutron yields P.
 
from Table A-2, the concentrations Ij from Table A-4, and Equation 6 or 7.
 
The total neutron yield from 1 gram of plutonium in PuO 2 is then 218 + 112 + 47 = 377 n/sec. Using the gravi metric dilution factor of 0.882, this is 333 n/sec for 1 gram of PuO 2 . If the PuO 2 is blended so that PuO2 / (PuO 2 +
UO ) = 0.03, the neutron yield from 1 gram of mixed oxide is 10 n/sec.
 
The impurity (ct,n) yields, Pi, used in this example are currently known to about 10 percent accuracy for most elements and 50 percent accuracy for the others. The oxide (atn) yields, Y.., are known to 10 percent or bette
 
====r. Both ====
.J
yield calculations must assume perfect mixing, however.
 
For these reasons, neutron yield calculations are accurate to
10 percent at best, and the neutron holdup measurement calibration should be based on representative standards rather than calculation wherever possible.
 
5.23-17 Table A-3
 
Table A-4 IMPURITY (ca,n) YIELD
Arbitrary Concentration I
Impurity (cn)
in PuO2 Yield (I12 n/sec)
Impurity (ppm by wgt)
(0.00 2 7 )Pjlj/Aj Li
9
1 Be
8
16 B
10
5 C
200
0
F
125
13
0 (moisture)
4600
4 Na
120
8 Total
47 (n/sec)
B. CONVERSION OF MEASURED M
D4(EFFECTIVE)
TO TOTAL PLUTONId"
To convert a measured effective 2 4 0 pu mass to actual total plutonium, one must use both the relationship between these two quantities, as shown in Equation 4, and the known isotopic composition of the samples being measured.
 
Let f238, f239' f240' f241, and f2 4 2 represent the weight fractions of the respective plutonium isotopes in the unknown sampl
 
====e.  The ====
2 4 0 pu effective weight fraction, f2 4 0(effective), can be defined as:
f2 4 0 (effective) = M2 4 o(effective)/Mpu(total)
where f2 4 0 (effective) = 2.50f 2 39 + f24 0 + 1.70f 2 4 2
(8)
(9)
Generally, as previously mentioned in this guide, the relative measurement uncertainty of M2 4 0 (effective) in a holdup measurement will be much larger than that of f;40(effective), so the relative error in Mpu(total) is essen 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:
f24 0 (effective) = 2.50(0.003) + 0.185 + 1.70(0.011)  
= 0.21 Thus, a holdup measurement of 35 + 10 grams 24&deg;pu effective corresponds to 166 + 47 grams total plutonium, where the relative error in the total plutonium result was taken to be equal to that of the M2 4 0 (effective) result.
 
5.23-18
 
VALUE/IMPACT STATEMENT
 
===1. PROPOSED ACTION===
1.1 Description No adverse impact on the public can be foreseen.
 
Licensees authorized to possess at any time more than
1 kilogram of plutonium are required by Part 70, "Domestic Licensing of Special Nuclear Material," of Title 10 of the Code of Federal Regulations to calculate a material balance based on a measured physical inventory at intervals not to exceed 2 months. Further, these licensees are required to conduct their nuclear material physical inventories in compliance with specific requirements set forth in Part 70.
 
Inventory procedures acceptable to the NRC staff are detailed in Regulatory Guide 5.13, "Conduct of Nuclear Material Physical Inventories."
Plutonium residual holdup is defined as the plutonium inventory component remaining in and about process equipment and handling areas after those collection areas have been prepared for inventory. This 'regulatory guide describes procedures acceptable to the NRC staff for the in situ assay of the residual plutonium holdup.
 
1.2 Need for Proposed Action Regulatory Guide 5.23 was published in 1974. The proposed action, a revision to this guide, is needed to bring the guide up to date with respect to advances in measure ment methods, as well as changes in terminology.
 
1.3 Value/Impact of Proposed Action
1.3.1 NRC Operations The regulatory positions will be brought up to date.
 
1.3.2 Other Government Agencies Not applicable.


Yn = YSF + Y(,t,n)                                  (1)
1.3.3 Industry Since industry is already applying the methods and procedures discussed in the guide, updating these methods and procedures should have no adverse impact.
                                                                            Y(an) Oxy            WiYi                        (3)
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).
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.


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.
1.4 Decision on Proposed Action The regulatory guide should be revised to reflect improve ments in measurement techniques and to bring the language of the guide into conformity with current usage.


(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:
===2. TECHNICAL APPROACH ===
      isotope. For reactor fuel applications, W2 3 8 + W239
Not applicable.
      + W240*'W241 +W242* I
      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
===3. PROCEDURAL APPROACH ===
                                                                5.23-14
Of the procedural alternatives considered, revision of the existing regulatory guide was selected as the most advanta geous and cost effective.


where                                                                          Pj    = (an) yield per ppm of the impurity j (see Table 2)
4. STATUTORY CONSIDERATIONS
      Y,,         = total a production Ii    = impurity j content, expressed in ppm (weight) of plutonium.
4.1 NRC Authority The authority for the proposed action is derived from the Atomic Energy Act of 1954, as amended, and the Energy Reorganization Act of 1974, as amended, and is implemented through the Commission's regulations, in particular 10 CFR Part 70.


=          WWiai    + WArnm'Am i                                                3.   Sample Calculation (PuO 2 -UO 2 )
4.2 Need for NEPA Assessment The proposed action is not a major action that may significantly affect the quality of the human environment and does not require an environmental impact statement.
      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
S. RELATIONSHIP TO OTHER EXISTING OR PROPOSED
    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%;.
REGULATIONS OR POLICIES
    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.
The proposed action is one of a series of revisions of exist ing regulatory guides on nondestructive assay techniques.


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)
6. SUMMARY AND CONCLUSIONS
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
Regulatory Guide 5.23 should be revised.
-- 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
1.3.4 Public
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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Latest revision as of 02:08, 17 January 2025

(Task SG 045-4), Revision 1, in Situ Assay of Plutonium Residual Holdup
ML003740013
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Issue date: 02/29/1984
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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

A. INTRODUCTION

Part 70,

"Domestic Licensing of Special Nuclear Material," of Title 10 of the Code of Federal Regulations requires licensees authorized to possess more than 1 kilo gram of plutonium to calculate a material balance based on a measured physical inventory at intervals not to exceed 2 months. Further, these licensees are required to conduct their nuclear material physical inventories in compliance with specific requirements set forth in Part 70.

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

Plutonium residual holdup is defined as the plutonium inventory component remaining in and about process equipment and handling areas after these collection areas have been prepared for inventory. Whenever possible, process equipment should be designed' and operated so as to minimize the amount of holdup. In this guide, proce dures acceptable to the NRC staff for the in situ assay of the plutonium residual holdup are described.

Assay information may be used in one of two ways:

1. When the standard error (estimator) of plutonium holdup is compatible2 with constraints on the overall standard error of the inventory difference (SEID), the mate rial balance can be computed using the measured contents of plutonium holdup. Additional cleanout and recovery for accountability will then not be necessary.

2. When the standard error of plutonium holdup is not compatible with constraints on the overall SEID, the

1 Design features to minimize holdup in process equipment are the subject of a series of regulatory guides (5.8, 5.25, and 5.42).

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

s. If the plant SEID

exceeds allowed limits because of an excessive contribution from the holdup standard error, compatibility does not exist and the remedial steps of paragraph 2 needto be taken.

USNRC REGULATORY GUIDES

Regulatory Guides are issued to describe and make available to the public methods acceptable to the NRC staff of Implementing specific parts of the Commission's regulations, to delineate tech niques used by the staff in evaluating specific problems or postu lated accidents or to provide guidance to applicants. Regulatory Guides are not substitutes for regulations, and compliance with them is not required. Methods and solutions different from those set out in the guides will be acceptable if they provide a basis for the findings requisite to the issuance or continuance of a permit or license by the Commission.

This guide was Issued after consideration of comments received from the public. Comments and suggestions for improvements in these guides are encouraged at all times, and guides will be revised, as appropriate, to accommodate comments and to reflect new Informa tion or experience.

information obtained in the holdup survey can be used to locate principal plutonium accumulations and to ensure that other areas of the process contain less than the detectable amount of plutonium. Once located, substantial accumulations can be recovered, transforming the pluto nium to a more accurately measurable inventory compo nent. Having reduced the amount of plutonium holdup, the standard error on the remeasurement of the remaining holdup may be sufficiently reduced to be compatible with overall SEID requirements.

Any guidance in this document related to information collection activities has been cleared under OMB Clearance No. 3150-0009.

B. DISCUSSION

Plutonium accumulates in cracks, pores, and zones of poor circulation within process equipment. The walls of process vessels and associated plumbing often become coated with plutonium during solution processing. Surfaces internal and adjacent to process equipment, especially glovebox walls and floors, accumulate deposits of pluto nium that can become appreciable. Plutonium also accu mulates in air filters and associated ductwork. The absolute amounts of plutonium holdup must be small for efficient processing and proper hazards control. However, the total holdup can be large relative to the plant inventory differ ence (ID) but have no significant impact on the ID if it remains reasonably constant. It is the change in the holdup between beginning inventory and ending inventory that may impact the ID.

The measurement procedures described in this guide involve the detection of gamma rays and neutrons that are spontaneously emitted by the plutonium isotopes. Because the gamma rays of interest are emitted by the major isotope,

23gpU, gamma ray assay is the preferred method whenever its acceptance criteria are satisfied. The amount of 2 3 9 pu The substantial number of changes in this revision has made it impractical to indicate the changes with lines in the margin.

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

The guides are issued in the following ten broad divisions:

1. Power Reactors

6. Products

2. Research and Test Reactors

7. Transportation

3. Fuels and Materials Facilities

8. Occupational Health

4. Environmental and Siting

9. Antitrust and Financial Review

5. Materials and Plant Protection 10. General Copies of issued guides may be purchased at the current Government Printing Office price. A subscription service for future guides in spe cific divisions is available through the Government Printing Office.

Information on the subscription service and current GPO prices may be obtained by writing the U.S. Nuclear Regulatory Commission, Washington, D.C. 20555, Attention: Publications Sales Manager.

holdup in a piece of equipment is proportional to the measured intensity of the emitted gamma rays after suitable corrections are made for attenuation by intervening mate rials, for self-attenuation by plutonium, for scattering, for geometrical factors, and for background radiation.

If plutonium is held up in furnaces, grinders, or other heavy equipment that is too dense to permit the escape of gamma rays, an assay based on spontaneous fission neutrons from the even isotopes of plutonium may be possible. This technique requires knowledge of the isotopic composition of the plutonium, some knowledge of its chemical form, and knowledge of the presence of other radionuclide impurities.

Thermoluminescent dosimetry is a third technique that can be used to measure holdup from the inside of large pieces of equipment. This technique is also useful for carrying out measurements in an unobtrusive manner outside normal plant operating hours.

For all three techniques, the proportionality factors between amount of holdup and detector response are best determined prior to the holdup measurement by assays of known quantities of plutonium distributed in well-defined and representative geometries, as discussed below.

1. DELINEATION OF COLLECTION ZONES

Typical plutonium process facilities comprise a number of interconnected gloveboxes that contain work areas and most process equipment, in-process storage areas, and self-contained process equipment. Also, solution process ing requires tanks, plumbing, and pumping equipment, which are often located in close proximity to the glovebox lines. Finally, storage areas for feed, scrap and waste, and final product are often located in close proximity to the plutonium process area.

To accomplish the holdup measurements, it is essential to consider the facility in terms of a series of zones that can be independently assayed.

Such zones are designated

"collection zones." Each plutonium-processing facility can be conceptually divided into a series of contiguous collection zones on the basis of process activities and collection geometries.

Individual machines, filters, pipes, tanks, gloveboxes, or surface areas that can be isolated from one another may be suitable discrete collection zones. Great care is needed to define all collection zones so that (1) the 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 distribution of material in the zone can be represented by one of the distribution geometries used in the calibration procedure described below.

Gamma ray assay for plutonium holdup is practical when the collection zone consists of a single structure of relatively uniform cross section. When a collection zone contains a complex item of equipment with significant self-shielding properties, the uncertainty in the holdup, measurement may be primarily due to attenuation of radiation in the internal structure. In such cases, neutron assay from the outside and thermoluminescent dosimeter assay from the inside may be applicable.

If delineation of collection zones is not possible, two alternatives are experiments with mockup geometries or complex numerical calculations.

2. APPLICABLE METHODS AND INSTRUMENTS

Two considerations are critical to the selection of methods and instruments. First, to perform an assay, one must ensure that the plutonium radiations reach the detector and are detected. Second, the observed response must be attributable to the collection zone being assayed. Therefore, the assay scheme is developed around penetrating radiations, and the detector is collimated to provide for sufficient directionality in the response to resolve a collection zone from its neighboring zones and from the background.

Finally, some effort may be necessary to employ external

"shadow shielding" to block radiation being produced in adjacent collection zones from the field of view of the collimated detector.

2.1 Gamma Ray Assay Under closely controlled conditions, the measured plutonium gamma ray spectrum can be interpreted in terms of the abundance of each gamma ray emitter present in the sample. Because of the large number of gamma rays (Refs. 1 and 2) present, many regions of the observed spectrum are characterized by overlapping lines. To accomplish the assay, it is necessary to select an appropriate spectral region and provide a detection system with sufficient resolution to measure the activity from the isotopes of interest.

Gamma ray assay has an advantage over neutron assay in that the emissions are primarily from the principal isotopes of interest. Because of the hiih emission rate of gamma rays, a detection sensitivity of less than 1 gram is generally attainable.

The most useful portion of the spectrum for holdup assay is the 2 3 9pu gamma ray complex in the 375- to

45 0-keV range. 3 The yields of these lines are given in Table 1.

31n typical Nal detectors (with energy resolution of 10 percent at 414 keV), the 414-keV photopeak will produce counts in the approximate energy range of 373 to 455 keV. Thus, an energy window from 375 to 450 keV will include most of the 414-keV

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

375-keV

P9P gamma rays (see Table 1), but will also exclude

'mos orftihe otentially interfering 332-keV gamma rays from

5.23-2 K

Table 1 PROMINENT GAMMA RAYS FROM 2 3 9 pu IN 375- to 450-keV ENERGY RANGE

Intensity Energy (y/sec-g 2 3 9Pu)

375.0

380.2

382.7

392.5

393.1

413.7

422.6

3.59 x 104

0.70 x 104

0.59 x 104

0.26 x 104

1.01 x 104

3.43 x 104

0.27 x 104 Total

9.85 x 104

2.1.1 Gamma Ray Detection Instruments Gamma ray detection systems consist of a scintillation or semiconductor gamma ray detector and appropriate electronics (Refs. 3 and 4). Required electronics include at least a single-channel analyzer and a timer-scaler unit. A

second single-channel analyzer viewing the same detector pulses used to determine the background radiation correc tion is a timesaving feature. A number of portable battery powered systems are commercially available for this applica tion.

The detection efficiency andresolution (10 to 15 percent)

of NaI(Tl) is generally adequate for holdup measurements.

CdTe, Ge(Li), and high-purity germanium (also known as intrinsic germanium) detectors have better resolution than NaI(Tl) but are more costly and more difficult to operate.

For more information on Ge(Li) and intrinsic germanium detectors, see Regulatory Guide 5.9,

"Guidelines for Germanium Spectroscopy Systems for Measurement of Special Nuclear Material," and the references cited therein.

The 332.3-keV gamma ray from 2 3 7 U, a short-lived

(6.75 days) daughter of 2 4 1pu, is usually the principal inter ference for 2 3 9Pu assay by Nal detection of the 375- to

450-keV complex. If the 2 3 7 U is in equilibrium with

2 4 1pu, the intensity of this gamma ray is 1.15 x 106 y/sec-g 2 4 1pu. Since this gamma ray is also emitted in the decay of 241 Am, the interference from this decay branch may also be important in case of preferential americium holdups. To avoid this interference when using Nal detectors, the assay energy window is adjusted to span the range from

390 to 450 keV for plutonium holdup with high americium content.

Detector crystal dimensions are selected to provide a high probability of detecting gamma rays from the 375- to

450-keV complex and a low probability of detecting high-energy radiation. For NaI, a crystal diameter of

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

For germanium detectors, a moderate-volume coaxial detector is recommended.

2.1.2 Collimators and Absorbers for Gamma Rays A shaped shield constructed of any heavy-element material is appropriate for gamma ray collimation. For cost, availability, and ease of fabrication, lead is recommended.

Less than 2 percent of all 400-keV gamma rays striking a

1.5-cm-thick sheet of lead will pass through without suffer ing an energy loss.

The collimator will be most effective when it is con centric about the crystal and photomultiplier and com pletely covers the photomultiplier base. Extending the collimator forward of the crystal at least a distance equal to 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 desirable to select collection zones and counting geometries so that one collimator setting will suffice for all measure ments. This will simplify the calibration procedures because the calibration constants depend strongly on the dimensions and placement of the collimator aperture.

The collimator not only defines the effective field of view but also shields the detector from unwanted radiation.

To effectively accomplish this latter purpose, the collimator material must also cover the rear of the detector. This is usually easy to achieve with portable Nal detectors but requires more effort when germanium detectors are used.

Intensive 50- to 100-keV X-ray radiation and 60-keV

241 Am gamma ray radiation are often emitted by process equipment, and this radiation can tie up the detector electronics unnecessarily. A 1.5-mm-thick layer of lead (on the outside) and a 0.75-mm-thick layer of cadmium (on the inside) may be placed against the front face of the detector to alleviate this problem. This graded energy shield will absorb most of the low-energy photons incident on the detector without substantially reducing the number of gamma rays detected in the 375- to 450-keV range.

2.1.3 Check Source for Gamma Ray Assay It is important to check the operation of the detection system each time the instrumentation is moved or otherwise disturbed (e.g., power outage) during the course of each inventory sequence. Either recalibrating one or more collection zones and comparing the results to previous analyses or testing the instrument with a check source is appropriate. When the response remains within the expected value, the previous calibration data are assumed to be valid.

If not, the energy window may have shifted, or the unit may be in need of repair and recalibration.

An appropriate check source enables the stability of the assay instrument to be tested at any location. Such a source can be prepared by implanting a small encapsulated plutonium sample (containing '.,0.5 gram of plutonium) in the face of

5.23-3

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 plug is in place.

The check source is fabricated in a manner that will ensure its internal stability. Other than radiations increasing from the ingrowth of 2 4 1 Am, the emission rate of the check source should remain constant.

2.2 Neutron Assay Neutrons are emitted in the spontaneous fission of 238Pu,

24°Pu, and 242Pu and through the interaction of emitted alpha particles with certain light nucleL These neutrons suffer little attenuation in passing through uranium or plutonium or through most structural and containment materials. Glovebox windows may reduce the energy of emerging neutrons, but, because of their regular and con stant shape, their effect can generally be factored into the assay calibration.

To be useful for the assay of plutonium holdup, the neutron production rate per gram of plutonium must be known. The spontaneous fission contribution to the total neutron production can be computed from basic nuclear data after the isotopic composition of the contained plutonium has been determined. Computing the (ca,n)

contribution requires a knowledge of the chemical form of the plutonium and the amount and distribution of certain high-(ca,n)-yield target materials.

The background count rate from neutron detectors may be a substantial part of the observed activity, often corre sponding to as much as 20 grams of plutonium in typical holdup assays. Thus, neutron assay is primarily applicable to the measurement of significant accumulations of pluto nium.

The measured neutron yield from prepared calibration standards is used to calibrate each neutron assay collection zone. In the appendix to this guide, a method is given to calculate the anticipated neutron yield. This method provides the ability to calculate the neutron yield when the isotopic or impurity composition of the plutonium holdup is different from that of the calibration standards. The method can be used to calculate a ratio of the neutron production rate of the unknown material to the neutron production rate of the standard material. The yield from the holdup material is then determined by multiplying the measured "known" material yield by the computed ratio.

2.2.1 Neutron Detection Instruments To effectively employ the spontaneous neutron yield as a measure of plutonium holdup, it is necessary to detect the neutrons in the presence of a more intense gamma ray background and to collimate the detector so that neu trons emanating from the collection zone under assay are preferentially detected.

Holdup assay is performed under in-plant conditions where ruggedness, portability, high neutron efficiency, and low gamma ray sensitivity in the detectors are important.

Gas-filled proportional counters containing He or BF 3 are suitable for this purpose. Typical fill pressures are 1 to 4 atmospheres. One advantage of 3 He for in-plant applications is that the operating voltage of 3He counters is about

75 percent of that required for BF 3 counters.

The efficiency of 3 He and BF 3 counters increases as the energy of the neutrons decreases. Embedding gas-filled counters in polyethylene to moderate the incoming neutrons to thermal or epithermal energies will improve their effi ciency. A nearly optimum design can be obtained by center ing the counters in 10 cm of polyethylene with 2 to 3 cm of polyethylene between adjacent counters.

To shield the detector from low-energy neutrons that may produce a complicated response pattern, the modera tor material is covered with a thermal neutron absorber.

Cadmium sheeting approximately 0.075 cm thick may be used for this application.

2.2.2 Collimators for Neutron Detectors To assay a specific collection zone in the presence of other distributed sources of plutonium, it is necessary to collimate the detector. This is accomplished by stopping neutrons coming to the detector from all directions except the desired one. The cadmium surrounding the detector will stop essentially all neutrons striking the detector with energies below 0.4 eV. By adding moderator material around the outside of the detector in all directions except for the collimator channel, neutrons coming from unwanted directions will lose energy in this shield and will be absorbed in the cadmium cover. For each 6 inches

(15 cm) of polyethylene added, the collimator assembly provides a factor of approximately 10 in the directionality of the response.

An example of a collimated neutron detector assembly for plutonium holdup assay is shown in Figure 1. This assembly has a polyethylene shield thickness of 6 inches

(15 cm) and a directionality of 10 to 1. The combined weight of the detector and collimator exceeds the require ments for a hand-held probe. For this reason and to provide for reproducible positioning at each assay, a sturdy cart housing such a detector and its associated electronics is recommended. In order to assay items at different heights, the capability to raise and lower the assembly to reproduc ible settings is needed.

An example of a small commercially available hand-held detector is given in References 5 and 6. This Shielded Neutron Assay Probe (SNAP) is 12 inches (30.5 cm) high and 10 inches (25.4 cm) in diameter and contains two 3He detectors. It includes a 2-inch-thick (5 cm) polyethylene shield that provides a directionality of 3 to

1. The SNAP

has been used to measure plutonium holdup, UO2 F2 holdup, and UF 6 enrichment. It is recommended for the assay of well-defined concentrations of plutonium in pumps, grinders, pipe elbows, or other items of equipment where portability and accessibility are more important than directionality.

5.23-4

DETECTOR CABLE ACCESS CHANNEL

(TOP SECTION ONLY) r-.

6

117

1 I5.1cm 15.1cm

3" I T

4.5cm

+

TOP VIEW

-- ,c

3c

"I

2.6cmnDIAl I I

I

I

I

I

IFRONT

I

II9cI

I

I

I

IIl I

m W9mITH 0,7I

CDIUMSI

E

N DETECTOR

"HANNELS

METER

(TYP)

VIEW

DETECTOR TUBE SUBASSEMBLY

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 FRONT VIEW

+

4

4

A third example of holdup measurement by neutron detection is given in Reference 7. In this case, a completely uncollimated polyethylene slab containing a row of 3He detectors was suspended in midair in some of the processing rooms of an industrial plutonium facility. The response of the detector was found to be proportional to the total room holdup if the plutonium was reasonably uniformly distributed and if the room was isolated from external sources. The calibration procedure for the use of. this detector will not be described here. However, it is recom mended as a means for quickly verifying total room holdup when measurements of the holdup in individual items or equipment are not needed.

2.2.3 Check Source for Neutron Assay To ensure the proper operation of the neutron assay system prior to making an assay, it is necessary to test the response of the instrument. An appropriate neutron assay check source can be measured, or the detector response for one or more collection zones can be recalibrated and compared to the results of previous calibrations.

An appropriate neutron assay check source can be prepared by implanting a small encapsulated plutonium source (containing about 5 grams of plutonium) into the face of a plug of neutron moderating material (see Figure 2). The plug is fabricated to fit and close the colli mator channel. When the response from the check source remains within the expected value, the previous calibration data are assumed to be valid.

2.3 Thermoluminescent Dosimeter (TLD) Assay Crystals of LiF, CaF2 , CaSe 4 , or other compounds can store energy at manganese or dysprosium impurity centers when they are struck by gamma or neutron radiation. At some later time, the crystals can be heated rapidly to several hundred degrees centigrade to induce thermo luminescence. The light output at this time is proportional to the amount of radiation received. Thermoluminescent detectors that are primarily gamma sensitive, graded X-ray shields, read-out instrumentation, and other accessories are commercially available.

TLDs have been used to measure the holdup in glove boxes by placing them at regular intervals on the outside surfaces. The TLDs are left in place overnight in order to accumulate a measurable dose. Accuracies of +20 percent relative to cleanout values are reported for plutonium of known isotopic composition. TLDs have also been used to measure the holdup in the interior of large furnaces that are not accessible by other means. For both of these examples, calibration requires either careful dose and geometry calculations or mockups of the actual collection zone.

Because their use is relatively new and only a few published references exist (Refs. 8 and 9), TLDs will not be discussed further in this guide. However, they could be useful for special applications.

3. ISOLATION OF COLLECTION ZONES

To ensure that each collection zone is independently assayed, it is necessary to shield the detector from all radiations except those radiations emanating from the collection zone being assayed. This is principally accom plished through the use of the collimators described in Sections B.2.1.2 and B.2.2.2. Two additional means exist to isolate a collection zone, detector positioning and shadow shielding.

3.1 Detector Positioning An unobstructed side view of a collection zone is pre

+/-erred. When plutonium is located behind the zone under assay in another collection zone or a storage facility, an additional background assay may be performed with the detector above or below the collection zone and pointing at the material behind the zone under assay. It is important to prevent, or account for, moving objects within the field of view. If this is not done, variations in shielding and scatter ing can affect the measurement.

3.2 Shadow Shielding It may not be possible to avoid interfering radiations by collimator design or by choosing the detector position for assay. In such cases, it may be possible to move a shield panel between the source of interfering radiations and the collimator zone under assay. If the shield panel is very thick and its dimensions match or exceed the back side of the 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 would limit its transportability. A rectangular panel mounted on wheels as an upright panel and containing %5 cm of neu tron moderator (e.g., benelex, WEP, or polyethylehe) and

,0.5 cm of lead sheet is recommended. To use such a panel, it is necessary to measure the response of the collec tion zone with and without the shield in place. Also, the gamma and neutron transmission factors of the shield itself must be measured beforehand with a representative pluto nium sample. From these measurements, the assay of the collection zone can be corrected for background radiation transmitted through the shield.

4. CALIBRATION FOR HOLDUP MEASUREMENTS

4.1 Basic Counting Geometries There are three fundamental counting geometries that can be used to represent most collection zones. These geometries are distinguished by the spatial distribution of the source material and the resulting dependence of the detector counting rate on the source-to-detector distance, r.

4.1.1 Point Source If the material being assayed is distributed over an area with dimensions that are small compared with the source-

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 within the detector field of view, the zone can be treated as a point source. The detector count rate for a point source varies inversely as the square of the source-to-detector distance (count rate is proportional to l/r 2 ). Any equip ment measured at great distances or any small pieces of equipment or equipment parts fall in this category. 4

4.1.2 Line Source If the material being assayed is distributed along a linear path so that only a segment of that distribution length is contained in the detector field of view, the zone can be treated as a line source. The detector count rate for a line source varies inversely as the source-to-detector distance (count rate is proportional to I/r). Examples of this type of holdup geometry include isolated sections of piping and long, narrow ducts or columns.

4.1.3 Area Source If the material being assayed is spread over an area so large that it covers the full field of view of the detector for a range of source-to-detector distances, the zone can be assayed as an area source. As long as the material being viewed is uniformly distributed, the detector count rate will be independent of the source-to-detector distance.

However, for holdup applications, uniform material distribu tion is rare; so the source-to-detector distance can affect the instrument response and needs to be specified. Furthermore, when there are several measurement locations covering a large area (such as a floor), it is important to maintain the same source-to-detector distance (even if material distribu tion is uniform within a given measurement area) so that the number of measurement areas needed to cover the entire area remains constant. Examples of this type of assay geometry include floors, walls, glovebox floors, and large rectangular ducting.

4.2 Calibration of Detector Response

4.2.1 Mockup of Known Material Distributions When a gamma ray assay is used and a collimator setting has been selected, the detector responses for the three basic source distribution geometries listed above need to be determined. For the point source, the response is expressed as (counts per minute)/gram of 2 3Pu at a specified source to-detector distance. For the line source, the response is expressed as (counts per minute)/(gram of 2 3 9 pu per unit length) at a specified source-to-detector distance. For the area source1 the response is expressed as (counts per minute)/

(gram of

39pu per unit area) at a specified source-to 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.

Calculation of 2 4 °pu effective from the plutonium isotopic composition is described in the appendix to this guide.

Analogous expressions can be given for line and area

4Caution: small deposits of plutonium could exhibit very large gamma ray self-attenuation and could therefore require great care in analysis or could require neutron assay.

sources although neutron assay is usually restricted to dense, isolated items of equipment that can be represented as point sources. For both neutron and gamma measure ments, corrections to the point and line source calibrations for different detector distances are made using the 1/r2 or

1 /r count-rate dependence, respectively. For further detailed discussion of the measurement of detector responses for these basic geometries, see Reference 10.

For gamma ray assay, the calibration of the point source response can be accomplished with a well-characterized encapsulated standard plutonium foil smaller in size than the detector collimator opening. This foil can also serve as the check source for verification of the continued stability of the instrument settings in the field. It is important that care be taken in the preparation of this calibration standard to ensure that the amount of encapsulated 2 3 9 pu is well known. It is also important to measure the gamma ray attenuation through the encapsulating material and the self-attenuation of the plutonium foil and to correct the calibration standard response to compensate for these effects. Enough 2 3 9Pu needs to be encapsulated in this standard to provide count rates that will ensure good statistical precision of the calibration in a reasonable period of time.

For neutron assay, it is probably necessary to encapsulate a larger amount of material in the calibration standard because the spontaneous neutron production rate is signifi cantly less than the 375- to 450-keV gamma ray production rate. A quantity of 50 to 100 grams of plutonium is ade quate for most applications. Again, it is important to know the exact quantity and isotopic composition of the pluto nium. Also, the neutron calibration standard may generate more neutrons than directly attributable to the spontaneous fission and (a ,n) reactions. Because a relatively large quantity of PuO2 Ls encapsulated in the neutron assay calibration standard, some spontaneous fission or (a,n) neutrons may be absorbed in 239pu or 24 1pu nuclei, producing additional neutrons through the induced fission reaction. The amount of multiplication depends in a complex manner on the amount and distribution of PuO2 and on the surrounding medium (Ref. 11). For 50 grams distributed in the bottom of a 4-inch-diameter (10 cm) can, a self-multiplication of

0.5 percent of the total neutron output would be typical.

At 100 grams, 1 to 2 percent may be expected. Thus, this effect is typically smaller than other errors associated with holdup measurements and can be neglected if the standard contains 100 grams or less of well-distributed material. The chemical and isotopic composition of the plutonium will have a larger effect, as described in the appendix to this guide.

The measurement of the line source response is best accomplished by constructing a cylindrical surface distribu tion of plutonium with the aid of large foils. It is also possible to establish the line source response using a point source, as described in Reference 4. The line source geom etry is closest to that of the pipes and ducts likely to be encountered in actual measurements.

5.23-8

The area source response can be measured with the same plutonium foils laid flat to simulate the expected distribu tion on surfaces such as walls and floors. The area response can also be established using a point source. The point source is measured at different radial distances from the center of the field of view of the collimated detector. The response at each radial distance is weighted by the area of a concentric ring at that radius. From these weighted re sponses, it is then possible to calculate the area of a circular region of uniform plutonium deposition that would yield the same total response as the point source. From this equivalent area, the expected response/(gram of 2 3 9Pu per unit area) can be derived. Further useful details on this procedure may be found in Reference 12. For both line and area calibrations, the self-attenuation of the foils or point sources also needs to be taken into account.

There may be special material distribution geometries in the facility that are not readily represented by one of the three basic configurations described above. These special geometries may be mocked up as carefully as possible with large plutonium foils and point sources to produce a usable detector response calibration for these special cases. Examples of special cases might be concave or convex equipment surfaces or the internal volume of a rectangular cavity (see Ref. 10). Because material particle sizes (or material deposit thicknesses) have a significant effect on the self-attenuation of the gamma ray signals, it is important to use (whenever practical) well-characterized process material for preparing calibration standards and to duplicate to the extent possible process holdup distribution relative to particle size or thick ness. Furthermore, holdup in floors is often deposited at various depths into the floor, rather than on the surface.

Thus, calibration standards for such measurements need to incorporate the appropriate geometry and matrix effects.

Core samples of a floor may be needed to establish typical concentrations at various floor depths.

Calibration of the holdup measurement system using this procedure is recommended until a history of comparisons between predicted and recovered holdup quantities is developed. If it is possible to take holdup measurements before and after the cleanout of a piece of shut-down process equipment, they can be used to establish this comparison history and improve the accuracy of the calibration for each collection zone.

4.2.2 Measurement of Calibration Sources in Actual Process Equipment One method for calibrating detector response to holdup radiation in process equipment is to place a known calibra tion source in various positions in that equipment and record the detector responses. In this way, the overall detector response (including all corrections for attenuation and geometry) is determined empirically. Unfortunately, this procedure is impractical, if not impossible, in process equipment already in operation. However, if those respon sible for holdup assays are made aware of occasions when new equipment is brought into the plant for installation in the process, calibration sources can be conveniently placed in the equipment before its installation and the empirical measurements of the detector responses can be made. This procedure would be a valuable supplement to calibration data obtained from mockups of standard counting geom etries and comparisons with cleanout recovery data.

5. HOLDUP MEASUREMENTS AND STANDARD ERROR

The measurement of holdup in a complex plant environ ment can involve a very large number of measurements. In a stable plant environment where the process behavior is well known and well characterized, it may be possible to arrange the holdup measurement program so that:

a. Careful and extensive holdup measurements are made infrequently (e.g., annually) and b. At more frequent intervals (e.g., at inventory times),

careful measurements are made in known problem areas, and "spot check" measurements are made in the other, less used, zones where accumulations are known to be low.

Such management of measurement resources can result in a very effective holdup measurement program at minimum costs 5.1 Holdup Measurements In performing the holdup measurements, one must be aware of the large variability in holdup assays arising primarily from variability in the measurement conditions (e.g., background, geometry, gamma ray or neutron attenua tion, material distribution). Accordingly, it is important to perform the assays from as many vantage points as possible for each collection zone. If this is impractical on a routine basis because of time or space constraints, one might consider multiple measurements initially on a collec tion zone, followed by fewer routine measurements at repre sentative assay sites. Careful thought in the selection of measurement points and measurement strategy will mini mize ambiguities in the interpretation of the data.

5.1.1 Selection of Collection Zones and Detector Positions Location and configuration of collection zones are established on the basis of a detailed physical examination and a radiation survey of the physical layout of the facility.

Preliminary measurements are needed to determine the optimum detector positions for the holdup assays. If nonuniform distribution of material in a collection zone is suspected or if the process apparatus is sufficiently compli cated to require extensive attenuation corrections for certain counting geometries, multiple measurements are advisable for the collection zone. More than one detector position may be necessary. In the cases where radiation surveys have pointed out zones of high holdup collection, extra care will be necessary in the holdup measurements for those zones to minimize their contribution to the overall holdup variability. Where radiation surveys show little hold up, proportionately less time need be budgeted. Selecting optimum detector positions includes consideration of the

5.23-9

need to conveniently measure the line-of-sight background by moving the detector to one side without changing its orientation.

5.1.2 Holdup Measurement Procedure The measurement and analysis of gamma or neutron radiation from a collection zone may be carried out by treating the material distribution as a point, line, or area source, as described in Section B.4.1, or as one of the special cases that may have been measured, as mentioned in Section B.4.2. If the nature of the material distribution is uncertain for a particular detector position, a measurement of the detector counting-rate dependence on the source-to detector distance, r, may reveal the most appropriate counting-rate geometry with which to interpret thc data.

After the assay positions for the detector and shadow shields are established for each collection zone, permanent markings that indicate detector location (including height)

and orientation will ensure reproducibility of subsequent measurements for these positions. Uniquely labeling each assay site will facilitate unambiguous reference to each measurement and its location in the assay log. Furthermore, assay site labels and markings can indicate whether neutron or gamma ray measurements are to be made. Alphabetic labels (for example, "G" for gamma and "N" for neutron)

and color-coded tape markings of the sites would be useful.

Protecting the markings (for example, with clear epoxy)

will ensure their long-term durability.

After measuring the gamma or neutron radiation intensity at each detector position in a given collection zone, the line-of-sight background is measured by moving the detector and collimator to one side (still pointing in the same direction as during the assay) and measuring the radiation intensity from the surrounding materials. During the background measurement, the vessel in which the holdup is being measured must not be in the field of view of the detector.

Because uncertainties in geometry, attenua tion, or sample matrix will usually dominate the total response variability, the counting period need not be long.

Having 1000 to 10,000 net counts is generally sufficient for most holdup applications.

This procedure is repeated at all measurement posi tions and in all counting geometries designated for each collection zone. The final holdup value for the zone is obtained from the average of the individual measurements (each one being corrected for the effects of attenuation and any variation in geometry relative to the calibration measure ment).

Whenever possible, the collection zone is assayed in a variety of ways. For example, one could measure an appara tus up close and treat it as an area source; the measurement could then be repeated at a large distance, treating the zone as a point source. It may be better to measure some zones from several different directions-especially if complicated attenuation corrections are called for in some of the count ing geometries. Several independent measurements of one zone can provide an average holdup value that is better than the individual measurements.

Further, the variability between these measurements can provide an indication of the measurement uncertainty.

5.1.3 Gamma Ray Attenuation Corrections To obtain useful assay results by detecting 375- to

450-keV gamma rays, it is necessary to correct each assay for attenuation of the signal, either within the plutonium holdup material or by structural materials. Without this critical correction, the assay is no more than a lower limit on the true holdup value. The attenuation correction may be based on calculations of known attenuation in uniform materials, on earlier measurements of materials similar to those found in the plant equipment, or on direct measure ments of gamma ray transmission through the actual equipment. Details on establishing an appropriate attenua tion correction are given in Laboratory Exercise No. 4 of Reference 4. Additional treatment of gamma ray attenua tion corrections is given in Reference 13.

5.1.4 Gamma Ray Interferences Variability in the observed gamma ray response may arise as a result of the presence of extraneous gamma ray emitters or as a result of fluctuations in the background from the Compton scattering of higher energy gamma rays.

The magnitude of this effect is generally small. It can be monitored by observing the spectrum with a multichannel analyzer, but, unless data on periodically recovered holdup accumulations are in error, this contribution can be ignored.

5.1.5 Matrix Effects on Neutron Assay A change in the neutron yield for a plutonium sample of fixed isotopic content can be caused by a change in the concentration of high-(a,n)-yield impurities in the matrix.

If it is possible to estimate the range of permissible impurity concentrations, the variation in a typical neutron yield can be calculated using the method given in the appendix to this guide.

5.1.6 Effect of Isotopic Uncertainty Gamma ray measurements of plutonium holdup provide a direct determination of the fissile plutonium (i.e., 2 3 9Pu and 241 Pu) holdup in the zone under consideration. On the other hand, neutron techniques measure only the 240Pu effective content, and chemical techniques provide elemental analysis without consideration of the isotopic makeup. Thus, knowledge of the isotopic composition of the plutonium is necessary to correlate holdup measure ments with chemistry and accountability values. Gamma ray assays must be divided by the 2 3 9 pu isotopic fraction, and neutron assays must be converted from 2 4 0 Pu effective to total plutonium in order to express holdup in terms of total plutonium.

If the process equipment is thoroughly cleaned each time the isotopic composition is changed, the holdup may consist primarily of the current material. In that case, the declared isotopic composition can be use

d. When mixing

5.23-10

occurs, use of the stream-averaged isotopic composition is appropriate. Bounds on the isotopic composition are esti mated by considering the batches of highest and lowest

.omposition and computing the corresponding range. This measure of variability must then be incorporated into the estimated holdup standard deviation before making direct comparisons with the chemical analyses. The variability in isotopic composition can be expressed as an estimated stan dard deviation defined as one-half the observed range and then combined in quadrature with the standard deviation given by Equation 1 in Section B.5.2. In general, gamma ray measurements of 239 pu will be less sensitive to isotopic variations than neutron measurement of 2 4 0 Pu.

5.2 Assignment of Standard Error The assignment of a standard error to a holdup measure ment is extremely difficult on a rigid statistical basis. This is because the only statistically predictable fluctuations (e.g.,

counting statistics) in this application are frequently negli gible in comparison with variability due to counting geo metry (including material distribution), gamma ray attenua tion, gamma ray background and interferences, neutron matrix effects, and instrument instabilities. It is important to recognize that the variability can be large and guard against underestimating the standard deviation of the overall holdup value in a collection zone. Careful measure ments must be carried out during the calibration procedure to determine the range of detector responses resulting from variations in measurement parameters. A useful discussion of these ideas is presented in Reference 10.

A reasonable estimate of the standard deviation of the measured holdup for a given collection zone may be obtained by consideration of the range of holdup values obtained from the variety of measurements performed on that collection zone, as suggested in the previous section.

The mean value for the holdup is defined as the average of the various (corrected) measurement results on the collec tion zone. The standard deviation, a, for that mean value is estimated as one-half the range of holdup values obtained in the measurements. This estimate is conservative if a large number of measurements have been made. For a small number of measurements, the actual standard deviation can be larger than one-half the range. In such cases the actual standard deviation of the holdup values must be calculated.

In some cases, it may be unavoidable that the counting statistics are so poor that they contribute significantly to the measurement variability. In such an instance, the overall holdup standard deviation, CF(h-u)' is defined as the square root of the sum of the squares of the standard deviation due to counting, O(stat), and the standard deviation due to measurement fluctuations, O(meas); that is,

0 (h-u) =

(stat) + a(meas)

(1)

5.3 Estimation of Bias When a single collection zone is cleared out, it is desirable to perform a holdup assay before, H

1 before, and after, H fter, the cleanout if possible. By comparing the amount oplutonium removed, Pur, to the recovery amount pre dicted through the in situ holdup assays, Pua, the collection zone calibration can be updated, and the calibration and assay standard deviations can be based on relevant data.

The amount of plutonium recovered, Pur, during the cleanout of a specific collection zone can be assayed through sampling and chemical analysis, through calorim etry, or through other applicable nondestructive assay methods (e.g., spontaneous fission coincidence detection or gamma ray assay).

The assay value for the recovered amount is computed as the difference in the holdup assays before and after the cleanout:

Pua = Hbefore - Hafter The percent difference, A, between the assay and recovery values for the plutonium holdup is then computed:

A = 100 (Pua - PUr)/Pur A running tabulation of the quantities Pua, Pur, and A (as well as their standard deviations, oa, ar, and OA) is kept in the assay log for each collection zone.

The average value, A, of the percent differences between Pu a and Pur will serve as an estimate of the bias in the holdup assay for that collection zone and will also provide quantita tive justification for revision of the assay calibration for that zone to remove the bias. The root-mean-square deviations, aA, of the percent differences, Ai from their mean value, A, serve as a check on the appropriateness of the size of the estimated standard deviation of the holdup measurements.

To the extent that the standard deviation of Pur is small compared with the uncertainty in Pua (usually an adequate assumption), the quantity sA should be comparable in size to the standard deviation of Pua. For K measurements of the percent differences, Ai, for a given collection zone, the quantity sA is given by:

FK

-E2

1/2 s

E (Ai

)2/(K - 1 A [i

1I

(4)

Equation 4 assumes that all the

0A's are equal. For a calculation of sA using weighted sums, see Reference 14.

Note that, if the holdup measurements (ie., Hbefore or Hafter) contain a constant bias, their difference can still provide useful information in the comparison with Pur.

However, a small difference between Pua and Pur does not necessarily mean that the bias associated with H is small.

5.23-11

(2)

(3)

This ambiguity is reduced in importance if the cleanout is such that Hafter is much smaller than Hbefore. In addition, the use of several holdup measurements from varying van tage points, as suggested earlier, will help to minimize the bias associated with incorrect geometrical or attenuation corrections in one measurement configuration.

C. REGULATORY POSITION

To develop a program for the periodic in situ assay of plutonium residual holdup as a method acceptable to the NRC staff for measuring this inventory component, it is necessary to consider the factors in the following sections.

Care must be exercised during the fabrication and use of check sources and calibration standards to ensure their continued integrity and to prevent contamination. In addition, the usual precautions for safeguarding plutonium should be taken.

1. DELINEATION OF COLLECTION ZONES AND

ASSAY SITES

Preliminary radiation survey measurements of the plutonium processing facility should be used to budget the measurement time to emphasize high-holdup areas, to establish independent collection zones, and to determine detector positions within the zones.

1. At each collection zone, detector positions (assay sites) should be chosen so that the material holdup can be measured from several vantage points around the zone. At each assay site, the detector should have an exclusive view of the collection zone being assayed. If necessary, shadow shielding should be used to isolate the region being assayed from other collection zones. Detector positions should be chosen to minimize the measurement ambiguities, as described in Section B.5.1.1.

2. Each assay site should be permanently marked with paint or colored tape on the floor to ensure reproducible assay positions. The markings should be protected (for example, with clear epoxy) to ensure their long-term durability. Detector height and orientation should be clearly indicated in the assay log for each measurement site and, if possible, included in the site markings.

3. Each assay site should be uniquely labeled to facilitate unambiguous reference to that site in the assay log. A

labeling and color-coding convention should be established to distinguish neutron assay sites from gamma ray assay sites.

4. Gamma ray assay should be used for collection zones containing less plutonium than the neutron detection limit.

Also, gamma ray assay should be used for all structures that do not contain irregularly shaped components capable of significantly attenuating the emerging gamma rays. Neutron assay should be used for all structures not suitable for gamma ray assay. There may be some large structures such as furnaces that can be measured only with small interior probes or with thermoluminescent dosimeters.

5. Areas may be denoted as problem areas so that careful holdup measurements will be made in these areas each time plant holdup is to be determined; or the area may be labeled as a spot-check zone, where accumulations are \\

known to be low and careful holdup assays are needed less frequently.

2. ASSAY INSTRUMENTS

Neutron and gamma ray assay capability can be provided, if desired, using separate or compatible electronics with interchangeable detector probes. Compatible electronics can provide for both 3He or BF 3 neutron detection and NaI(T1) gamma ray detection. The electronics unit should have a temperature coefficient of less than 0.1 percent per degree centigrade. Battery-powered electronics can expedite assays.

2.1 Gamma Ray Assay Gamma ray assay should be based on the activity observed in the energy range from 375 to 450 keV, excluding the composite gamma ray complex centered at 333 keV. Yield data for appropriate gamma rays are presented in Sec tion B. 2.1 of this guide.

2.1.1 Detector Selection Gamma ray detectors for holdup measurements should have FWHM (full width at half maximum) resolution better than 10 percent at 662 keV (1 3 7 Cs gamma ray). NaI(Tl)

detectors can exhibit resolutions as good as 7 percent and are suitable for this application. The crystal depth should be sufficient to detect a significant percentage of 400-keV

gamma rays. For NaI(T1), the minimum depth should be

1 inch (2.5 cm); a 2-inch (5-cm) depth is recommended.

The crystal should be stabilized with a suitable radioac tive source. An internal seed containing 241Am is recom mended for this application. The electronics should be capable of stabilizing on the reference radiation emitted by the seed. The crystal face (external to the cover) should be covered with 0.75 mm of cadmium and 1.5 mm of lead to filter low-energy radiations.

Two single-channel analyzers should be provided with lock-set energy windows. One channel should be set to admit gamma rays from 375 to 450 keV. Unless equilibrium of the 2 3 7 U and 24 1 pu can be ensured, the 333-keV region of the gamma ray spectrum should be completely excluded.

The second channel should be set above the first window to provide a background subtraction for the assay window.

This second window should be set from approximately 475 to 575 keV. The width and position of this window is a matter of personal preference in how the background subtraction should be done. These analyzers should be packaged as one integral unit.

2.1.2 Gamma Ray Collimator A cylinder of shielding material such as lead should be made concentric with the gamma ray detecto

r. The end of

5.23-12

the cylinder opposite the crystal should be blocked with the shielding material. The thickness of the collimator should be chosen to provide sufficient directionality for the specific facility (1.5 cm of lead thickness should be sufficient for most applications). The collimator sleeve should be fixed over the end of the detector crystal at a reproducible setting identical to that used in the calibration measure ments.

2.1.3 Gamma Ray Calibration and Check Sources Standard sources of 2 3 9 pu should be provided for calibration of the measurement system for the basic measure ment geometries described in Section B.4. A small encapsu lated plutonium sample can be used both as a calibration standard for the point source counting geometry and as a check source for verification of instrument stability. For the line and area calibrations, large plutonium foils can be used, or the calibrations can be derived from a series of measurements made with the point source. The gamma ray self-attenuation correction should be clearly specified for all foils and samples.

2.2 Neutron Assay

2.2.1 Neutron Detector Selection Neutron detectors should have high detection efficiency and be capable of operating in the presence of gamma radiation. BF 3 and 3He neutron detectors are recommended for this application. Neutron detectors should be surrounded by a layer of neutron moderator material to enhance their detection efficiency. The neutron moderator layer should be covered with a low-energy neutron absorber to filter out extraneous neutrons from the desired signal.

2.2.2 Neutron Collimator A slab collimator or concentric cylinder collimator of polyethylene should completely surround the detector, leaving open only a detection channel in one directio

n. The

.moderator thickness should be selected to provide the directionality required for each facility. A directionality profile providing a 10:1 response ratio is desirable. However, for portable detectors a 3:1 ratio may be used.

2.2.3 Neutron Calibration and Check Source A 50- to 100-gram sample of plutonium should be adequate both as a point source calibration standard and as a check source. The isotopic composition, 241Am content, and high-(c,n)-yield impurity composition should be representative of the plutonium being processed. The neutron yield of the standard should be independently measured, if possible, and also computed using the method described in the appendix of this guide. If the measured and calculated yields differ by more than 20 percent, any future yield calculations should be normalized to be consistent with this measurement.

2.3 Service Cart A cart carrying electronics and both detector probes should be provided. The capability to raise or lower the probes to reproducible settings should be included.

3. CALIBRATION

3.1 Instrument Check The stability of the neutron and gamma ray detection systems should be tested prior to each inventory by compar ing the observed counts obtained from the check source, minus the counts with the shaped shield in place but with out the check source, to the readings obtained prior to previous inventories. If the measurement is consistent with previous data (i.e., is within + 2 single-measurement standard deviations of the mean value of previous data), all previously established calibrations using this detection system should be considered valid. If the measurement is not consistent, the operation of the unit should be checked against the manufacturer's recommendations and repaired or recali brated, as required. These check source measurements should be supplemented with regular remeasurements of instrument calibrations to ensure continued proper instru ment performance over the entire operating range.

3.2 System Response Calibration The response of the detection system should be deter mined with well-known quantities of plutonium in the basic measurement geometries described in Section B.4. If there are special counting geometries in the facility that are not readily represented by one of the basic configurations, these geometries should also be mocked up and measured during the calibration procedure.

4. ASSAY PROCEDURES

4.1 Assay Log An assay log should be maintained. Each collection zone should have a separate section in the assay log, with the corresponding calibration derived on the page facing the assay data sheet. Recording space should be provided for the date of measurement, gross counts, corrected counts, and the corresponding grams of plutonium from the calibration in addition to position and instrument electronic setting verification. There should also be provision for recording data from recovery operations and holdup assay comparisons, as described in Section B.5.3.

4.2 Preassay Procedures Prior to inventory, the isotopic composition of the plutonium processed during the current operational period should be determined. Variations in the neutron and gamma ray yield data from the calibration standard should be calculated. Either the calibration data or the predicted holdup should then be corrected to reflect this difference.

5.23-13

Prior to each inventory, the operation of the neutron and gamma ray assay detection systems should be checked, as described in Regulatory Position 3.1.

Prior to any assay measurements, feed into the process line should be stopped. All in-process material should be processed through to forms amenable to accurate account ability. All process, scrap, and waste items containing plutonium should be removed to approved storage areas to minimize background radiations.

4.3 Measurements 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 tions are the highest (and therefore where the most careful measurements should be made). In zones where accumula tions are shown to be very low by the survey, spot-check measurements may be adequate, as pointed out earlier.

Before assaying each collection zone, the operator should verify the floor location, probe height, and probe orientation. The electronic settings should be verified every

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 of the collection zones, the check source should be removed or shielded so as not to interfere with the measurement.

Prior to taking a measurement, a visual check of the zone and the line of sight of the detector probe should be made to ensure that no obvious changes have been made to the process area and that no unintended accumulations of plutonium remain within the collection zone. The operator should initial the measurement log to ensure compliance for each collection zone.

When the preceding steps have been completed, the measurement at each collection zone should be taken, recorded, and converted to grams of plutonium. If each value is within an expected or permissible range, the assayist can proceed to the next collection zone. However, if the collection zone contains an unexpectedly large amount of plutonium, it should be cleaned to remove the accumulation for conversion to a more accurately accountable material category. After the cleanout has been completed, the zone should be reassayed.

5. ESTIMATION OF HOLDUP ERROR

During the initial implementation of the holdup measure ment program, the holdup uncertainty for each collection zone should be estimated from the range of values obtained in the various measurements on that zone, as described in Section B.5.2. As a history of comparisons between holdup measurements and cleanout recovery data becomes avail able, these data should be used to adjust for bias and to revise the magnitudes of the holdup uncertainties, as described in Section B.5.3.

During each physical inventory, the calibration in at least 10 percent of the collection zones should be updated on the basis of the comparison between holdup and cleanout recovery measurements. In any case, all calibrations should be updated at least once per year.

5.23-14 t

I

REFERENCES

R. Gunnink et al., "A Re-evaluation of the Gamma Ray Energies and Absolute Branching Intensities of U-237, Pu-238 , -239,-240, -241, and Am-241:' Lawrence Livermore Laboratory, UCRL-52139, 1976.

2.

J. E. Cline, R. J. Gehrke, and L. D. Mclsaac, "Gamma Rays Emitted by the Fissionable Nuclides and Asso ciated Isotopes," Aerojet Nuclear Co., Idaho Falls, Idaho, ANCR-1069, July 1972.

3.

L. A. Kull, "Catalogue of Nuclear Material Safeguards Instruments," Battelle National Laboratories, BNL

17165, August 1972.

4.

R. H. Augustson and T. D. Reilly, "Fundamentals of Passive Nondestructive -Assay of Fissionable Material,"

Los Alamos Scientific Laboratory, LA-5651-M, 1974;

also T. D. Reilly et al., "Fundamentals of Passive Nondestructive Assay of Fissionable Material: Labora tory Workbook," Los Alamos Scientific Laboratory, LA-5651-M, Suppl., 1975.

5.

R. B. Walton et al., "Measurements of UF Cylinders with Portable Instruments," Nuclear Technology, Vol. 21, p. 133, 1974.

6.

C. H. Kindle, "In Situ Measurement of Residual Pluto nium," Nuclear Materials Management, Vol. 5, No. 3, p. 540, 1976.

7.

J. W. Tape, D. A. Close, and R. B. Walton, "Total Room Holdup of Plutonium Measured with a Large-Area Neutron Detector," Nuclear Materials Management, Vol. 5, No. 3, p. 533, 1976.

8.

H. E. Preston and W. J. Symons, "The Determination of Residual Plutonium Masses in Gloveboxes by Remote Measurements Using Solid Thermoluminescent Dosimeters," United Kingdom Atomic Energy Author ity, Winfrith, England, AEEW-R13 59, 1980.

9.

A. Ohno and S. Matsuura, "Measurement of the Gamma Dose Rate Distribution in a Spent Fuel Assembly with a Thermoluminescent Detector,"

Nuclear Technology, Vol. 47, p. 485, 1980.

10.

W. D. Reed, Jr., J. P. Andrews, and H. C. Keller, "A

Method for Surveying for 2 3 sU with Limit of Error Analysis," Nclear aterials Management, Vol. 2, p. 395 ,

1973.

11.

N. Ensslin, J. Stewart, and J. Sapir, "Self-Multiplication Correction Factors for Neutron Coincidence Counting,"

Nuclear Materials Management, Vol. VIII, No. 2, p. 60,

1979.

12.

M. S. Zucker et al, "Holdup Measurements for Nuclear Fuel Manufacturing Plants," Nuclear Materials Manage ment, Vol. X, p. 239, 1981.

13.

J. L. Parker and T. D. Reilly, "Bulk Sample Self Attenuation Correction by Transmission Measure ment," Proceedings of the ERDA X- and Gamma-Ray Symposium, Ann Arbor, Michigan (Conf. 760639),

p. 219, May 1976.

14.

P. R. Bevington, Data Reduction and Error Analysis for the Physical Sciences, McGraw-Hill, 1969.

5.23-15

APPENDIX

A. NEUTRON YIELD COMPUTATIONS

The following model for the calculation of the total spontaneous neutron yield from plutonium-bearing materials assumes that the plutonium is widely dispersed. With this condition, there will be no significant neutron production through induced fission of

39pu or 2 4 1 Pu. The total neutron yield of plutonium holdup will then be the sum of the spontaneous fission and (ct,n) contributions:

Yn= YSF + Y(a,n)

(1)

1. SPONTANEOUS FISSION NEUTRONS

To determine the spontaneous neutron yield of pluto nium, the isotopic composition must be known. (The contribution from 23SU spontaneous fission is usually negligible even if uranium is present in large quantities.)

The yield from the plutonium isotopes is given by:

YSFp M2 3 8Q2 38 + M2 4 0 Q2 4 0 + M2 4 2 Q2 4 2

(2)

where Mi is the total mass of the ith plutonium isotope, and Qi is the spontaneous fission neutron yield per gram of the ith isotope. Using the yield data from Table A-l, Equation 2 can be rewritten as:

YSF = (1030 n/sec-gram)M 24 0 (effective)

(3)

where M2 4 0 (effective) = 2.50M2 38 + M240 + 1.70M2 4 2

(4)

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

of effective 2 4 0 pu mass reflects the fact that most of the spontaneous fission yield is due to that isotope.

2. (cz,n) NEUTRONS

When the plutonium holdup is in the form of oxide, the major contribution from (ct,n) reactions will be due to the

0-18(a,n) 21Ne reaction. The additional neutron yield is typically 50 to 100 percent of the spontaneous fission yield. The (a,n) yield can be calculated from the yields per gram of each isotope of Pu(Yi) given in Table A-1:

Y(ay,n) oxide = 1 MiYi

1y

(5)

The summation over Mi should also include 24 1 Am, which is a strong alpha emitter.

In addition to (c4n) production in the oxide itself, certain low-Z impurities in the oxide can contribute substan tially. Values for the yields of neutrons obtained in bombard ing thick targets of these elements with 5.2-MeV alpha particles are given in Table A-2. Further research may change these values somewhat, but they are sufficient for computing the approximate effect of these elements if they exist as impurities in PuO . One method for doing this is to compute the impurity (x,n) yield relative to the oxide (ct,n) yield:

Y(an)impurity - Y(cn)oxide f (WiAoI )/(Pok io)

(6)

ble A-1 ALPHA PARTICLE AND SPONTANEOUS FISSION NEUTRON YIELDS

Half-Life (yr)

87.78

24,150

6,529

14.35**

379,000

433.8

2.47 x 105

7.1 x 108

4.51 x 109 Alpha Activity (a/sec-gram)

6.33 x 1011

2.30 x 109

8.43 x 109

9.39 x 107

1.44 x 108

1.27 x 10"1

2.29 x 108

7.93 x 104

1.23 x 104 Qi Spontaneous Fission (n/sec-gram)

2.57 x 103

2.22 x 10-2

1.03 x 103

5.00 x 10-2

1.75 x 103

6.05 x 10f'

5.67 x 10-3

5.96 x 10-4

1.12 x 10-2

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

5.23-16 Nuclide

238pu,

239pu

2 4 0 pu

2 4 1 pu

2 4 2 pu

2 4 1 Am

234U

235U

238 U

Y.

PuO2 U0 2 (a,n) Yield*

(n/sec-gram)

1.4 x 104

42.5

157

1.3

2.2

2957

4.65

1.37 x 10- 3

1.93 x 10-4

Table A-2 (a,n) NEUTRON YIELDS FOR

VARIOUS LIGHT ELEMENTS

P.

Neutron Yield Element per 106 Alphas Be

58 B

18 F

6.4

7 Li

1.3 Na

-1.5 NatMg

0.89 Al

0.44 Natsi

0.077 C

0.05 O

0.050

  • Most of these yields are based on:

J. K. Baki and J. Gomez del Campo,

"Neutron Yields from Alpha-Particle Bombardment,"

Nuclear Science and Engineering, Vol. 71, p.

18,

1979.

In Equation 6, P. is the (a,n) neutron yield in the impurity element, and P0 is the yield in oxygen (0.050 neutrons/106 alphas); A. is the atomic weight of the impurity element, and Ao that for oxygen (16); Ij is the concentration of the 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, it can be converted to ppm of plutonium oxide by multi plying by the gravimetric dilution factor, 0.882.

To summarize the calculation of (ct,n) neutron yields in oxide that also contains impurities, Y(a,n) from all sources is given by:

Y(

Z,n)

MiYf + .0027EP_-I-/A-)

(7)

i

"

I

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 summation over i must include 241 Am and that the summa tion over j includes only the oxygen that is not bound up as plutonium oxide.

3. SAMPLE CALCULATION FOR PuO 2 -UO2 Consider the case of I gram of recycle plutonium blended to 3 percent by weight of PuO 2 in a UO 2 matrix where the isotopic composition is as given in Table A-3. For mixed oxides, the oxygen density is approximately the same as in PuO2 alone. Also, plutonium and uranium have similar atomic numbers. For these reasons, it may be assumed that the oxygen (ca,n) yield in mixed oxide is the yield in PuO 2, further reduced by the blending ratio, PuO2 /(PuO 2 + U0 2 ).

SAMPLE CALCULATION FOR 1 GRAM OF PLUTONIUM

Isotopic Nuclide Composition YSF

(n/sec)

Y(a, n)o ide (n/sec)I

23 SPU

0.003

8

42

2 3 pt

0.756

0

32

24 PU

0.185

191

29 S1 Pu

0.045

0

0

242

0.011

19

0

241 Am

0.003

0

9 Total Yields

218

112 Using the isotopic composition given in Table A-3 and using Equation 3, the spontaneous fission neutron yield can be found to be 218 n/sec for 1 gram of plutonium. Then the neutron production in the oxide can be calculated using the masses Mi of each isotope and the yields Y- from 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 particle yield of ylutonium is nearly constant in time, but that, because 24 Am builds up in time, the total alpha production increases at a rate of roughly 0.3 percent per month in typical reactor fuel The impurity (ct,n) yields are calculated in Table A-4.

The calculation is based on impurities in PuO 2 only. The mixed oxides are assumed to consist of blended PuO 2 and UO 2 particles approximately 40 pm in diameter where most alpha particles stop within the PuO2 particles. If the particle size were smaller or the mixed oxide were created through coprecipitation, the uranium impurity content would also contribute to the plutonium (a,n) yield. In the present example, it is sufficient to use the neutron yields P.

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

The total neutron yield from 1 gram of plutonium in PuO 2 is then 218 + 112 + 47 = 377 n/sec. Using the gravi metric dilution factor of 0.882, this is 333 n/sec for 1 gram of PuO 2 . If the PuO 2 is blended so that PuO2 / (PuO 2 +

UO ) = 0.03, the neutron yield from 1 gram of mixed oxide is 10 n/sec.

The impurity (ct,n) yields, Pi, used in this example are currently known to about 10 percent accuracy for most elements and 50 percent accuracy for the others. The oxide (atn) yields, Y.., are known to 10 percent or bette

r. Both

.J

yield calculations must assume perfect mixing, however.

For these reasons, neutron yield calculations are accurate to

10 percent at best, and the neutron holdup measurement calibration should be based on representative standards rather than calculation wherever possible.

5.23-17 Table A-3

Table A-4 IMPURITY (ca,n) YIELD

Arbitrary Concentration I

Impurity (cn)

in PuO2 Yield (I12 n/sec)

Impurity (ppm by wgt)

(0.00 2 7 )Pjlj/Aj Li

9

1 Be

8

16 B

10

5 C

200

0

F

125

13

0 (moisture)

4600

4 Na

120

8 Total

47 (n/sec)

B. CONVERSION OF MEASURED M

D4(EFFECTIVE)

TO TOTAL PLUTONId"

To convert a measured effective 2 4 0 pu mass to actual total plutonium, one must use both the relationship between these two quantities, as shown in Equation 4, and the known isotopic composition of the samples being measured.

Let f238, f239' f240' f241, and f2 4 2 represent the weight fractions of the respective plutonium isotopes in the unknown sampl

e. The

2 4 0 pu effective weight fraction, f2 4 0(effective), can be defined as:

f2 4 0 (effective) = M2 4 o(effective)/Mpu(total)

where f2 4 0 (effective) = 2.50f 2 39 + f24 0 + 1.70f 2 4 2

(8)

(9)

Generally, as previously mentioned in this guide, the relative measurement uncertainty of M2 4 0 (effective) in a holdup measurement will be much larger than that of f;40(effective), so the relative error in Mpu(total) is essen 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:

f24 0 (effective) = 2.50(0.003) + 0.185 + 1.70(0.011)

= 0.21 Thus, a holdup measurement of 35 + 10 grams 24°pu effective corresponds to 166 + 47 grams total plutonium, where the relative error in the total plutonium result was taken to be equal to that of the M2 4 0 (effective) result.

5.23-18

VALUE/IMPACT STATEMENT

1. PROPOSED ACTION

1.1 Description No adverse impact on the public can be foreseen.

Licensees authorized to possess at any time more than

1 kilogram of plutonium are required by Part 70, "Domestic Licensing of Special Nuclear Material," of Title 10 of the Code of Federal Regulations to calculate a material balance based on a measured physical inventory at intervals not to exceed 2 months. Further, these licensees are required to conduct their nuclear material physical inventories in compliance with specific requirements set forth in Part 70.

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

Plutonium residual holdup is defined as the plutonium inventory component remaining in and about process equipment and handling areas after those collection areas have been prepared for inventory. This 'regulatory guide describes procedures acceptable to the NRC staff for the in situ assay of the residual plutonium holdup.

1.2 Need for Proposed Action Regulatory Guide 5.23 was published in 1974. The proposed action, a revision to this guide, is needed to bring the guide up to date with respect to advances in measure ment methods, as well as changes in terminology.

1.3 Value/Impact of Proposed Action

1.3.1 NRC Operations The regulatory positions will be brought up to date.

1.3.2 Other Government Agencies Not applicable.

1.3.3 Industry Since industry is already applying the methods and procedures discussed in the guide, updating these methods and procedures should have no adverse impact.

1.4 Decision on Proposed Action The regulatory guide should be revised to reflect improve ments in measurement techniques and to bring the language of the guide into conformity with current usage.

2. TECHNICAL APPROACH

Not applicable.

3. PROCEDURAL APPROACH

Of the procedural alternatives considered, revision of the existing regulatory guide was selected as the most advanta geous and cost effective.

4. STATUTORY CONSIDERATIONS

4.1 NRC Authority The authority for the proposed action is derived from the Atomic Energy Act of 1954, as amended, and the Energy Reorganization Act of 1974, as amended, and is implemented through the Commission's regulations, in particular 10 CFR Part 70.

4.2 Need for NEPA Assessment The proposed action is not a major action that may significantly affect the quality of the human environment and does not require an environmental impact statement.

S. RELATIONSHIP TO OTHER EXISTING OR PROPOSED

REGULATIONS OR POLICIES

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

6. SUMMARY AND CONCLUSIONS

Regulatory Guide 5.23 should be revised.

5.23-19

1.3.4 Public

UNITED STATES

NUCLEAR REGULATORY COMMISSION

WASHINGTON, D.C. 20555 FIRST CLASS MAIL

POSTAGE & FEES PAID

USNAC

WASH 0 C

PERMII No SiIL

OFFICIAL BUSINESS

PENALTY FOR PRIVATE USE, $300


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