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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
February 1984 U.S. NUCLEAR REGULATORY COMMISSION
Y G U I D[E'DIRECTORATE
REGULATORY GUIDE
OF REGULATORY  
OFFICE OF NUCLEAR REGULATORY RESEARCH
STANDARDS REGULATORY  
REGULATORY GUIDE 5.23 (Task SG 0454)
GUIDE 5.23 IN SITU ASSAY OF PLUTONIUM  
IN SITU ASSAY OF PLUTONIUM RESIDUAL HOLDUP
RESIDUAL HOLDUP  


==A. INTRODUCTION==
==A. INTRODUCTION==
Part 70, "Special Nuclear Material," of Title 10 of the Code of Federal Regulations requires licensees authorized to possess more than one kilogram of plutornium to calculate a material balance based on a measured physical inventory at intervals not to exceed two 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 Regulatory staff are detailed in Regulatory Guide 5.13, "Conduct of Nuclear Material Physical Inventories." Plutonium residual holdup is defined as theplutonium inventory component remaining in and about process equipment and handling areas after those collection areas have been prepared for inventory.
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.


Whenever possible, process equipment should be designed*
Inventory procedures acceptable to the NRC staff are detailed in Regulatory Guide 5.13, "Conduct of Nuclear Material Physical Inventories."
and operated so as to minimize the amount of holdup. In this guide, procedures are detailed for the in situ assay of the residual plutonium holdup.Assay information can be used in one of two ways: I. When the limit of error of plutonium holdup is compatible with constraints on the overall limit of error on the facility MUF (LEMUF), the material balance can be computed using the measured contents of Pu holdup.Additional cleanout and recovery for accountability will then not be necessary."Design features to minimize holdup in process equipment are the subject of a seriý of rgulatory guides.2. When the limit of error of Pu holdup is not compatible with constraints on the overall LEMUF, the information obtained in the holdup survey can be used to locate principal Pu accumulations and to assure that other areas of the process contain less than the detectable amount of plutonium.
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.


Once located, substantial accu-mulations can be recovered, transforming the plutonium to a more accurately measurable inventory component.
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.


Having reduced the amount of plutonium holdup, the limit of error on the remeasurement of the remaining holdup may be sufficiently reduced to be compatible with overall LEMUF requirements.
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 accumulates in cracks, pores, and zones of poor circulation within process equipment.
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.


The walls of process vessels and associated plumbing often become coated with plutonium during solution processing.
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.


Surfaces internal and adjacent to process equipment, especially glove box walls and floors, accumulate deposits of plutonium which can become appreciable.
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.


Plutonium also accumulates in air filters and associated ductwork.
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.


The absolute amounts of plutonium holdup must be small for efficient processing and proper hazards control. However, the total amount of plutonium holdup may be significant in the context of the tolerable facility MUF.The measurement procedures detailed in this guide are based on the controlled observation of gamma rays and neutrons which are spontaneously emitted by the plutonium isotopes.
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.


Because the gamma rays of interest are emitted by Pu-239, garnma ray assay is the preferred USAEý REGULATORY
3.1 Detector Positioning An unobstructed side view of a collection zone is pre
GUIDES Regulatory Guides we issued to describe and make avaiille to the public methods acceptable to the AEC Regulatory staff of implementing specific parts of the Commission's regulations, to delineate techniques
+/-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.
.-.ed by the staff in evaluating specific problems or postulated accidents:
or to provlde guidance to epplicents.


Regulatory Guides we not substitutes for regulations arnd comoliancs with them is not required.
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.


Methods and solutions different from those sit out in the guides will be acceptable if they provide a basls for the findings requisot to the issuanc or continuance of a pearmil or licemni by the Comnission.
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.


* Published guidet will be revised periodically, as appropriate, to accommodatei comments end to reflict new information or experience.
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


Copies of published guides may be obtained by rsquast indicating the divisions dosircd to the US. Atomic Enrgty Commission, Washington, D.C. 2054'.Attention:
COLLIMATOR
Director of Regulatory Standards.
TOP VIEW
-CHECK
SOURCE
COVER
CHECK SOURCE
FRONT VIEW
Figure 2. Neutron Collimator Channel Plug and Check Source
5.23-7


Comments and suggestions for inmprovements in thes guides ere encouraged and should be sent to the Secretary of the Commission, U.S. Atomic Energy Commission.
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.


Washington.
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.


D.C. 20645.Attention:
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.
Chief. Public Promedinga Staff.The guidas ea issued in the following ton broad divisions:
1. Power eactors 2. Resmrch and Test Reactors 3. Fuels and Materials Facilities
4. Envwonnmental and Siting S. Materials and Plant Protection S. Produects 7. Transportation S. Occupational l'slooh 9. Antitrust Revow 10. General assay method whenever its acceptance criteria are satisfied.


To accomplish either gamma ray, or neutron assay, it is essential to consider the facility in terms of a series of zones which can be independently assayed.Such zones are designated as "collection zones." 1. Delineation of Collection Zones Typical plutonium process facilities comprise a number of interconnected glove boxes which contain work areas and most process equipment, in-process storage areas, and self-contained process equipment.
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.


Also, solution processing requires tanks, plumbing, and pumping equipment, which are often located in close proximity to.the glove box lines. Finally, storage areas for feed, scrap and waste, and final product are also often located in close proximity to the plutonium process area.Each facility can be divided into a series of collection zones on the basis of a logical understanding of process activities.
Calculation of 2 4 °pu effective from the plutonium isotopic composition is described in the appendix to this guide.


Individual glove boxes can be subzoned to improve assay performance, but for most applications, individual glove boxes are -examples of suitable size areas for discrete collection zones.Gamma ray assay for plutonium holdup measurement is practical when a collection zone consists of a single structure of relatively uniform cross section.When a collection zone contains an item of equipment having significant shielding properties and capable of contributing to the holdup, the uncertainty in the holdup prediction based on the observed response may become primarily due to attenuating the radiations in the internal structure.
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.


In such cases, neutron assay is applicable.
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.


2. Applicable Methods and Instruments Two ,considerations are critical to the selection of methods and instruments.
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.


First, to perform an assay, the plutonium radiations must reach the detector, and be detected.
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.


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 neighbor zones and from the background.
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.


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', 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 one or'two~isolopes o-Thinterest.
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.


Gamma ray assay has an ' advantage , over neutron assay in that the emissions are primarily from the principal isotopes qf linterest. -Because of the high emission rate of gammna rays, a detection sensitivity of less than one gram is generally attainable..
5.23-8
The most useful portion of the spec trum for holdup assay is the Pu-239 gamma ray complex in the 375-440 keV range. The-yields of these lines are given in Table B.l.Table B.1 PROMINENT
GAMMA',RAYS
FROM Pu-239 in ENERGY RANGE 375-440 keV Energy Intensity
(- /sec-g Pu-239)375.0 ........................
3.59 x J04.l 380.2 ......................
0.70 x 10 382.7 .......................
0.59 x 104 392 ..5 ................
...... 0.26 x,10 4 393.1 ..........
.... ..... 1.01 x10 4.413.7 ......................
3.43 x I04 422.6 ...............
..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 detector sensitive to gamma rays and .appropriate
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.
-.electronics.


3 Required electronics include lat least a single-ýchannel analyzer and a timer-scaler unit.- A second :single- channel analyzer used to determine the background radiation correction is a time-saving feature. Battery powered systems are commercially.
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.


available and can provide operational convenience, particularly in this application.
Thus, calibration standards for such measurements need to incorporate the appropriate geometry and matrix effects.


The detection efficiency and res6lution of good Nal(Tl) detectors is'generally adequate for this application.
Core samples of a floor may be needed to establish typical concentrations at various floor depths.


CdTe, Ge(L), and-intrinsic
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.
'Ge:detectors have better resolution than Nal(TI) but: cost more, are generally less available, and are more difficult to operate.-' -The 332.3 keV- gamma-ray from U-237, a short-lived
(6.75 d) daughter -of Pu-241, is usually the principal interference for. Pu-239 assay by Nal detection of the 375-440 keV complex. If the U-237 is in equilibrium with Pu-241, the intensity of this gamma ray is 1.15 x 106 7t/sec-g Pu 1 24l.Since this gamma ray is also emitted inthe decay of Am-241., 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 440 keV.5.23-2 Detector dimensions are selected to provide a high probability for detecting the appropriate gamma rays. The geometric detection efficiency increases as the square of the detector radius; however, the weight of the gamma ray shielding material required to collimate the detector also increases
;when larger detectors are used. The crystal depth is chosen such that most of the gamma rays of interest will lose all their energy within the crystal;To reduce the pile-up of low energy radiations, the crystal face can be covered with an appropriate shield (e.g., 0.075 cm cadmium).
This procedure will reduce counter dead time effects without significantly affecting assay results.2.1.2 Collimators for Gamma Rays A shaped shield constructed of any dense material is appropriate for gamma ray collimation.


For cost, availability, and ease of fabrication, lead is recommended.
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.


Less ,than 2% of all 400 keV gamma rays striking a 1.5-cm-thick sheet of lead will pass through without having suffered an energy loss.The collimator will be most effective when it is concentric about the crystal and photomultiplier and completely 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.
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.


4 Making this distance variable to reproducible settings will permit adjustment over a range of collection zone sizes.2.1.3 Check Source for Gamma Ray Assay It is important to check the operation of the detection system prior to each inventory sequence.Either recalibrating one or more collection zones and comparing the results'to previous analyses or testing the instrument with an appropriate check source is appropriate.
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.


When the performance remains within the expected value,, the previous calibration data are assumed to be valid. If not, theenergy window may have shifted, or the unit may be in need of repair and recalibration.
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.


An appropriate check source enables the stability of the assay instrument to be tested at any location.
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


Such a source can be prepared by implanting a small encapsulated plutonium source (containing
need to conveniently measure the line-of-sight background by moving the detector to one side without changing its orientation.
-0.5 g 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 plug is in place.The check source is fabricated in a manner to ensure its internal stability.
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.


Other than .radiations increasing from the ingrowth of Am-241, the emission rate of the check source should remain constant.2.1.4 Calibration Source for Gamma Ray Assay To calibrate a collection zone, the observed assay -response is compared to the response obtained when the zone contains a known amount of plutonium.
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.


Because of the complexity of the assay, the response is assumed to be linear. To be representative of typical holdup situations, the calibration standard is prepared as an encapsulated disk with a bed thickness of less than 0.2 cm. Care must be exercised in the preparation of the calibration standard to ensure that the amount encapsulated of total plutonium, Pu-239, and the amount of Amn-241, is known. It is important to measure the gamma ray attenuation
Protecting the markings (for example, with clear epoxy)
'through the encapsulating material and correct the calibration standard response to compensate for that attenuation.
will ensure their long-term durability.


The amount of plutonium encapsulated in 'the gamma ray calibration standard is selected to be representative of typical accumulations.
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.


2.2 Neutron Assay Neutrons are emitted in the spontaneous fission of Pu-238, Pu-240, and Pu-242 and through the interaction of emitted a particles with certain light nuclei. These neutrons suffer little attenuation in passing through uranium or plutonium or through most structural and containment materials.
Because uncertainties in geometry, attenua tion, or sample matrix will usually dominate the total response variability, the counting period need not be long.


Glove box windows may reduce the energy of emerging neutrons, but because of their regular and constant shape, their effect can generally be factored into the assay calibration.
Having 1000 to 10,000 net counts is generally sufficient for most holdup applications.


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, once the isotopic composition of the contained plutonium has been determined.
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.


Computing the (an) contribution requires a knowledge of the chemical form of the plutonium and the amount and, distribution of certain high (an) yield target materials.
Further, the variability between these measurements can provide an indication of the measurement uncertainty.


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


The measured neutron yield from prepared calibration standards is used to calibrate each neutron assay collection zone. In the Appendix, 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 5.23-3 calibration standards.
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 method can be used to calculate a ratio of the neutron production rate of the unknown material to the standard material neutron production rate. 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 vresence of a more intense gamma ray background and to collimate the detector so that the only neutrons being counted are emanating from the collection zone under assay._,Holdup assay -is performed under in-plant conditions where ruggedness, high detection efficiency, and high (-y,n) rejection, performance in the detectors is important.
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.


He-3 has one advantage over BF 3 detector*tubes in that the operating voltage for He-3' tubes does not increase as rapidly with increased gas pressure.To increase the efficiency of the system, detector gas pressure in the tubes may be increased or multiple detectors can be connected in parallel to feed a common preamplifier.
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.


He-3 and BF 3 detectors have efficiencies which increase as 'the energy of the neutrons decrease.To take advantage of this characteristic, the detectors can be surrounded by a 'neutron moderating material (see Figure B1I). Polyethylene is recommended.
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.


The thickness of the moderator is important.
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.


When the moderating distance is short, a fraction of the higher energy neutrons pass through the gas chamber without being detected.
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


Conversely, when the moderating distance is too long, a substantial number of low-energy neutrons are absorbed by the hydrogen contained in the moderator.
====d. When mixing====
5.23-10


A balance -between these, two effects is reached when -the spacing between adjacent tubes is approximately one-inch of polyethylene, and the -spacing between the front of the unit and the detectors and the back of the unit and the detectors is approximately I1%inch when one-inch-diameter tubes are used, and approximately one inch when two-inch-diameter tubes are used.'To -shield the detector, from low-energy neutrons which may produce a complicated response pattern, the moderator material is covered .with a thermal neutron absorber.
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.


Cadmium sheeting approxi-mately .0.075 cm thick can be used for this application.
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.


by stopping neutrons coming to the detector from all directions
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.
--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 Cd cover. For each six inches of polyethylene added, the collimator assembly provides a factor of approximately ten: in -the directionality of the response.


An example of a collimated ,neutron detector assembly for plutonium holdup assay is shownin Figure B-I.The weight of the .combined detector and collimator assembly.
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.


can easily exceed requirements for a hand-held detector probe.4 For this reason, and to provide for reproducible positioning at each assay, a sturdy cart housing both the detector/collimator and the associated-'electronics is recommended;
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,
Further, as the items to be assayed will be at different, heights, the ability to raise .and lower. the assembly to reproducible settings is ,recommended to expedite the assay and reduce the possibility of errors.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.
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.


An appropriate , neutron .assay check source can be measured, Por one or more :collection zones can be recalibrated and compared to the results of previous calibrations.
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.


An appropriate neutron assay check source can be prepared by implanting a small encapsulated plutonium source (containing about 5 g Pu).into the face of a plug. of neutron moderating material .(see Figure B-2). The plug is .fabricated to fit and close the collimator channel.-2.2.4 Calibration Source for, Neutron Assay To calibrate a neutron assay collection zone, the observed response is compared to the response obtained when the zone contains an additional known amount of plutonium.
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.


Neutron assay is less sensitive to attenuation than.is.,gamma ray assay.. It is important to know how:..much plutonium is dencapsulated in the neutron assay calibration standard, and the isotopic composition of that plutonium.
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.


The spontaneous neutron production rate from typical reactor plutonium is significantly less than the production rate of 375440 keV gamma rays. To provide. ,.an adequate response for calibration, it is therefore necessary, to encapsulate a larger amount of plutonium.
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.


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


This is accomplished
5.23-11
5.23-4 COLLIMATED
(2)
NEUTRON DETECTOR ASSEMBLY FOR PLUTONIUM
(3)
HOLDUP ASSAY DETECTOR CABLE ACCESS CHANNEL (TOP SECTION ONLY) --.--T-17.5cm I FRONT VIEW 68-5-1cm 1cmI " I TOP VIEW 4.5cm.. .. .\NEUTRO TUBE C I I 2.6cm DIA I II I I " , I .I I I I I I I I~I I I I !I I I I' I I I I I I I t l I.I II I I I .1 FRONT&#xa2;r 39cm I I I I II I I I I *I I I I I I I II I..1 I I I I I.I I I I I I I II -I I I I " I I lIl, I I 11 I II 1 1


===4. l POLYETHYLENE ===
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.
BLOCK, COVERED ALL SIDES'WITH 0.0756m CADMIUM SHEET VIEW FIGURE B-1 N DETECTOR HANNELS METER&#xfd; ITYP)DETECTOR TUBE SUBASSEMBLY
NEUTRON DETECTOR/COLLIMATOR
ASSEMBLY.


ASSEMBLY INCLUDES THREE BF 3 OR He-3 TUBES (2.54cm DIAMETER)
==C. REGULATORY POSITION==
UNIT CAN BE MODIFIED TO INCREASE ORDECREASE
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.
THE NUMBER OF TUBES.MODERATOR
THICKNESS
OF 15cm PROVIDES,10:?
DIRECTIONALITY.


ADDITIONAL
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.
POLYETHYLENE
CAN BE ADDED TO IMPROVE DIRECTIONALITY
Io.p., POLYETHYLENE
PROVIDES~100:1 DIRECTIONALITY).
COMPONENTS
ARE BOLTED OR STRAPPED TO REMAIN IN A FIXED CONFIGURATION.


5.23-5 MODERATOR\
1. DELINEATION OF COLLECTION ZONES AND
NEUTRON COLLIMATOR
ASSAY SITES
/CHANNEL PLUG 1 CHECK SOURCE TOP' VIEW CHECK SOURCE COVER CHECK SOURCE FRONT VIEW FIGURE B-2 NEUTRON COLLIMATOR
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.
CHANNEL PLUG AND CHECK SOURCE While the amount needed is best determined through an evaluation of typical accumulations, 100 g Pu is adequate for most applications.


The neutron assay calibration standard may generate more neutrons than directly attributable to the spontaneous fission and (an) reactions.
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.


Because a relatively large quantity of PuO 2 is encapsulated in the neutron assay calibration standard, some of the spontaneous fission or (an) neutrons may be absorbed in Pu-239 or Pu-241 nuclei, producing additional neutrons through the induced fission reaction.
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.


The amount of multiplication depends in a complex manner on the amount and distribution of PuO 2 and on the surrounding medium. The potentially significant calibration error arising by having too large a neutron yield per gram of plutonium will be corrected in the long term through assay verification tests. In the initial phase of assaying holdup, a rough correction for this yield can be measured by preparing two additional PuO2 sources containing
3. Each assay site should be uniquely labeled to facilitate unambiguous reference to that site in the assay log. A
1/3 and 2/3 of the neutron assay calibration standard mass. These samples need not be encapsulated, as they will be measured only once and can then be returned to the process stream.The PUO2 used in this test is taken from the same batch used to prepare the neutron assay calibration standard.
labeling and color-coding convention should be established to distinguish neutron assay sites from gamma ray assay sites.


After weighing out the proper quantities, the PuO 2 is put into containers having close to the same geometry as found in the neutron assay calibration standard.
4. Gamma ray assay should be used for collection zones containing less plutonium than the neutron detection limit.


Each test sample is transferred to an empty glove box and positioned next to the window for measurement.
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.


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 transferred in and positioned in the identical location for measurement.
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.


A plot of counts minus background as a 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.
===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.


A correction term is obtained by determining the increase in counts per gram at the mass value corresponding to the neutron 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 calibration standard mass. All measurements relating to this standaid are thereafter reduced by the ratio of the difference in counts to the observed counts.3. Isolation of Collection Zones To ensure that each collection zone is independently assayed, it is necessary to screen all radiations from the detector except those radiations emanating from the collection zone being assayed. This is principally accomplished through the use of the collimators described in Sections B.2.1.2 and B.2.2.2.Two additional means exist to further isolate a collection zone.3.1 Detector Positioning An unobstructed side view of a collection zone is preferred.
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.


When plutonium is located behind the zone under assay in another collection
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)
*zone or a storage facility, either consider positioning the detector above or below the collection zone, or consider the use of shadow shielding.
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.


3.2 Shadow Shielding It may not be possible to avoid interfering radiations through the collimator design or through 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.
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.


A rectangular
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.
5.23-6 panel containing
-5 cm of neutron moderator (e.g., benelex, WEP, or polyethylene)
and -0.5 cm lead sheet is recommended, mounted on wheels as an upright panel. To use such a panel, two measurements are required.*
--R1 -Rcz + Rlnterference
(1)R2 = Rcz + TRinterference
(2)where R 1 is the assay response obtained before the shadow shield is moved into position, R 2 is the assay response obtained with the shadow shield in position, Rez is the response component attributable to the collection zone under assay, Rinterference is the response component attributable to the interfering radiations, and T is the transmission through the shadow shield.Note that T represents a measured transmission-T.r for gamma rays or Tn for neutrons.


Ty. and Tn are measured by counting radiations from any arbitrary source of plutonium with the shield between the source and detector and again with the shadow shield removed: T = (R.?,) shield in/(R,,) shield out (3)T, = (Rn) shield in/(Rn) shield out (4)To correct for the interference, subtract R 2 from R 1 , and solve for 'Rlnterference: (R 2 -R) (" Rlnterference
The second channel should be set above the first window to provide a background subtraction for the assay window.
(1 -T) " (5)To ensure that this correction is sufficiently accurate, it may be necessary to extend the length of the normal counting period .to accumulate sufficient counting statistics
(1% statistics are generally adequate for this application).
4. Calibration of Collection Zones Euch collection zone is independently calibrated, as hackground-lfaclors and the of each zone vary widely from zone to zone. A collection zone is best calihlaled throngh the in situ measurementi of' known t'iilih)lU
n i lanltdads.


When such a program is not piositlve,.  
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.
Ihli callibration can it , based on the calculalion of Ithe anticipated response or through measuring a mockup ot the collection zone of interest.5*Response tierms refer to neutron or gamma response, as appropriate.


The calibration obtained through this procedure is recommended until a history of comparisons between predicted and recovered holdup quantities is developed, as described in Section B.5 of this~guide.
2.1.2 Gamma Ray Collimator A cylinder of shielding material such as lead should be made concentric with the gamma ray detecto


4.1 Detector Positioning To calibrate each collection zone, the best position or series of positions is selected to observe the collection zone with .the least amount of interference from principal structural components.
====r. The end of====
5.23-12


It is important to view the collection zone with the detector located between the collection zone and all areas used for Pu storage during inventory.
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.


A three-dimensional approach can be investigated, positioning the detector on top of or below the collection zone if it is not possible to have an unobstructed, interference-free side view of the collection zone. The use of shadow 'shielding can be explored if it is not possible to get a clear view of each collection zone for assay.On the basis of a detailed examination of the physical layout of the facility, some preliminary measurements are made to determine optimum detector positions for holdup assay. Once the assay positions for the detector and shadow shields -are established, permanently marking the assay positions will. facilitate subsequent measurements.
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.


4.2 Calibration Sources Since this assay is to measure the amount of plutonium holdup, it is appropriate to use plutonium as the calibration standard material.
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.


Further, as the plutonium holdup will generally be distributed over a large surface area, it is recommended that the gamma ray calibration standard be fabricated to resemble this characteristic, as described in Section B.2 of this guide.4.3 Calibration Procedures Once the principal items containing plutonium have been removed and the detector located in its assay position, the response from a calibration standard combined with the plutonium already held up is obtained.
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


When the collection zone is appropriately isolated, two factors influence the observed response from the calibration standard: I. the location of the calibration standard within the collection zone, and 2. the shielding of radiations from the calibration standard caused by the items comprising the collection zone.The gcomelric response variation is measured by observing lie response from-one calibration standard with the other standard removed from the collection zone under investigation.
====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.


The calibration standard response is measured with the standard positioned in various parts of the collection zone. avoiding internal items which may attenuate the radiation emanating .from the standard.5.23-7 When neutron assay is employed or when the collection zone consists of a hollow box, pipe, or duct, attenuation is either relatively uniform or negligibly small. The calibration of each collection zone then becomes a matter of appropriately averaging the geometric response variations.
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.


The average response of the entire collection zone is assumed to properly represent that zone. -'If, however, it is known that plutonium accumulates in one particular location within a collection
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.
'zone, the response of the standard is emphasized when located near the principal collection site.If the item to be assayed consists of a large unit, assay performance may be enhanced by subdividing the unit into smaller contiguous measurement zones.The repeat dimensions of the subzones are determined by measuring the rcsponse while moving the standard along an axis perpendicular to the detector centerline.


By studying the response curve, the distance D is selected as the point beyond which sufficient activity is detected to flatten the response within the subzone.Each subzone will measure 2D across its face. An example is illustrated
===3. CALIBRATION ===
'in Figure B-3. As the response about the centerline is assumed to be symmetrical, only half of the traverse is indicated.
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.


In Figure B-3, D is selected such that the area under the curve to the right of D is approximately equal to the area above the curve to the leftof D (Area A 1 = Area A 2). Note: the distance from the collection zone to the detector or the distance from the crystal face to the end of the collimator, or both, can be varied to divide the collection zone into an integral number of subzones.AREA A 1 100, S A MEASUREMENT
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.
DATA POINTS VISUAL FIT TO MEASUREMENT
DATA A S RA .To use this relationship, the detector is first positioned at point d and a reading is taken. Point d is the center of the first subzone, selected to coincide with the physical edge of the calibration'
zone. The detector is then moved a distance 2D along the traverse to the center point of the second subzone, and the second measurement taken. The cycle*is repeated to include all of the larger collection zone. The value interpreted for calibration for each subzone *.corresponds to the maximum of the traverse across each subzone because the response has been flattened.


The content of the entire collection zone is the sum of the contributions from the subzones.5. Estimation of the Holdup Error The overall uncertainty associated with the measured plutonium holdup is due to (1) the uncertainty in. the observed response and (2) the uncertainty in the interpretation of that response.
===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.


The random uncertainty components in this application are.frequently negligible in comparison with the geometric uncertainty and the uncertainty in the isotopic composition.
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.


In this assay application, it is appropriate to estimate the assay error components by assuming the measured range -(Ri) of the ith fluctuation constitutes an interval four standard deviations wide. The midpoint of the range estimates the mean effect, and the' distance from the "midpoint to each extreme comprises an estimated
5.23-13
95% confidence interval.


'The error attributable to this effect is then approximately
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.
2=(R) 2 (6)If a.severe effect is~noted, the response can often be corrected for the variation in the corresponding 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
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.
'5.1.1 Counting Statistics The magnitude of the uncertainties attributable to variations in the geometric distribution and in the attenuation of the radiations are expected to dominate the total 'response uncertainty.


'The relative standard deviation due to counting statistics can usually be made as small as desired through '(I) using more efficient detectors or (2) extending the counting period.Having 1000 to't0,000
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.
net counts is generally sufficient for most holdup assay applications.


5.1.2 Instrument Instabilities
Before assaying each collection zone, the operator should verify the floor location, probe height, and probe orientation. The electronic settings should be verified every
-Fluctuations in ambient temperature, humidity, electronic noise, and line voltage (for AREA A 2 0 25 5D 75 100 DISTANCE FROM DETECTOR CENTERLINE
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.
TO POINT SOURCE, CENTIMETERS
FIGURE B-3 EQUIVALENT
DIAMETER SUSZONE TO ACHtEVE A FLAT PLANAR RESPONSE.


SELECT D SUCH THAT AREA A 1= A 2.5.23-8 non-battery-powered electronic units) generally affect the stability of electronic systems. The magnitude of this uncertainty can be estimated by monitoring the check standard response and determining the range of variability as described in Section B.5 of this guide, 5.1.3 Geometric Uncertainty The geometrical variation in the observed response is measured by moving the calibration source within the bounds of each collection .zone. Two -cases are described below.5.1.3.1 Isolated Collection Zones When a single unit comprises a collection zone, the standard is moved .to all .sites within 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 neighbor zones. The average of the response, weighted to reflect 'prejudgments on the likelihood of accumulation sites, is then used as the calibration point. As shown in Section B.5, the range of values can be assumed to comprise an expectation interval four standard deviations wide. The geometric error is then estimated using Equation 6.5.1.3.2 Overlapping Collection Zones When a collection zone is subdivided into overlapping subzones, the geometric uncertainty due to the dimension perpendicular to the detector collection zone centerline is eliminated through the area-averaging calibration method described in Section 4.3.The uncertainty in the depth dimension in each subzone can be determined through the procedure outlined 'for isolated collection zones.Judgment can be used to weight the calibrationdata to emphasize principal accumulation sites.5.1.4 Attenuation Uncertainty If the attenuation is not extreme, it can be measured in situ, mocked up, or computed for the different conditions encountered.
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.


The worst and best cases can be assumed 'to determine the range of permissible effects. Using Equation 6, the magnitude of Ihis uncertainty component can then be estimated.
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.


Again, judgmaent is appropriate to weight the correction factor.5.2 Interpretation Uncertainties Two factors are central to the issue here, assuming that the calibration standard material is similar to the held-up material.5.2.1 Interfering Radiations
5. ESTIMATION OF HOLDUP ERROR
5.2.1.1 Gamma Ray Assay An uncertainty in the observed gamma ray response may arise due to the presence of extraneous gamma ray emitters or due to fluctuations in the background from the' Compmon scattering of higher-energy gamma rays. The shape of the background gamma ray spectrum may change in such cases to such an extent that even with the energy windows stabilized.
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.


the background correction is irregular and uncertain.
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.


The magnitude of this effect is generally small. It can be.monitored by observing the spectrum with a multichannel analyzer, but unless the data on periodically recovered
5.23-14 t
'holdup accumulations are in error, this contribution can be ignored.5.2.1.2 Neutron Assay A change in the neutron yield for a plutonium sample of fixed isotopic content is primarily attributable to the fluctuation in the concentration of high (an) yield impurities.*
I
Judgment can be used to determine the range *of permissible impurity concentrations.


The variation in a typical neutron yield can then be predicted using the methods discussed in the Appendix of this guide. Again, the range of permissible variations is assumed to constitute an acceptance interval from which the component error is computed using Equation 6.5.2.2 Isotopic Uncertainties If the process equipment is cleaned each time the isotopic composition of the plutonium feed is varied, the holdup will consist primarily of the current material.
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.


New calibration standards can be prepared or the previous yield data can be normalized using the methods presented in tht Appendix to correct tor  effect. When mixing occurs, use of the stream-averaged isotopic composition is appropriate.
2.


The uncertainty bounds are estimated by considering the highest .,id lowest fissile isotopic batches and computing the corresponding range.5.3 Holdup and Its Associated Error The amount of Pu holdup can be ,measured through the systematic application of the program developed in conjunction with the principles and pitfalls discussed herein. For each collection zone, measured holdup and its error can be determined.
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.


*Over a long period of time the a-particle production ratc increases due to the ingrowth of Am-24 1.5.23-9
3.
5.3.1 Initial Operations During the initial phase of operations, the error associated with the in situ assay of plutonium holdup is estimated by combining the component errors determined in the preceding sections of this guide (B3.5.1 and B.5.2).5.3.2 Routine Operations To ensure the validity of assay predictions and to more realistically estimate the uncertainty in those predictions, it is necessary to establish a program to measure, the amount of plutonium recovered when a collection zone is cleaned out. By comparing the aniount of plutonium recovered to the recovery amount predicted, the collection zone calibration can be updated and the assay error can be based on relevant verification tests.The update data is computed as the difference in the assays before and after cleanout: (PU)assay
= Rbefore -Rafter , (7)The difference.(A)
in assay and recovery, A = (PII)assay
-(Pu)recovery
(8)is then computed.The standard deviation in the A values (s.)is computed separately for" each collection zone, including no more than the twelve preceding measurement tests: sA (K- 1 (9)When a value of A is determined, it is used to update the estimate sb. The -standard, deviation estimate s. can be used to estimate the~error in. the assay prediction for the collection zone for which it has been established.


The amount of plutonium collected during the cleanout of a specific collection zone can be assayed through sampling and chemical analysis, through calorimetry, or through other applicable nondestructive assay methods (eg.,. spontaneous fission coincidence detection or gamma ray assay). Each of these topics is the subject of a Regulatory Guide.C. REGULATORY
L. A. Kull, "Catalogue of Nuclear Material Safeguards Instruments," Battelle National Laboratories, BNL
POSITION To develop a program for the periodic in situ assay of plutonium residual holdup as an acceptable measurement method for this inventory component, it is necessary to consider -the -factors'
17165, August 1972.
in -the -following sections.Note: Care must be exercised during the fabrication and use of check sources and-calibration.


standards to ensure their continued integrity and to prevent contamination.
4.


4. Delineation of Assay Collection Zones A plan of each plutonium processing facility should be examined.,to establish, independent collection zones.Individual glove boxes and similar containment structures should be so-identified..
R. H. Augustson and T. D. Reilly, "Fundamentals of Passive Nondestructive -Assay of Fissionable Material,"
Using the layout and touring the facility, -an. assay. site(s) for. each collection zone should be selected: 1. Assay site(s) -should afford a clear, unobstructed view ,9f-the collection zone with no other collection or....storage. areas in_ the line- of sight of the .collimator assembly.
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.


Location of the. detector probe above or below, the -collection zone- should be considered if an unobstructed side , view is not possible.
5.


If an-unobstructed view is not, possible, shadow, shielding should be used to isolate the collection zone, for assay.2. The assay site should be set back as far as possible from each collection zone to reach a compromise between interference from neighbor zones and efficient counting..
R. B. Walton et al., "Measurements of UF Cylinders with Portable Instruments," Nuclear Technology, Vol. 21, p. 133, 1974.
3.' Gamma ray assay should ,be applied to measure the plutonium- held up in all collection.


zones containing less than the neutron- detection limit- and for' single containment structures which do not contain irregularly shaped structural components capable of significantly the emerging gamma rays. Neutron assay should: be applied to measure the accumulation of plutonium holdup in all structures not suitable, for gamma ray assay. -.4. Each collection zone should be uniquely numbered.(Neutron collection zones could be preceded by an "N", gamma ray collection zones by a "G". Subzones should be identified by an alphabetic suffix to the collection
6.
-zone identification.)
..5. Each assay site should be' marked with paint or colored tape on the floor. (To be consistent, blue tape should be used for neutron assay sites, orange for gamma ray sites.) The height setting for midpoint assay should be recorded in the measurement log corresponding to each assay site....2.., Assay Instruments Neutron and gamma ray assay capability should be provided using separate or compatible'
electronics with interchangeable detector probes. Compatible electronics
5.23-10
should provide for both He-3 or BF 3 neutron detection and Nal(TI) gamma ray detection.


The electronics unit should have a temperature coefficient of less than 0.1%per 'C. Battery-powered electronics should be provided to expedite assays.2.1 Gamma Ray Assay Gamma ray assay should be 'based on the activity observed in the energy range from 375 keV to 440 keV, excluding the composite gamma ray complex centered at 333 keV. Yield data for appropriate gamma rays are presented in Section B.2.1 of this guide.2.1.1 Detector Selection Gamma ray detectors should have FWHM resolution equal to or better than 7.5% at 662 keV (Cs- 137 gamma ray). NaI(TI) can meet such specifications and is suitable for this application.
C. H. Kindle, "In Situ Measurement of Residual Pluto nium," Nuclear Materials Management, Vol. 5, No. 3, p. 540, 1976.


The crystal depth should be sufficient to detect a significant percentage of 400-keV gamma rays. For NaI(TI), the minimum depth should be one inch. A two-inch depth is recommended.
7.


The crystal should be stabilized with a suitable radioactive source. An'&#xfd; internal Cs] seed containing Am-241 is recommended for this application.
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.


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 0.075 to 0.150 cm cadmium sheet to filter low-energy radiations.
8.


Two single-channel analyzers should be.provided with lock-set energy windows. One channel should be set to admit gamma rays from 390 keV to 440 keV unless equilibrium of the U-237 and Pu-241 can be assured. The 333-keV region of the gamma ray spectrum should be excluded.
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.


With Nal detectors, it is necessary to exclude the 375 keV gamma ray to ensure that the tail from the 333 keV complex is not added. The second channel should be set above the first window to provide a background correction for the assay window. This second window should be set from approximately
9.
450 keV to 600 keV.2.1.2 Gamma Ray Collimator A cylinder of shielding material such as lead should be made c(ncentric with the gamma ray detector.


The end of the cylinder opposite the crystal should be blocked with the shielding material.
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.


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).
10.
The collimator sleeve should be extendible over the end of the crystal to reproducible settings to vary the degree of collimation for different collection zones.2.1.3 Gamma Ray Check Source To ensure the continued normal operation of each system, an encapsulated plutonium check source should be provided.


The source should be small enough to be implanted in a section of shielding material so shaped as to close off the collimator opening. The check source should be positioned adjacent to the detector.The source should contain an amount of plutonium sufficient to provide a gross count rate of 1000 to 10,000 counts per second.2.1.4 Gamma Ray Calibration Source To permit the calibration of gamma .ray assay collection zones, a calibration standard should be fabricated by encapsulating plutonium oxide in a disk.The isotopic composition of the plutonium and the abundance of Am-241 should be measured and be chosen to be nominally representative of the plutonium being processed.
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.


The total amount of plutonium encapsulated should be closely monitored.
11.


Attenuation losses within the bed of PuO 2 and through the encapsulating material should be measured and the calibration standard response normalized to counts per gram incorporating these corrections.
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.


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.
12.


He-3 and BF 3 neutron detectors are recommended for this application.
M. S. Zucker et al, "Holdup Measurements for Nuclear Fuel Manufacturing Plants," Nuclear Materials Manage ment, Vol. X, p. 239, 1981.


Multiple detector tubes with matdhed operating performance should be connected in parallel to a single preamplifier to increase the overall detection efficiency obtainable from a single detector tube. Neutron detectors should be surrounded by a layer of neutron moderator material to enhance their detection efficiency.
13.


The neutron moderator layer should be covered with a low-energy neutron absorber to filter out extraneous neutrons from the desired signal. A recommended configuration is diagrammed in Figure B-I.2.2.2 Neutron Collimator A slab collimator or concentric cylinder collimator of a suitable neutron moderator material (e.g., polyethylene)
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),  
should be constructed to completely surround the detector with its associated moderator and filter assembly, 'leaving open orly the collimator channe
p. 219, May 1976.


====l. A recommended ====
14.
'configuration is shown in Figure B-1.The moderator thickness should be selected to provide. the directionality required for each facility.


A directionality profile providing a 10:1 5.23-11 response ratio (six inches of polyethylene)
P. R. Bevington, Data Reduction and Error Analysis for the Physical Sciences, McGraw-Hill, 1969.
should be adequate for most applications;
however, each situation should be evaluated as discussed in Part B of this guide.2.2.3 !NeutronCheck Source Any neutron source which emits approximately
100-10,000
neutrons/second is acceptable for this application.


The source should be small enough to be contained within a section of, neutron moderator material so shaped as to completely fill the collimator channel of the detector assembly.
5.23-15


The source should be implanted ,directly adjacent to the neutron detectors, outside the cadmium thermal neutron filter. A recommended configuration for this assembly is diagrammed in Figure B.2.2.2.4 Neutron Assay Calibration Standard To permit the, calibration of neutron assay* collection zones, a calibration standard should be-fabricated by encapsulating PuO 2.The PuO 2 should be nominally representative of the plutonium being processed in isotopic composition, in Am-241 content,"and in the content of high (a,n) yield target materials.
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&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.


The amount of plutonium to be encapsulated should be chosen to be representative of the amounts of plutonium estimated to be held up in typical neutron assay collection zones.'The' neutron yield of the calibration standard should be measured and also computed using the. method described in the Appendix.
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.


The observed neutron. count rate should be normalized.
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.


6 If the ,predicted response differs by more than 10%76, the response should be normalized as discussed in Section B.2.2.4.2.3 Service Cart A cart carrying electronics and both detector probes should be provided.
*
**&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.


The capability to raise or lower the probes to reproducible settings should be included.2.4 Notation of Operating Parameters When compatible electronics are used to facilitate neutron and gamma ray assay, a notation of athe respective settings should be affixed to the electronics unit. To decrease the likelihood of incorrect settings, the neutron probe and the -appropriate electronics settings should be color-coded blue; the gamma ray probe and :corresponding electronics settings should be coded orange.3. Calibration Each collection zone should be independently calibrated when all in-process material has been located so that the response from the calibration standards will not be influenced by the in-process material.3.1 Instrument Check The stability of the neutron and gamma ray detection systems should be tested prior to each inventory by comparing the observed counts obtained from the check source, minus the counts with the shaped shield in place but without the check source, to the readings obtained prior to previous inventories.
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


If the measurement is consistent with previous data (i.e., is within plus or minus two 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 recalibrated, as required.3.2 Zone Calibration The geometric response profile for each collection zone should be determined by measuring the variation in the response as a calibration standard is moved within the defined limits of the collection zone.The. response variation should then be averaged to determine the response per gram of plutonium for that collection zone. The averaging should be weighted to reflect known local accumulation sites within each collection-zone.
Table A-2 (a,n) NEUTRON YIELDS FOR
VARIOUS LIGHT ELEMENTS
P.


The response per gram should be used to directly translate the observed response to grams oi plutonium, after the response is corrected for background.
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.


3.2.1 Subzone Calibration When a collection zone is too large to be accurately measured in a single assay, the collection zone should be divided into overlapping subzones.
18,  
1979.


The repeat dimensions of each subzone perpendicular to the detector-to-collection-zone line should be determined so that the response variation across that distance is nulled.Using this procedure, the residual geometric uncertainty should be determined by measuring the response as a calibration standard is moved along the depth coordinate.
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.


The calibrated response should then reflect the average of the depth response, weighted to reflect known accumulation sites.
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.


===4. Asmy Procedures===
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 ).
4.1 Ammy LoA An assay log should be maintained.
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.


Each collection zone or subzone should have a separate page in the amy log, with the corresponding calibration derived on the page facing the assay data sheet.Recording space should be provided for the date of 5.23-112 measurement, gross counts, corrected counts, and the corresponding grams plutonium from the calibration in addition to position and instrument electronic setting verification.
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.


4.2 Preassay Procedures Prior to inventory, the isotopic composition of the plutonium processed during the current operational period should be determined.
from Table A-2, the concentrations Ij from Table A-4, and Equation 6 or 7.


Variations in the neutron and gamma ray yield data from the calibration standard should be calculated.
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.


Either the calibration data or the predicted holdup should then be corrected to reflect this difference.
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


Prior to each inventory, the operation of the neutron and gamma ray assay detection systems should be checked.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 accountability.
====r. Both ====
.J
yield calculations must assume perfect mixing, however.


All process, scrap, and waste items containing plutonium should be removed from the process areas to approved storage areas to minimize background radiations.
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.


4.3 Measurements The assay cart should:be moved in sequence to the assay site(s) corresponding to each collection zone.Assaying all gamma ray sites before assaying neutron sites (or Vice versa) is recommended.
5.23-17 Table A-3


Before assaying each collection zone, the operator should verify the floor location, probe selection, probe height, and electronics settings.
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.


All check and calibration sources should be sufficiently removed so as not to interfere with the measurement.
Let f238, f239' f240' f241, and f2 4 2 represent the weight fractions of the respective plutonium isotopes in the unknown sampl


Prior to taking a measurement, a visual check of the zone and the line of sight of the detector probe should be made to assure 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 assure conmpliance for each collection zone.Having met all preceding requirements, the measurement at each site should be taken, recorded, and converted to grams plutonium.
====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.


If each value is within an expected or permissible range, -the cart should be moved to the next site and the cycle repeated.
5.23-18


If a high response is noted, the cause should be investigated.
VALUE/IMPACT STATEMENT


If the collection zone contains an unexpectedly large content of plutonium, that collection zone should be cleaned to remove the accumulation for conversion to a more accurately accountable material category.
===1. PROPOSED ACTION===
1.1 Description No adverse impact on the public can be foreseen.


After the cleanout has been completed, the zone should be reassayed and the recovered material quantity used to test the validity of the zone calibration.
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.


5. Estimation of the Holdup Error During the initial implementation of this program, the error quoted for the holdup. assay should be computed on the basis of estimating the error components, as described in SectionsB.5.1 and B.5.2.Prior to the cleanout of any collection zone for whatever purpose, that zone should be prepared for assay and measured as described in:Section C.4 of this guide. Following this assay, the collection zone should be cleaned out and the collected plutonium should then be assayed using an appropriately accurate assay method. When the collection zone has been cleaned and the collected plutonium removed,, the collection zone should be reassayed.
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.


The recovered plutonium should be used to update the calibration and,. from the. sixth test on, should serve as the assay error estimate.
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.


Separate records should be maintained for each collection zone to estimate the error in assaying the plutonium holdup.To ensure that error predictions remain current, only data of the twelve preceding independent tests should be used to estimate the assay error. Collection zones not cleaned for other purposes should be cleaned for assay verification at intervals not to exceed two month
1.3 Value/Impact of Proposed Action
1.3.1 NRC Operations The regulatory positions will be brought up to date.


====s. REFERENCES====
1.3.2 Other Government Agencies Not applicable.
1. R. Gunnink and R. J. Morrow, "Gamma Ray Energies and Absolute Branching Intensities for 2 3 8 , 2 3 9 , 2 4 0 , 2 4 1Pu and 2 4 1 Am," UCRL,51087 (July 1971).2. J. E. Cline, R. J. Gehrke, and L. D. Mclsaac,"Gamma Rays Emitted by the Fissionable Nuclides and Associated Isotopes," ANCR-1069 (July 1972).3. L. A. Kull, "Catalogue of Nuclear Material Safeguards Instruments," BNL-17165 (August 1972).4. An example of a collimator for uranium gamma ray assay is found in R. B. Walton, et al, "Measurements of UF 6 Cylinders with Portable Instruments," Nucl.Technol., 21, 133 (1974).5. W. D. Reed, Jr., J. P. Andrews, and H. C. Keller, "A Method for Surveying for Uranium-235 with Limit of Error Analysis," Gulf-GA-A12641 (June 1973).5.23-13 APPENDIX 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 created through induced fission of Pu-239 or Pu-241. The total neutron yield per gram of plutonium holdup will then be the sum of the spontaneous fission and (an) contributions:
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.
Yn = YSF + Y(,t,n) (1)1. Spontaneous Fission Neutrons To determine the spontaneous neutron yield per gram of plutonium held up within a collection zone, the isotopic composition of the plutonium and uranium (if present) must be known. The contribution from spontaneous fission can generally be calculated by neglecting the contribution from U-238: YSF = W 2 3 8 Q 2 3 8 + W240Q240 + W242Q242 (2)where Wi = weight fraction of the ith plutonium isotope. For reactor fuel applications, W 2 3 8 + W239+ I Qi = spontaneous fission neutron yield per gram of the ith plutonium isotope (see Table 1).2. (a,n) Neutrons The maior contribution to the total neutron production from (ax) reactions will typically be due to the 048 (an) Ne-21 reaction when the plutonium exists as the oxide. The yield from this reaction per gram of plutonium can be calculated using the isotopic weight fractions (Wj) and the Yi yield data given in Table 1.Y(an) Oxy WiYi (3)The yield per gram of Put 2 is calculated by multiplying the yield per gram of plutonium by the gravimetric dilution factor (Pu/PuO 2 -0.882).The presence of certain impurities can contribute substantially to the total (atn) production rate.Approximate values of (a,n) impurity yields for the highest yield (an) target materials are given in Table 2.To compute the impurity (an) contribution, the total a particle production is determined.


Production rates of a particles per gram of the principal nuclides of interest are shown in Table 1. This contribution to the total neutron yield can be computed using the relationship:
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.
Y(a,n) Impurity = Y 0 TPjlj i (4)TABLE 1 a Particle and Spontaneous Fission Neutron Yields Half-life Alpha Activity PuO 2 (mn) Yield 8  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 1 4.3 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 1 1  3.46 x 10 3  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 10 9  1.23 x 10 4  1.93 x 10--4  !.12 x 10-2 a -Oxygen yield from PuO 2 form only.b -&-branching ratio -2.46 x 10-5 5.23-14 where Y,, = total a production
= WWiai + WArnm'Am i Wi = Pu isotopic weight fractions WAm = Am weight fraction = Am/Pu ai = a yield per gram of nuclide i (see Table 1)TABLE 2 (Q,n) Yield Rats of Low-Z Impurities in Pu02a P.I mpurity (n/a-ppm)Li .........................
6.29 x 10-12 Be .........................
2.00 x 10 -'0 B .... ......................
4.63 x 10-11 C ..........................
2.77 x 10-13 0b ....... ..................
1.56 x b0-"13 F ...........................
2 .44 x 1O-Il Na .........................
3.00 x 10-12 g...........................
2.67 x 10-12 i f .........................
1.45 x 10-1 2 Si .........................
3.25 x 10-13 aAssumnes zero yield from all other impurities.


bOxygen not contained in oxide.Pj = (an) yield per ppm of the impurity j (see Table 2)Ii = impurity j content, expressed in ppm (weight) of plutonium.
===2. TECHNICAL APPROACH ===
Not applicable.


3. Sample Calculation (PuO 2-UO 2)Consider the case of recycle plutonium blended t6 3 wt %Pu in a normal U0 2 matrix, Where the isotopic composition is Pu-238 (.25%). Pu-239 (75.65%), Pu-240 (18.48%), Pu-241 (4.5%), Pu-242 (1.13%), and Am-241 (.28% of Pu).For mixed oxides, the oxygen density is approximately the same for the case ofPuO.. This fact, together with the atomic similarity of uranium and 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, Pu/(Pu + U).Using the values given in Table I, the spontaneous fission yield and total a production per gram of plutonium can be computed.
===3. PROCEDURAL APPROACH ===
Of the procedural alternatives considered, revision of the existing regulatory guide was selected as the most advanta geous and cost effective.


Results are shown in Table 3.The a particle yield of plutonium is constant in time for all intents. However, the Am-241 a production in-creases at a rate which results in approximately a 0.3%;.increase per month in the total a production, for the range of plutonium isotopic compositions intended for reactor fuel application.
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.


In the present example, the impurity levels of the principal (a.n) target materials are shown in Table 4. The 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)Pu7238 .0025 6.4 1.58 x 199 42.6 Pu-239 .7565 <.05 1.74 x 109 41.3 Pu-240 .1848 189.4 1.56 x l09 37.3 Pu-241 .0450 <.05 4.23 x 106 0. i Pu-242 .0113 19.8 1.63 x 106 <0.05 Ain-241 .0028 <.05 3.56 x 108 9.7 rotal Yields 215.6 5.26 x 109 131.0-- oxygen yield only.
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.


impurities are also shown in Table 4, calculated using the a particle production rate of 5.3 x 109 a/sec-g Pu, computed above. In this example, the mixed oxides are composed of blended PuO 2  and U0 2  particles approximately
S. RELATIONSHIP TO OTHER EXISTING OR PROPOSED
40 microns in diameter.
REGULATIONS OR POLICIES
The proposed action is one of a series of revisions of exist ing regulatory guides on nondestructive assay techniques.


If the particle size were smaller or the mixed oxide was created through coprecipitation, the uranium impurity content would also contribute to~the plutonium(an)
6. SUMMARY AND CONCLUSIONS
yield. This contribution can be ignored for large particles and estimated by combining the impurities for small particles and coprecipitatedoxides.
Regulatory Guide 5.23 should be revised.


The total neutron yield in this example is 380 n/sec-g Pu. In this example, the percentage of plutonium tq the total Pu + 0 is 0.8835. Using this gravimetric dilutign factor, the neutron yield is 336 n/sec-g PuO 2.If the PuO 2 is blended with U0 2 to 3%, i.e., PuO 2/PuO 2 +U0 2 = 0.03, the neutron yield. from the blend will be 10.1 n/sec-g MO.TABLE 4 Impurity (.,n) Yield Arbitrary Concentration (a,n) Yield Impurity (ppm) Wnisec-g.
5.23-19
1.3.4 Public


Pu)Li 9 0.30 Be 8 8.42 B 10 2.44 C 200 .30 F 125 16.0 OR ... 4600 3.77 Na 120 1.90 Total 33.1 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
Person / Time
Issue date: 02/29/1984
From:
Office of Nuclear Regulatory Research
To:
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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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