Regulatory Guide 5.38: Difference between revisions

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{{Adams
{{Adams
| number = ML003740000
| number = ML13064A076
| issue date = 10/31/1983
| issue date = 09/30/1974
| title = (Task SG 048-4) Revision 1 Nondestructive Assay of High-Enrichment Uranium Fuel Plates by Gamma Ray Spectrometry
| title = Nondestructive Assay of High-Enrichment Uranium Fuel Plates by Gamma Ray Spectrometry
| author name =  
| author name =  
| author affiliation = NRC/RES
| author affiliation = US Atomic Energy Commission (AEC)
| addressee name =  
| addressee name =  
| addressee affiliation =  
| addressee affiliation =  
Line 10: Line 10:
| license number =  
| license number =  
| contact person =  
| contact person =  
| document report number = RG-5.38 Rev 1
| document report number = RG-5.038
| document type = Regulatory Guide
| document type = Regulatory Guide
| page count = 14
| page count = 7
}}
}}
{{#Wiki_filter:Revision 1 U.S. NUCLEAR REGULATORY COMMISSION                                                                   October 1983 REGULATORY GUIDE
{{#Wiki_filter:U.S. ATOMIC ENERGY COMMISSION
                                OFFICE OF NUCLEAR REGULATORY RESEARCH
REGULATORY GI
                                                              REGULATORY GUIDE 5.38 (Task SG 0484)
DIRECTORATE OF REGULATORY STANDARDS
                                  NONDESTRUCTIVE ASSAY OF HIGH-ENRICHMENT URANIUM
REGULATORY GUIDE 5.38 NONDESTRUCTIVE ASSAY OF HIGH-ENRICHMENT URANIUM
                                            FUEL PLATES BY GAMMA RAY SPECTROMETRY
FUEL PLATES BY GAMMA RAY SPECTROMETRY
September 1974 JIDE


==A. INTRODUCTION==
==A. INTRODUCTION==
Part 70 of Title 10 of the Code of Federal Regula- tions requires each licensee authorized to possess more than 350 grams of contained U-235 to conduct a physical inventory of all special nuclear material in his possession at intervals not to exceed 12 months. Each licensee authorized to possess more than one effective kilogram of high-enrichment uranium is required to conduct measured physical inventories of his special nuclear materials at bimonthly intervals. Further, these licensees are required to conduct their nuclear material physical inventories in compliance with specific require- ments set forth in Part 70. Inventory procedures acceptable to the Regulatory +,aff for complying with thesc pi 'wisions of Part 70 are detailed in Regulatory Guide 5.13. "Conduct of Nuclear Material Physical Invcntories."
For certain nuclear reactors, the fuel consists of highly enriched uranium fabricated into flat or bowed plates. Typically, these plates are relatively thin so that a significant percentage of the U-235 gamma rays ptle- trate the fuel cladding. When the measurement condi- tions are properly controlled and corrections are made for variations in the attenuation of the gamma rays, a measurement of the U-235 gamma rays can be used as an acceptable measurement of the distribution and the total U-235 content of each fuel plate. In lieu of assaying the product fuel plates, fuel plate core compacts may be assayed through the procedures detailed in this guide, provided steps are taken to ensure the traceability and integrity of encapsulation of each assayed fuel plate core compact. This guide describes features of a gamma ray spectrometry system acceptable to the Regulatory staff for nondestructive assay of high-enrichment uranium fuel plates or fuel plate core compacts.


==B. DISCUSSION==
==B. DISCUSSION==
Part 70 of Title 10 of the Code of Federal Regulations                  The number, energy, and intensity of gamma rays requires each licensee authorized to possess more than                  associated with the decay of 2 3 5 U provide the basis for
The number, energy, and intensity of gamma rays associated with the decay of U-235 provide the basis for nondestructive assay of high-enrichment fuel plates by gamma ray spectrometry (Ref. 1). The 185.7-keV
        350 grams of contained 235U to conduct a physicalinven                  nondestructive assay of high-enrichment fuel plates by tory of all special nuclear material in its possession at              gamma ray spectrometry (Ref. 1). The 185.7-keV gamma intervals not to exceed 12 months. Each licensee authorized            ray is the most useful 2 3 5 U gamma ray for this application;
gamma ray is the most useful U-235 gamma ray for this application; it is emitted at the rate of 4.25 x 104 gamma rays per second per gram of U-235. Lower- energy gamma rays emitted by U-235 are less pene- trating and more sensitive to errors due to fluctuations in clad and core thickness. In general, more accurate fuel plate assays may be made by measuring only the activity attributable to the 185.7-keV U-235 gamma ray.
        to possess more than one effective kilogram of high                    it is emitted at the rate of 4.25 x 104 gamma rays per enrichment uranium is required to conduct measured                      second per gram of 2 3 5 U. Lower energy gamma rays physical inventories of special nuclear materials at bimonthly          emitted by 235 U are less penetrating and more sensitive to intervals. Further, these licensees are required to conduct            errors due to fluctuations in cladding and core thickness. In their nuclear material physical inventories in compliance              general, more accurate fuel plate assays may be made by with specific requirements set forth in Part 70. Inventory              measuring only the activity attributable to the 185.7-keV
        procedures acceptable to the NRC staff for complying with              235 U gamma ray.


these provisions of Part 70 are detailed in Regulatory Guide 5.13, "Conduct of Nuclear Material Physical Inven                    Assay measurements are made by integrating the response tories."                                                                observed during the scanning of single fuel plates and comparing each response to a calibration based on the The fuel for certain nuclear reactors consists of highly          response to known calibration standards.
Assay measurements are made by integrating the response observed during the scanning of single fuel plates and comparing each response to a calibration based on the response to known calibration standards.


enriched uranium fabricated into flat or bowed plates.
1. GAMMA RAY MEASUREMENT SYSTEM
1.1 GAMMA RAY DETECTION SYSTEM
1.1.1 Gamma Ray Detector High-resolution gamma ray detectors, i.e., intrinsic or lithium-drifted germanium, provide resolution beyond that required for this assay application. While the performance of such detectors is more than adequate, their low intrinsic detection efficiency, extensive opera- tional and maintenance requirements, and high cost make them unattractive for this application.


Typically, these plates are relatively thin so that a signifi          1. GAMMA RAY MEASUREMENT SYSTEM
Most ,sodium iodide [Nal (TI)] scintillation detectors are capable of sufficient energy resolution to be used for the measurement of the 185.7-keV gamma rays. The detector diameter is determined by the fuel plate width and the scanning method selected (see Section B.I.2 of this guide). The thickness of the Nal crystal is selected to avoid unnecessary sensitivity to gamma rays above the
        cant percentage of the 235U gamma rays penetrates the fuel and cladding. When the measurement conditions are properly              1.1 Gamma Ray Detection System controlled and corrections are made for variations in the attenuation of the gamma rays, a measurement of the 2 3 5 U                1.1.1 Gamma Ray Detector gamma rays can be used as an acceptable measurement of the distribution and the total 2 3 5 U content of each fuel                High-resolution gamma ray detectors, i.e., high-purity plate. In lieu of assaying the product fuel plates, fuel plate        germanium, HPGe, also referred to as intrinsic germanium core compacts may be assayed through the procedures                    (IG), or lithium-drifted germanium [Ge(Li)] detectors, detailed in this guide provided steps are taken to ensure the         provide resolution beyond that required for this assay traceability and integrity of encapsulation of each assayed            application. While the performance of high-resolution fuel plate core compact. This guide describes features of a           detectors is more than adequate, their low intrinsic detec gamma ray spectrometry system acceptable to the NRC                    tion efficiency, higher maintenance requirements, and high staff for nondestructive assay of high-enrichment uranium              cost make them unattractive for the measurements dis fuel plates or fuel plate core compacts.                              cussed here.
185-keV region which produce a background in the
185-keV energy region as a result of Compton scattering.


Any guidance in this document related to information                    Most sodium iodide [NaI(TI)] scintillation detectors are collection activities has been cleared under OMB Clearance            capable of sufficient energy resolution to be used for the No. 3150-0009.                                                        measurement of the 185.7-keV gamma rays. For plate USNRC    REGULATORY GUIDES                            Comments should be sent to the Secretary of the Commission, U.S. Nuclear Regulatory Commission, Washington, D.C. 20555, Regulatory Guides are issued to describe and make available to the      Attention: Docketing and Service Branch.
For measurements to be reproducible, it is necessary to assure that the detection system is stabilized on the USAEC REGULATORY GUIDES
Cop.- of published guids may be obtained by request indicintng the dinirsom deired to the US. Atomic Erw Commismson. Vlhington. D.C. 20545.


public methods acceptable to the NRC staff of implementing specific parts of the Commission's regulations, to delineate tech        The guides are issued in the following ten broad divisions:
RegulatmrV Guide are ssued to iescribe and make availabte to the puublic Attlntion: Director of Regulatory Standstills. Comments wed suggestions for nalhods c.acptable to tht AEC Regulatory st&ff of imiplemnting specific parts of ifl3hmtflts in thes guides we o fiurqlpd and should be sent to the Secretary the Commnisson* s regulations, to delineate technqlus
      niques used by the staff in evaluating specific problems or postu lated accidents or to provide guidance to applicants. Regulatory        1. Power Reactors                  6. Products Guides are no? substitutes for regulations, and compliance with          2. Research and Test Reactors     
.sead by the staff in of the Comnssmion, US. Atomic EnoqW Commission. Washington, D.C. 20545.


===7. Transportation===
oealuat-0n specific problems or postulated accidents, or to provide guidenws to Attention: Do-kteting and Sorvece Section.
.___/ them is not required. Methods and solutions different from those set    3.  Fuels and Materials Facilities  8. Occupational Health out in the guides will be acceptable if they provide a basis for the    4.  Environmental and Siting        9. Antitrust and Financial Review findings requisite to the issuance or continuance of a permit or        5. Materials and Plant Protection  10. General license by the Commission.


Copies of issued guides may be purchased at the current Government This guide was issued after consideration of comments received from      Printing Office price. A subscription service for future guides in spe the public. Comments and suggestions for improvements in these          cific divisions is available through the Government Printing Office.
aml)icents. Regulatory Guides wit not substitutes for regulations and comNpflanio with them is not re*uired. Methods and sofutions diffrt from tfeor lit out in The guides are
,ssued in the following ten broaed divisions:
t.


guides are encouraged at all times, and guides will be revised, as      Information on the subscription service and current GPO prices may appropriate, to accommodate comments and to reflect new informa        be obtained by writing the U.S. Nuclear Regulatory Commission, tion or experience.                                                    Washington, D.C. 20555, Attention: Publications Sales Manager.
guides well bea asipgtobl if they piroind a basis for the findingl requisits to the iiufnt*i or oiminuenOt of a permit or licnese by the Commtissson.


assays by scanning techniques, the detector diameter                      plate. To minimize this detection nonuniformity and to is determined by the fuel plate width and the scanning                    minimize the sensitivity to vibration, the detector-to-plate method selected (see Section B.l.2 of this guide). For                    distance can be made large, especially with respect to the passive counting of the total fuel plate (see Section B. 1.3              dimensions of the slit opening. As an alternative means of K-
===1. Powr Reectorn ===
of this guide), the detector diameter is not a critical param            reducing the detection nonuniformity across the slit, the eter, and detectors suitable for plate scanning would also                slit opening can be divided into channels by inserting a be adequate for the passive counting measurements. In both                honeycomb baffle into the slit or by fabricating the colli cases, the thickness of the Nal crystal is selected to provide            mator by drilling holes through the disk in a pattern that a high probability of detecting the 185.7-keV gamma rays                  ensures that each hole is surrounded by a minimum wall and a low probability of detecting higher energy radiation.              thickness of 0.2 mean free path length. A 7.0-cm-thick iron A crystal thickness of 1/2 to 1 inch (13 to 25 mm) is                      disk with holes less than 0.5 cm in diameter drilled in a recommended.                                                              pattern having 0.2 cm of wall between adjacent holes is one example of a collimator that would perform satisfactorily.


For measurements to be reproducible, it is recommended                A large number of small-diameter holes is preferable to a that the detection system be energy stabilized. Internally                few large-diameter holes.
===6. Products===
2. Research and Test Ractors


"seeded" NaI crystals that contain a radioactive source (typically 24'Am) to produce a reference energy pulse are                      1.1.2.2 Collimation for Total Plate Counting. For total commercially available. The detection system is stabilized                plate counting (see Section B. 1.3), the collimator opening is on the reference, and the amplifier gain is automatically                  circular with a diameter less than that of the NaI crystal.
===7. Tranpomrtation===
3. Fuels isid Materi- Fac;itisi a. O=upetiona! Meeilh Pubfb*ied guides will be revised periodicafly. as approprat


corrected to ensure that the reference energy and the rest                Furthermore, the collimator diameter and detector-to-plate of the spectrum remain fixed.                                              distance are chosen so that the field of view includes the entire fuel plate. (Note that in this more relaxed counting
====e. to accomn oal ====
    1.1.2 Gamma Ray Collimator                                            geometry, the viewing area may have to be isolated from nearby sources of the 185.7-keV gamma rays in the line of The detector collimator is intended to shield the detector            sight of the detector. This can be accomplished by shadow from radiation from all sources except those that are to be                shielding with small pieces of lead or tungsten.)
4. Environmenetal end Siting
measured. Thus the collimator shielding not only defines the front area of the detector crystal to be exposed but it                    1.1.3 Multiple Detectors also shields the sides and, if possible, the rear of the detec tor. The front opening of the collimator is designed to                        Several detectors may be used to shorten the measurement define the field of view appropriate for the measurement                  time. The detectors can be positioned to measure different technique to be employed. Once a measurement system is                    segments of a single fuel plate or several separate fuel plates  3/4 calibrated with a particular collimator configuration, that                simultaneously. In some cases it may be useful to sum the configuration must be maintained for all subsequent assays.                response from two detectors positioned on opposite sides Any change in the collimation system will necessitate                      of a plate to increase counting efficiency. In such cases, it is recalibration of the measurement system.                                  essential that the relative response of such detectors be known and checked at frequent intervals for continued
9. Antitrust Review comments and to reflect new ,ntormition or expoerine.
    1.1.2.1 Collimation for Scanning Techniques. To ensure                stability.


that the only gamma ray activity detected originates from a well-defined segment of the fuel plate, the detector is                    1.2 Scanning Techniques shielded from extraneous background radiations and collimated to define the plate area "seen" by the detector                    It is critical that the scanning apparatus for moving the crystal. The collimator consists of a disk of appropriate                plates relative to the detector provide a uniform and shielding material. A slit is machined through the center of              reproducible scan. The importance of a well-constructed the disk to allow only those gamma rays emitted within                    mechanically stable conveyor cannot be overemphasized.
5. Mot'earits and Plant Protection
1


the slit opening to strike the detector. The disk thickness is            Either the detector can be moved and the plate held station a minimum of six mean free path lengths to effectively stop                ary, or the plate can be moved past a fixed detector. If the all 185.7-keV gamma rays emitted from outside the field of                detector collimator field of view extends beyond the edges view. For more compact counting geometries, higher                        of the fuel plate, care must be exercised to maintain the density shielding materials (such as tungsten or lead) can be            detector-to-plate spacing within close tolerances to minimize used. The linear dimensions of other shielding materials                  errors caused by the resulting dependence of count rate on scale down according to the decrease in mean free path                    this spacing. This is especially important in the case of close length. 1                                                                  spacing, which is sometimes desirable to maximize the count rate. However, a superior collimator configuration The probability of detection for gamma rays emitted at                from this point of view would be one in which the field of the center of the collimator slit is greater than that for                view is filled with active material over a range of detector-to gamma rays emitted near the ends of the slit. This effect                  plate distances. In this case, the measured material acts as becomes increasingly important at small detector-to                      an area source for which the counting rate is nearly independ plate spacing, especially when scanning near the edge of a                ent of the detector-to-plate spacing. Therefore, in the
===0. Generaf===
    1 For the 185.7-keV gamma ray from 235U, a thickness equi            "sweeping spot scan" technique discussed in Section B. 1.2.2, valent to six mean free path lengths in lead is approximately              the spacing is not as critical a measurement parameter.


0.45 cm; in tungsten it is approximately 0.33 cm; and in iron it is approximately 4.9 cm.                                                    Various commercial conveying systems have been used and
intended portion of the gamma ray spectrum during measurements. Internally "seeded" Nal crystals which contain a radioactive source (typically Am-241) to produce a reference energy pulse art commercially available. The detection system is stabilized on the reference, and the amplifier gain is automatically cor- rected to assure that that energy and the rest of the spectrum remain fixed in position.
                                                                    5.38-2


found to be adequate. Such systems may significantly                  is then determined by averaging the results of sample spot reduce the cost of designing and building new scanning                measurements      of the  235 U content per unit area at a mechanisms. High-precision tool equipment such as milling            number of sites along the plate and multiplying this average
.1.2 Gamma Ray Collimator To ensure that the only gamma ray activity detected originates from a well-defined segment of the fuel plate, the detector is shielded from extraneous background radiations and collimated to define the area "seen" by the detector crystal. The collimator consists of a disk of appropriate shielding material.
*,  machines, lathes, and x-y scanning tables can be investigated.        value by the measured area of the fuel core. The radiograph Numerically controlled units offer additional advantages              of each plate is examined to ensure that the core filler is when they can be incorporated into a scanning system. This            uniform since nonuniformities would invalidate this type of is particularly true when an automated scanning system is            assay.


being developed.
A slit is machined through the center of the disk which will allow only those gamma rays emitted within the slit opening to strike the detector. The disk thickness is a minimum of six mean free path lengths to effectively stop all gamma rays emitted from outside the view area. To prevent gamma rays from striking the crystal around the edges of the collimator disk, the disk diameter exceeds the crystal diameter by at least twice the crystal depth.


The collimator shape and dimensions can be selected to Fuel plate core compacts may be sufficiently small to             provide compatible information on the uniformity of the permit total assay in a fixed-geometry counting system                fuel plate.
The probability of detection for gamma rays emitted at the center of the collimator slit is greater than that for gamma rays emitted near the ends of the slit. This effect becomes increasingly important at small detector-to- plate spacing, especially when scanning near the edge of a plate. To minimize this detection nonuniformity and to minimize the sensitivity to jitter, the detector-to-plate distance can be made large, especially with respect to the dimensions of the slit opening. As an alternative means of reducing the detection nonuniformity-across the slit, the slit opening can be divided into channels by inserting a honeycomb baffle into the slit or by fabricating the collimator by drilling holes through the disk in a pattern which ensures that each hole is surrounded by a minimum wall thickness of 0.2 mean free path length. A
7:0-cm-thick iron disk with holes less than 0.5 cm in diameter drilled in a pattern having 0.2 cm of wall between adjacent holes is one example of a collimator that would perform satisfactorily. A large number of small-diameter holes is preferable to a few large-diameter holes.


without scanning (see Section B.1.3). The scanning tech niques for fuel plates discussed in the following subsections        1.3 Passive Total Counting Techniques can also be used for core compacts when total fixed com pact counting is not possible.                                            A single passive gamma count of a fuel plate can be used to obtain the information of primary concern, namely the total 23SU content of the plate. The detector response in a
1.1.3 Multiple Detectors Several detectors may be used to shorten the mea- surement time. The detectors can be positioned to simultaneously measure different segments of a single fuel plate or to simultaneously measure additional fuel plates. In some cases it may be useful to sum the response from two detectors positioned on opposite sides of a plate to increase counting efficiency. In such cases it is essential that the response of such detectors be balanced-and checked at frequent intervals.
        1.2.1 Linear Total Scan                                            "wide-angle" counting geometry can be converted to grams of 2 3 5 U in the plate if the response with standard The detector collimation consists of a rectangular                fuel plates is known for the same counting geometry and if opening that extends across the width of the fuel plates              appropriate attenuation corrections are made with suitable beyond the edges of the uranium core contained within the            transmission sources.


plate cladding. Scanning the total plate is accomplished by starting the count sequence on the end of a plate and                      The detector collimator and detector-to-plate distance continuing to count until the entire length of plate has been          defines a field of view that (a) includes the entire fuel plate scanned.                                                             and (b) is isolated from other sources of radiation in the line of sight of the detector. Provide a measurement platform To ensure that gamma rays emitted anywhere across the            to facilitate the reproducible placement of the fuel plates, face of the fuel plate have an equal probability of being            transmission sources, detector, and collimator shielding in a detected, it is necessary that the diameter of the detector          standard measurement configuration.
1.2 SCANNING TECIINIQUES
It is critical that the scanningapparatds for ,'oviog t:,
plates relative to the detector provide a uniforn;.
reproducible scan. The importance of a well-constructed, mechanically stable conveyor cannot be overemphasized.


crystal exceed the plate width or that the detector be
Either the detector can be moved and the plate held stationary, or the plate can be moved past a fixed detector. Care must be exercised to maintain fl detector-to-plate spacing within close tolerance, minimize errors caused by the inverse-square dcpen(!
.*- positioned away from the plate.                                           Core compacts are also to be assayed in this way provided representative standards are used to calibrate the measure Use of the spot or circular collimator scan technique            ment for the geometry pertaining to these items.
of detection on distance. This is especially important in the case of close spacing, which is sometimes desirable to maximize the count rate. Various commercial conveying systems have been used and found to be adequate. Such.


eliminates or reduces to insignificance most of these edge effects.                                                                 Additional details on passive total sample counting and the associated attenuation corrections for assay of special
systems may significantly reduce the cost of de..,7!i, g and building new scanning mechanisms. High-precision tool equipment such as milling machines, lathes, and x-y scanning tables can be investigated. Numerically con- trolled units offer additional advantages when they can be incorporated into a scanning system. This is particu- larly true when an automated scanning system is being developed.
        1.2.2 Sweeping Spot Scan                                          nuclear materials are given in References 3 and 4.


If the collimator channel width is smaller than the fuel          1.4 Computer Control plate width, the viewing area (spot) can be swept across the plate as the detector scans along the length of the plate                Computer control of the plate scanning techniques can (Ref. 2). This scanning technique can be readily adapted to          greatly reduce the associated manpower requirements and scanning bowed plates through the use of a cam that is                improve measurement reproducibility. The computer can designed to maintain the detectoi-to-plate distance constant          be used to control data acquisition by accumulating counts over the entire fuel plate. The collimator channel dimensions        according to a predetermined scheme. Also, the computer can be selected to provide compatible information on the              can be used for data analysis, including background and uniformity of the fuel plate, which is frequently obtained            attenuation corrections and intermachine normaliza by comparing fixed (static) spot counts at a variety of              tion, calibration, error analysis, and diagnostic test measure locations to reference counts.                                        ments and analyses. Report preparation and data recording for subsequent analysis are also readily accomplished
Fuel plate core compacts may be sufficiently small permit total assay without scanning in a fixed-geometry counting system. The scanning techniques for fuel plates discussed in the following subsections can also be used for core compacts when total fixed compact counting is not possible.
        1.2.3 Sampled Increment Assay                                    through an appropriately designed computer-controlled system. Use of a computer can be of great value in many of When used in conjunction with radiographic dimensional            these functions for the total passive gamma counting measurements performed on all fuel plates, the 235 U                  technique as well.


content of a fuel plate can be measured by scanning the ends of each fuel plate and sampling the balance of the               2. INTERPRETATION OF MEASUREMENT DATA
1.2.1 Linear Total Scan The detector collimation consists of a rectangular opening which extends across the width of the fuel plates beyond the edges of the uranium core contained within the plate cladding. Scanning the total plate is accomplished by starting the count sequence on the end of a -plate and continuing to count until the entire length of plate has been scanned.
    plate. It is necessary to measure the dimensions of the fuel core loading radiographically through gamma ray scanning                  The raw measurement data from either a scanning or a
___ along the length of the plate or by spot-scanning the fuel          total passive counting technique can be distorted by several plate ends and measuring the distance between end spots              effects for which corrections should be made for accurate where the fuel loading stops. The 235 U content of the plate         assays. The three factors discussed below are the most
                                                                    5.38-3


important potential sources of measurement error that can            ing the clad thickness over the range of thicknesses to be give rise to significant misinterpretation of the data.                encountered in normal product variability.
To ensure that gamma rays emitted anywhere across the face of the fuel plate have an equal probability of being detected, it is necesary that the diameter of the detector crystal exceed the plate width or that the detector be positioned away from the plate.


2.1 Enrichment Variations                                                2.2.3 Core FillerAttenuation Licensees authorized to possess highly enriched uranium              Radiation intensity measurements may be made of are required to account for each element and isotope as              plates fabricated with different ratios of uranium to filler to prescribed in § 70.51. Under the conditions detailed in this          show the effects of this type of attenuation. If significant guide, the 2 3 5 U content of individual plates is measured.          effects are noted, plates can be categorized by core composi To determine the total uranium content of each plate, the            tion characteristics and the assay system can be independ
Use of the spot oi cizcalar collimator scan technique eliminates or reduces to insignificance most of these edge effects.
  235U enrichment of the core filler must be known from ently calibrated for each category of fuel plates.


separate measurements.
1.2.2 Sweeping Spot Scan (Ref. 2)
If the collimator channel width is smaller than the fuel plate width, the viewing area (spot) can be swept across the plate as the detector scans along the length of the plate. This scanning technique can be readily adapted to scanning bowed plates through the use of a cam which is designed to maintain the detector-to-plate distance constant over the entire geometry of the fuel plate. The collimator channel dimensions can be selected to provide compatible information on the uniformity of
5.38-2


2.2.4 Attenuation Corrections Enrichment variations may also alter the radiation background in the gamma ray energy region of interest and                  When the thickness of the core and cladding and the cause fluctuations in the 2 3 5 U assay. The 2 3 8 U decays by        composition of the core material are known, an attenuation alpha particle emission to 2 3 4 Th. The 2 3 4 Th then decays          correction can be calculated and applied to improve the by    beta particle emission with a half-life of 24.1 days to          accuracy of the assay. These corrections must also be
the fuel plate which is frequently obtained by comparing fixed (static) spot counts at a variety of locations to reference counts.
  2 34 Pa which, in turn, decays by beta particle emission to applied to the assays of the standards in the calibration
  234U. Approximately 1 percent of the 234Pa decays are        procedure. Ultrasonic gauging may provide such a measure followed by high-energy (e.g., 1001 keV, 766 keV) gamma                if the metallographic zones within the plate are sufficiently rays. These gamma rays frequently lose energy through                  defined to provide a detectable interface.


Compton scattering and may appear in the 185-keV spec tral region. It is important to note that activity from                    The alternative attenuation correction can be based on a
1.2.3 Sampled Increment Assay When used in conjunction with radiographic dimen- sional measurements performed on all fuel plates, the U-235 content of a fuel plate can be measured by scanning the ends of each fuel plate and sampling the balance of the plate. It is necessary to measure the dimensions of the fuel core loading radiographically, through gamma ray scanning along the length of the plate, or by spot scanning the fuel plate ends and measuring the distance between end spots where the fuel loading stops. The U-235 content of the plate is then determined by averaging the results of sample spot measurements of the U-235 content per unit area at a number of sites along the plate and multiplying this average value by the measured area of the fuel core. The radiograph of each plate is examined to ensure that the core filter is uniform.
234Pa may be altered by disturbing the equilibrium between
23 5                                                                  micrometer measurement of the total thickness of each U and 2 3 4 Th, as frequently occurs in uranium chemical plate. The clad thickness of a plate is estimated by subtract conversion
238 processes. The interference due to variations in        ing the mean core thickness of the product plates, which U daughter activity becomes less important as the is determined by periodically sampling product plates and enrichment of 2 3 5 U increases. At enrichment levels above            cutting a cross section to permit visual measurement of clad
90 percent, this problem can essentially be ignored.                   and core thickness.


2.2 Radiation Attenuation                                                  As long as the gamma ray attenuation corrections are computed on the basis of declared component thicknesses Attenuation of gamma radiation mayrange from complete              and composition (or on the basis of occasional measure absorption of the radiation by the intervening material to            ments of these quantities), unnoticed plate-to-plate fluctua partial energy loss of the emitted radiation through scatter          tions in these parameters will undermine the accuracy of ing processes. Both effects reduce the number of full-energy          the assays. A far more reliable approach to the application gamma ray events that are detected. Gamma rays from                  of attenuation corrections is to measure the gamma ray
The collimator shape and dimensions can be selected to provide compatible information on the uniformity of the fuel plate.
235 U are attenuated in the uranium, in the cladding, and in transmission property of each plate (standard as well as the inert material that may be added with the uranium to             unknown) as it is being assayed. This approach increases the form the core of the fuel plate. Through well-controlled              complexity of the assay procedures, but poses the further product tolerance limits, each of these potential sources of          advantages of ýl)rendering the calibration dependent signal variability can be controlled to permit accurate                only upon the 35U loading of the standard plates and accountability assays.                                                independent of other plate properties and (2) making the sample plate assays insensitive to possible fluctuations in
    2.2.1 Self-Attenuation                                            cladding thicknesses and core composition and thickness.


General discussions of gamma ray attenuation corrections The photon attenuation coefficient of uranium for                 accompanying passive assays are given in References 3, 4, gamma ray energies corresponding to 2 3 5 U emissions is              and 6. Specific details of a correction procedure for Materi quite large (Ref. 5). Small changes in uranium density                als Testing Reactor (MTR) fuel plates are given in the resulting from increased fuel loading or from variations in            appendix to this guide.
1.3 COMPUTER CONTROL
The reproducibility of measurements can be im- proved and the measurement time per fuel plate can be reduced by using a computer to control the fuel plate scanning operation. The computer can be used to control data acquisition by accumulating counts ac- cording to a predetermined scheme. Also, the computer can be used for data analysis, including background corrections and intermachine normalization, calibration, error analysis, and diagnostic test measurements and analyses. Report preparation and data recording for subsequent analysis are also readily accomplished through an appropriately designed computer-controlled system.


the manufacturing process can therefore significantly change the number of gamma rays that escape from the fuel              2.3 Interfering Radiations plate.
2. INTERPRETATION OF MEASUREMENT DATA
The three factors discussed below may give rise to significant errors in interpreting measurement data.


As noted in Section B.2.1 of this guide, an internal
2.1 ENRICHMENT vARIATIONS
    2.2.2 CladdingAttenuation                                        background variation may arise from changes in the amount of 2 3 8 U present in a fuel plate or from changes in the ratio Small variations in cladding thickness may cause signif          of 234Th to 238U resulting from fuel manufacturing icant variations in attenuation. These variations in attenua          processes. Fluctuations in the internal background cause tion can be measured by a simple gamma ray absorption test            the response of the unknown items to be different from the using thin sheets of cladding material as absorbers and vary-        calibration standards, thereby creating a fluctuating measure-
Licensees authorized to possess highly enriched uranium are required to account for element and isotope as prescribed in §70.51. Under the conditions detailed in this guide, the U-235 content of individual plates is measured. To determine the total uranium content of each plate, the U-235 enrichment must be known from separate measurements.
                                                                5.38-4


ment bias. In addition, some discrete gamma ray inter                  more accurate measurements of the content of typical ferences may be present at energies near 185.7 keV. For                fuel plates (see Regulatory Postion 4 of this guide). Guid further information on these possible interferences, see                ance on methods to relate this assay to the national measure Reference 7. Both the background and discrete gamma ray               ment system and to reconcile verification measurements is interferences are generally of minor importance, but they              addressed in Regulatory Guide 5.58, "Considerations for can be corrected for by measurement of additional regions              Establishing Traceability of Special Nuclear Material of the gamma ray spectrum. Pertinent nuclear data for such              Accounting Measurements."
Enrichment variations may alter the radiation back- ground in the gamma ray energy region of interest.
      measurements are available in Reference 1.


==C. REGULATORY POSITION==
Uranium-238 decays by alpha-particle .emission to Th-234. Thorium-234 then decays by beta-particle emis- sion with a half-life of 24.1 days to Pa-234 which, in turn, decays by beta-particle emission to U-234. Ap- proximately 1% of the Pa-234 decays are followed by high-energy (e.g., 1001 keV, 766 keV) gamma rays.
Other interfering radiations may come from external sources, from fuel plates awaiting assay, or from nearby                    The content and distribution of 23 5U in high-enrichment radiation sources used for other measurements. This is not              uranium plates can be measured through the gamma ray expected to be a major problem and can be controlled                    assay methods discussed in this guide. Combining this through (1) removing radiation sources, (2) shielding the              measurement with the results of an independent measure detectors, and (3) monitoring the background at frequent                ment of the 2 3 SU enrichment enables the total uranium intervals.                                                             content of the fuel plates to be determined. The factors presented below should be taken into consideration for this
      3. CALIBRATION AND VERIFICATION                                        assay method to be acceptable to the NRC staff.


3.1 Initial Operations                                                 
These gamma rays frequently lose energy through Compton scattering and may appear in the 185-keV
spectral region. It is important to note that activity from Pa-234 may be altered by disturbing the equilibrium between U-235 and Th-234, as frequently occurs in uranium chemical conversion processes. The interference due to variations in U-238 daughter activity becomes less important as the enrichment of U-235 increases. At enrichment levels above 90%, this problem can essen- tially be ignored.


===1. MEASUREMENT SYSTEM===
2.2 RADIATION ATTENUATION
          Calibration and the verification of assay predictions is an        1.1 Gamma Ray Measurement System ongoing effort where performance is periodically monitored and the calibration relationship is modified to improve the
The number of U-235 gamma rays which escape from the fuel plate (and are thus available for detection)
                                                                                  1.1.1 Gamma Ray Detector accuracy of assay predictions. During initial operations, two means of basing preliminary calibrations are appropriate.                 Thallium-activated sodium iodide [NaI(TI)] scintillation detectors are recommended for this assay applicatio
without losing energy depends on the characteristics of the fuel plate core and cladding. Gamma rays from U-235 are attenuated in the uranium, in the cladding, and in the inert material that may be added with the uranium to form the core of the fuel plate. Through well-controlled product tolerance limits, each of these potential sources of signal variability can be controlled to permit accurate accountability assays.


====n. When====
2.2.1 Self-Attenuation The uranium photon attenuation coefficient for gamma ray energies corresponding to U-235 emissions is quite large (Ref. 3). Small changes in uranium density resulting from increased fuel loading or from variations in the manufacturing process can significantly change the number of gamma rays which escape from the fuel plate.
          3.1.1 Foil Calibration Technique                                    more than one detector is to be incorporated into the measurement system, the performance characteristics of Methods for calibrating scanning systems for high                  the detectors should be matched as closely as possible, and enrichment uranium fuel plates through the assay of the relative response of the detectors should be checked prepared clad uranium foils are described in Reference 2.


periodically to verify continued stability of the system. The These methods may be used in place of or in addition to the             diameter of the crystal should be larger than the projected technique described in the following subsection.                        view onto the crystal face through the collimator channel.
2.2.2 Gadding Attentuation Small variations in cladding thickness may cause significant attenuation variations. Variations in cladding attenuation can be measured by a simple gamma ray absorption test using thin sheets of cladding material as absorbers and varying the clad thickness over the range of thicknesses to be encountered in normal product variability.


A crystal thickness of 1/2 to 1 in. (13 to 25 mm) is recom
2.2.3 Core Friler Attenuation Radiation intensity measurements may be made of plates fabricated with different ratios of uranium to filler to show the effects of this type of attenuation. If significant effects are noted, plates can be categorized by core composition characteristics and the assay system can be independently calibrated for each category of fuel plates.
          3.1.2 FabricatedCalibrationPlates                                  mended. The crystal should contain an internal seed that is doped with a suitable alpha emitter (typically 2 4 1 Am) to Calibration standard fuel plates can be fabricated using          produce a reference energy peak for spectral stabilization.


special  precautions to ensure that the amounts of uranium,
5.38-3
      2 35 U, inert matrix, and cladding are accurately measured            The seed should produce approximately 1,000 counts per second at the reference energy.


and that these parameters fall within manufacturing toler ances for product plates.
2.2.4 Attenuation Corrections When the thickness of the core and cladding of each plate is known, an attenuation correction can be applied to improve the accuracy of the assay. Ultrasonic gauging may provide such a measure, provided the metallo- graphic zones within the plate are sufficiently defined to provide a detectable interface.


1.1.2 Collimatorand Detector Shielding
The alternative attenuation- correction is based on a micrometer measurement of the total thickness of each plate. The clad thickness of a plate is estimated by subtracting the mean core thickness of the product plates, which is determined by periodically sampling product plates and cutting a cross section to permit visual measurement of clad and core thickness.
      3.2 Routine Operations The collimator should be fabricated of appropriate The performance of the assay system is periodically               gamma ray shielding material such as iron, lead, or tungsten.


monitored to ensure that the response of the assay system The shielding should completely surround the detector and has not shifted since its last calibration. Control limits for photomultiplier assembly and should be sufficiently thick acceptable performance can be established for the response to completely shield the detector from extraneous radiation.
2.3 INTERFERING RADIATIONS
As noted in Section B.2.1 of this guide, an internal background variation may arise from changes in the amount of U.238 present in a fuel plate or from changes in the ratio of Th-234 to U-238 resulting from fuel manufacturing processes. Fluctuations in the internal background cause the response of the unknown items to be different from the calibration standards, thereby creating a measurement bias. Such interferences can be compensated by measuring additional regions of the gamma ray spectrum.


to an appropriate working standard. The control chart of the responses to the working standard can be checked for indications of short-term instrument drift or malfunction.
Other interfering radiations may come from external sources, from fuel plates awaiting assay, or from nearby radiation sources used for other measurements. This is not expected to be a major problem and can be controlled through
(1) removing radiation sources,
(2) shielding the detectors, and (3) monitoring the back- ground at frequent intervals.


1.1.3 Electronic Apparatus The control chart can also be analyzed to detect long term shifts -within the measurement-to-measurement All electronic systems should be powered by filtered, control limits that may be corrected by recalibrating the highly regulated power supplies. The ambient temperature system. In general, however, it is important that observed and humidity in the vicinity of the scanning system should instrument drifts and performance changes be investigated be controlled so that permitted fluctuations do not signifi and remedied rather than compensated for by recalibration.
3. CALIBRATION AND VERIFICATION
3.1 INITIAL OPERATIONS
Calibration and the verification of assay predictions is an ongoing effort where performance is periodically monitored and the calibration relationship is modified to improve the accuracy of assay predictions. During initial operations, two means of basing preliminary calibrations are appropriate.


cantly affect the assay measurements. All electronic circuitry in signal-processing components should feature temperature To ensure that the calibration remains valid during compensation. Residual sensitivity to fluctuations in the
3.1.1 Foil Calibration Technique Methods for calibrating scanning systems for high- enrichment uranium fuel plates through the assay of prepared uranium and clad foils are described in Refer- ence 2. This method may be used in place of or in addition to the -technique described in the following subsection.
"-' normal operations and that accuracy estimates are rigorously ambient environment should be tested and monitored justified, assay predictions are periodically compared with periodically.


5.38-5
uranium, U-235, inert matrix, and cladding are accu- rately measured and that these parameters bracket the nominal range of product plates anticipated to fall within manufacturing tolerances.


The capability for multichannel gamma ray pulse height                monitoring the variations in plate cladding thicknesses and analysis with cathode ray tube spectral display should be                core composition and thickness. For further detail on such provided. Signal-processing electronics capable of stabilizing            corrections, see References 4 and 6 as well as the appendix on the reference energy peak produced by the alpha-emitter                to this guide.
3.2 ROUTINE OPERATIONS
The performance of the assay system is periodically monitored to ensure that the performance of the assay system has not shifted since its last calibration. Control limits for acceptable performance can be established for the response to an appropriate working standard. The control chart of the responses to the working standard can be checked for indications of short-term instrument drift or malfunction. The control chart can also be analyzed to detect long-term shifts within the measurement-to-measurement control limits that may be corrected by recalibrating the system. Severe changes in instrument performance are investigated promptly and their causes remedied.


doped seed should be provided to stabilize the energy spectrum.                                                                  2.3 Radiation Interferences
To ensure that the calibration remains valid during normal operations and that accuracy estimates are rigorously justified, assay predictions are periodically compared with more accurate measurements of the content of typical fuel plates (see Section C.4 of this guide). Guidance on methods to relate this assay to the national measurement system and to reconcile verifi- cation measurements will be addressed in separate regulatory guides.*
1.2 Measurement System                                                      A graphic record of an acceptable (reference) gamma ray spectrum display (i.e., free of interferences and exhibiting Plate scanning should be accomplished by one of the                  nominal background) should be prepared. When radioactive three techniques discussed in Section B.1.2 of this guide.               interference may be encountered, the assay spectrum With these techniques, a mechanically sound, highly repro                should be compared at appropriate intervals to the reference ducible, automated scanning system should be employed.                    spectrum for indications of interference. Background When more than one scanning system is employed, the                      radiation should be measured periodically during each assay responses of each system should be normalized so                    operating shift.


that each instrument provides consistent results. Verification data to estimate the bias for each assay system should be                  3. MEASUREMENT CALIBRATION AND CONTROL
==C. REGULATORY POSITION==
obtained with the same standard plate.
The content and distribution of U-235 in high- enrichment uranium plates can be measured through the gamma ray assay methods described in this guide.


During initial operations, the assay system should be If a passive total counting technique is used, a stable,              calibrated either by the foil calibration method or with carefully constructed measurement platform should be                      specially prepared sample fuel plates as described in Sec employed to ensure the achievement of a reproducible                      tion B.3.1 of this guide. Instrument response to appropriate measurement geometry.                                                      working standards should also be checked periodically to verify the continued stability of the assay system calibration.
Combining this measurement with the results of an independent measurement of the U-235 enrichment enables the total uranium content of the fuel plates to be measured. The factors presented below should be taken into consideration for this assay method to be acceptable to the Regulatory staff.


1.3 Computer Control
I. MEASUREMENT SYSTEM
                                                                          4. SOURCES OF VARIATION AND BIAS
1.1 GAMMA RAY MEASUREMENT SYSTEM
    A dedicated minicomputer to control data acquisition, calibration, diagnostic testing, and report preparation                    4.1 Random Assay Standard Deviation Estimation should be employed for fuel plate assay operations.
1.1.1 Gamma Ray Detector A
thallium-activated sodium iodide scintillation detector or series of detectors is recommended for this assay application. When more than one detector is to be incorporated into the scan system, the performance characteristics of the detectors should be matched. The diameter of the crystal should be larger than the projected view onto the crystal face through the
*For example, regulatory guides related to measurement quality amurance and calibration of nondestructive a&say systems are being developed.


A replicate assay program should be established to
3.1.2 Fabricated Calibration Plates Calibration standard fuel plates can be fabricated using special precautions to ensure that the amounts of
2. MEASUREMENT INTERPRETATION                                            generate data for the evaluation of the random assay standard deviation during each material balance period.
5.38-4


I--
collimator channel. The thickness of the crystal should hc noi more than onc inch. The crystal should contain an internal cesium iodide seed which is doped with a suilable alpha-emittcr for spectral stabilization. The seed should produce approximately 1,000 counts per second at the reference energy.
2.1 Enrichment Variations                                                During each bimonthly interval, a minimum of fifteen plates should be selected for replicate assay. The second Procedures should be developed to ensure that the                      assay of each plate selected for replicate assay should be enrichment of the plates being scanned is known through                    made at least four hours after the first assay. Replicate separate measurements. Fuel plates generally satisfy the                  assay data should be collected and analyzed at the end of gamma ray penetrability criteria for quantitative 2 3 SU                  the material balance period. The single-measurement assay; they do not satisfy the criteria for nondestructive2                standard deviation of the replicate assay differences should enrichment measurement through gamma ray spectrometry.                    be computed as described in Reference 8. Replicate measure Facilities processing more than one uranium enrichment                    ments should be made under the same conditions as routine should maintain strict isotopic control and characterize the              measurements, performed throughout the production run, enrichment through appropriate measurement methods.                        and checked for consistency. If the probability distributions for the data are not different, pooling of results from
2.2 Attenuation Corrections                                              previous inventory periods can improve the random assay standard deviation estimates.


If computed attenuation corrections are used, attenuation variations arising from plate-to-plate changes in core thick              4.2 Calibration Standard Deviation Estimation ness, core composition, and clad thickness should be determined over the range of product tolerance specifica                      The calibration standard deviation associated with the tions. When such variations cause the assay standard devia                assay of all fuel plates assayed during each calibration tion to exceed the standard deviation realized without the                 period throughout the material balance period can be variations by 33 percent or more, procedures should be                     determined through one of the procedures presented below.
1.1.2 Collimator A collimator should be fabricated of appropriate gamma ray shielding material such as iron, lead, or tungsten. The shielding should completely surround the detector and photomultiplier assembly and should be sufficiently thick to completely block extraneous radi- ations from the detector. The response variation from the center of the collimator opening to its edge should be less than 1%.
1.1.3 Electronic Apparatus All electronic systems should be powered by filtered, highly regulated power supplies. The ambient tempera- ture and humidity in the vicinity of the scanning system should be controlled so that permitted fluctuations do not significantly affect the assay measurements. All electronic circuitry in signal-processing components should feature temperature compensation. Residual sen- sitivity to fluctuations in the, ambient environment should be tested and monitored periodically.


implemented to measure and apply a correction to the                      These methods are discussed in detail in ANSI 15.20-1975, assay of each plate. It should be noted that routine measure              "Guide to Calibration of NDA Systems," 3 and in Regulatory ment of attenuation corrections for each plate is recom                  Guide 5.53, "Qualification, Calibration, and Error Estima mended since such a procedure will remove the necessity of                tion Methods for Nondestructive Assay" (a proposed revision to this guide has been issued for comment as Task
The capability for multichannel gamma ray pulse height analysis with cathode ray tube spectral display should be provided. Signal-processing electronics capable of stabilizing on the alpha radiations emitted within the doped cesium iodide seed should be provided to stabilize the energy spectrum.
    2 Criteria for gamma ray uranium enrichment measurements are          SG 049-4).
given in Regulatory Guide 5.21, "Nondestructive Uranium-235                    3 Available from the American National Standards Institute, Enrichment Assay by Gamma Ray Spectrometry." A proposed revision to this guide has been issued for comment as Task SG 044-4.        1430 Broadway, New York, New York 10018.


5.38-6
1.2 SCANNING SYSTEM
A mechanically sound, highly reproducible scanning system should be employed. Scanning should be accom- plished by one of the three techniques discussed in Section B. 1.2 of this guide.


To estimate the standard deviation arising from the            random assay standard deviation associated with the less calibration procedure, the calibration should be based on a        accurate measurement method. To determine precisely the least-squares fitting of the calibration data to an appropriate      bias in the nondestructive assay measurement, the fuel model, then part of the calibration standard deviation              plates selected for comparative measurements should be
1.3 COMPUTER CONTROL
*-*-    can be derived using the residual mean square. The standard        randomly selected but should span tile range of 2 3 5 U
A dedicated minicomputer to control data acquisi- tion, analysis, calibration, diagnostic testing, and report preparation -should be employed for this assay appli- cation.
        deviation for the calibration standards includes the standard      contents encountered in normal production. The fuel deviation of the reference values for the calibration stand        plates could have been selected from those rejected from ards. See ANSI 15.20-1975.                                          the process stream for failing to meet quality assurance requirements. Each plate should be repeatedly assayed to To ensure the validity of the measurements, the stable          reduce the random assay relative standard deviation (coeffi performance of the instrument should be monitored and                cient
                                                                            2 35 of variation) to less than 10 percent. To determine its normalized through the response to appropriate working                    U and total uranium content, the plate should be standards that are assayed at frequent intervals. The fre            completely dissolved and the resulting solution should be quency for assaying working standards should be deter                analyzed by high-accuracy assay procedures such as chemi mined through testing but should -not be lower than one              cal and mass-spectrometric analyses.


test during each two-hour assay interval for spot response stability and one full scan test during each operating shift.           For one material balance period during the initial For total passive counting techniques, assay of working              implementation of this guide, a product fuel plate should standards should take place during each four-hour assay              be randomly selected twice each week for an accuracy interval during each operating shift. Indications of shifting        verification measurement. Following this initial implementa instrument performance should be investigated and the cause          tion period, facilities manufacturing 100 or more fuel plates should be remedied. The instrument should then be recali            per week may reduce the verification frequency to one brated to ensure the validity of subsequent measurements.            plate per week and pool the verification data (provided the two distributions can be tested to show no differences) for In order to ensure that the calibration standards continue      two consecutive material balance periods. Low-throughput to adequately represent the unknown fuel plates, key                facilities manufacturing less than 100 plates per week production parameters that affect the observed response            should verify at least 4 plates per material balance period should be monitored through separate tests. (If transmis            through the procedures described above. At the close sion corrections are being measured for each plate assayed,          of each material balance period, data should generally be the monitoring of plate parameters is less critical for assay        pooled (if allowable) to include the 15 most current data accuracy.) Data should be compiled and analyzed at the              points. However, if the data are demonstrably stable over close of each material balance period. When a production            longer periods, using additional data points from previous S      parameter shifts from previously established values, the            compatible results is one method of reducing the random impact of the shift on the response of the assay instrument          assay standard deviation estimate.
1.4 MULTIPLE SCANNING ASSAY SYSTEMS
When more than ornc scanning system is employed, assay response should be normalized so that each instrument provides consistent results. Verification data to establish the systeniatic assay error for each assay system should be obtained with the same plate.


should be determined through an appropriate experiment or calculation (Ref. 9). A bias correction should be deter              Two methods are presented for estimating the bias.
2. MEASUREMENT INTERPRETATION
2.1 ENRICHMENT VARIATIONS
Procedures should be developed to ensure that the enrichment of the plates being scanned is known through separate measurements. Fuel plates generally satisfy the gamma ray penetrability criteria for quantita- tive U-235 assay; they do not satisfy the criteria for nondestructive enrichment measurement through gamma ray spectrometry.* Facilities processing more than one uranium enrichment should maintain strict isotopic control and characterize the enrichment through appri.,-
priate measurement methods.


mined and applied to all items assayed from the point of             When the 23SU content of the plates assayed using a common the parameter change. The variance of the bias estimate              calibration relationship varies over a range of +/-5 percent or should be combined with the variance due to the calibra              more of all plate loadings, the bias should be estimated by tion procedure. When the bias exceeds 3 percent of the              Method No. 1. When plate loadings are tightly clustered plate contents in a single material balance period, when a          about a nominal value, the bias should be estimated by trend of .1.5 percent or more is observed in three consecutive      Method No. 2.
2.2 ATTENUATION CORRECTIONS
Attenuation variations arising from plate-to-plate changes in core thickness, composition,-and clad thick- ness should be determined over the range of product tolerance specifications. When such variations cause the assay error to exceed the error realized without the variations by 50% or more, procedures should be implemented to measure and apply a correction to the assay of each plate.


material balance periods, or when the standard deviation in the estimated bias is sufficient to increase the standard error Method No. 1. At the close of the reporting period, the (i.e., twice the standard deviation) of the assay above              assay value for each plate is plotted against the verified
2.3 RADIATION INTERFERENCES
      0.5 percent, new calibration standards should be obtained            quantity. The verification data plot is examined for indica and the scanning measurement system should be recalibrated.          tions of nonlinearity or obvious outlier data. Anomalous indications should be investigated and remedied. Further As a further check on the continued validity of the              details on handling outlier data are contained in Regulatory calibration standards, a program to introduce new calibration Guide 5.36, "Recommended Practice for Dealing with standards periodically should be implemented. A minimum Outlying Observations." The comparison data should of one new calibration standard fuel plate should be intro          be analyzed as described in Regulatory Position 7.3 of duced during each six-month period.                                  Regulatory Guide 5.53, "Qualification, Calibration, and Error Estimation Methods for Nondestructive Assay." A
A clear plastic template which shows an acceptable spectrum display should be prepared. When radioactive interference may be encountered, the assay spectrum should be compared at appropriate intervals to the reference spectrum for indications of interference. Bick- ground radiation should be measured periodically during each operating shift.
      4.3 Bias Estimation                                                  proposed revision to this guide has been issued for comment as Task SG 049-4.


When two sets of measurements are made on each of a series of items and the accuracy of one of the methods used Method No. 2. When all plates contain essentially the is considerably better than the other, the corresponding            same 235U content, the difference in the mean content estimates can be compared to establish an estimate of bias values should be tested against zero as an indication of bias,
3. MEASUREMENT CALIBRATION
--    between the measurement methods and to estimate the and the standard deviation associated with an inventory of
During initial operations, the assay system should m, calibrated either by the foil calibration method or with specially prepared sample fuel plates as described mn Section B.3.1 of this guide.
                                                                    5.38-7


plates should be estimated as the standard deviation of the      compact. The fuel plate should carry an identification mean difference. For individual plates, the standard devia      corresponding to the compact identification.
4. RANDOM AND SYSTEMATIC ASSAY ERRORS
4.1 RANDOM ERROR ESTIMATION
A replicate assay program should be established to generate data for the evaluation of random assay errOrs during each material balance period. During each bi- monthly interval, a minimum of fifteen plates should be selected for replicate assay. The second assay of each plate selected for replicate assay should be made at least four hours after the first assay. Replicate assay diffet- ences should be collected and analyzed at the end (if the
*Criteria for uranium gamma ray enrichment measurenicTi*r.


tion should be estimated as the standard deviation of a single measurement.
arff given in Regulatory Guide 5.21, "Nondestructrvr ltiranwurn.235 Enrichment Assay by Gamma Ray Spectromelr\\*
"
5.38-5


2. Each fuel plate should be radiographically examined
material balance period. The single-measurement stan- dard deviation of the relative replicate assay differences should be computed as described in Reference 4.
5. CORE COMPACT ASSAY                                            to ensure that the entire compact has been encapsulated.


Final product assay in high-enrichment fuel plate manu facturing can also be accomplished through assaying each            3. Each fuel plate should be checked with a gamma ray core compact following the procedures detailed in this          probe to ensure qualitatively that the plate core is uranium guide and the following supplemental criteria:                  of the normal product enrichment.
4.2 SYSTEMATIC ERROR ESTIMATION
The systematic error associated with the assay of all fuel plates fabricated during a material balance period should be determined through one of the procedures*
presented below.


1. Each core compact should carry a unique identifica          4. Calibration and error evaluation should follow the tion. Accountability records should be created for each          procedures for fuel plate assay.
4.2.1 Propagation through the Calibration Function To estimate the systematic assay error through the calibration function, the calibration should be based on the regression analysis of an appropriate function to the calibration data. Uncertainties in the reference values of the calibration standards should be factored into the fit, and the errors propagated as demonstrated in Reference
5.


5.38-8
To ensure the validity of the predictions, the stable performance of the instrument should be monitored and normalized through the response to appropriate working standards which are assayed at frequent intervals. The frequency for assaying working standards should be determined through testing, but should not be lower than one test during each two-hour assay interval for spot response stability and one full scan test during each operating shift. Indications of shifting instrument perfor- mance should be investigated and remedied, and the instrument should be recalibrated to ensure the validity of subsequent measurements.


APPENDIX
In order to ensure that the calibration standards continue to adequately represent the unknown fuel plates, key production parameters which affect the observed response should be monitored through separate tests. Data should be compiled and analyzed at the close of each material balance period. When a production parameter shifts from previously established values, the impact of the shift on the response of the assay instrument should be determined through an appropriate experiment or. calculation (Ref. 6). A bias correction should be determined and applied to all items assayed from the point of the parameter change. The uncertainty in the bias should be combined with the systematic error predicted through the calibration function. When the bias exceeds 3% of the plate contents in a single material balance period, when a trend of 1.5% or more is observed in three consecutive material balance periods,
                                        SAMPLE ATTENUATION CORRECTION BY
,)r when the uncertainty in the observed bias is sufficient to increase the limit of error of the assay above 0.5%,
                                    TRANSMISSION MEASUREMENT FOR MATERIALS
new calibration standards should be obtained, and the scanning system should be recalibrated.
                                            TESTING REACTOR FUEL PLATES
1. BACKGROUND                                                      where lic is the mass absorption coefficient of the cladding at 185.7 keV (for aluminum, pc = 0.126 cm 2/g) and pc is Gamma ray assay data are subject to distortions due to        the cladding density (for aluminum, pc = 2.7 g/cm 3 ). If the the attenuation of the gamma ray flux by the intervening          cladding thickness varies by as much as 10 percent, the sample material and sample container. The data must be            corresponding variation in Tc will be only 0.2 percent. Thus corrected for this effect or the amount of nuclear material       an assumption of invariant cladding attenuation for a being assayed will be underestimated. The measured inten          particular type of fuel plate will contribute very little to the sity I for the 185.7-keV gamma radiation from a Materials          assay variance when the constant cladding attenuation Testing Reactor (MTR) fuel plate is related to the 23SU            correction is applied. One then determines Tc for the fuel content MU of the fuel plate by:                                  plates from careful measurement of the cladding thickness and application of Equation 4.


MU = kI/C                                (1)
As a further check on the continued validity of the calibration standards, a program to periodically intro- duce new calibration standards should be implemented.
                                                                      Under the above assumption, one can then determine where k is a calibration constant that includes effects such      the transmission of the core material TU from the measured as detector efficiency, counting geometry, and nuclear            total plate transmission T, knowledge of Tc, and Equation 3.


properties of uranium. The factor C is the correction factor      The attenuation correction factor in Equation 1 is then that adjusts the raw data for the attenuation of the              given by (see References 3 and 4):
*These methods will be discussed in detail in a regulatory guide
18,5.7-keV gamma ray by the plate cladding and core materia
,n preparation entitled "Calibration and Error Estimation Pr:ccduir"s fcr Nondestructi-e Asay."
A minimum of one new calibration standard fuel plate should be introduced during each six-month period.


====l. In TU====
4.2.2 Comparative Evaluation When two measurements are made on each of a series of items and the accuracy of ono of the methods used is considerably greater than the other, the corresponding predictions can be compared to establish an estimate of bias between the measurement methods and to estimate the error. associated with the lens-accurate measurement method. To precisely determine the systematic error in the nondestructive assay, the fuel plates selected for comparative measurements should be randomly selected but should span the range of U-235 contents en- countered in normal production. The selected fuel plates may be rejected from the process stream for failing to meet quality assurance requirements. Each plate should be repeatedly assayed to reduce the random asay error to less than
                                                                                                                              (5)
10% of the estimated or previously established systematic error. To determine its U-235 and total uranium content, the plate should be completely dissolved and the resulting solution should be analyzed by high-accuracy chemical and mass spectrometric pro- cedures.
    Determination of this attenuation correction factor can                             Tc (1- TU)
be accomplished using an external gamma ray source.


(Ideally, this should be a 23 5U source. For details on how       
For one material balance period during the initial implementation of this guide; a product fuel plate should be randomly selected twice each week for an accuracy verification measurement. Following this initial implementation period, facilities manufacturing 100 or more fuel plates per week may reduce the verification frequency to one plate per week and pool the verifi- cation data for two consecutive material balance periods.


===2. IMPLEMENTATION===
LoW-throughput facilities manufacturing lesa than 100
to use a transmission source with a gamma ray energy dif ferent from that measured in the assay, see Reference 4.)          2.1 The Scanning Techniques The radiation from this source is detected after it passes through the fuel plate, and that transmitted gamma inten              A small transmission source should be placed behind the sity I I is compared with the source intensity with no plate      fuel plate as shown in Figure 1. The transmission correction present 10 to obtain the gamma ray transmission T through          must be measured and applied at each scan point so that the plate materials:                                              nonuniformities in core composition within a plate can be corrected for. The transmission of the plate T(i) is measured T = 11/1 0                              (2)      at each scan point i by determining (1) the plate count rate with the transmission source shielded I(i) (Figure 1A),
plates per week should verify at least 4 plates per material balance period through the procedures de- scribed above. At the close of each material balance period, data should be pooled to include only the 15 most current data points.
    This total plate transmission can be subdivided as            (2) the total counting rate of the plate and unshielded follows:                                                          transmission source IT(i) (Figure 1B), and (3) the transmis sion source count rate with no intervening plate 10 (i)
                T=TT 2                                  (3)      (Figure 1C).
                    U c T(i) = [IT(i) - I(i)l /1o(i)                (6)
where Tc is the gamma ray transmission through one thickness of the plate cladding, and TU is the transmission            If the transmission source is a small localized 2 35 U
through the core material. (The same cladding thickness on        source, a plate assay with the attenuation correction will both sides of the plate is assumed.)                              require two scans: one to get the I (i) values and one to get the I(i) values. (The quantity IoTi) will be constant at The gamma ray transmission through a plate is dominated        all scan points and can be measured at a separate time.) If by the effect of the core material (i.e., TU < Tc), so it is      the transmission source is another fuel plate that remains convenient to treat the cladding transmission Tc as a              stationary with respect to the plate being assayed, the I0 (i)
constant. Furthermore, variations in the core composition          must be measured by scanning the transmission source will cause more drastic fluctuations in the gamma ray              fuel plate. That is, an unattenuated transmission source attenuation than the small variations in the cladding thick        plate intensity 10 (i) must be measured at the same scan ness tc. For example, a 20-mil (0.051 cm) aluminum                points i and associated with the corresponding IT(i) and 1(i)
cladding thickness attenuates the 185.7-keV gamma intens          from the measurements with the unknown plate. The ity according to:                                                  count arrays IT(i), I(i), and 10 (i) must be stored in the computer memory as they are measured. The counts I(i) are then corrected by the factor in Equation 5 for each total Tc= e c ctc    = 0.983                  (4)      plate transmission T(i).
                                                            5.38-9


2.2 Total Passive Count Technique                                  3. MEASUREMENTS WITH HIGH-RESOLUTION
When the U-235 contents of the plates assayed using a common calibration relationship varies over a ranpg of
                                                                        SYSTEMS
+/-5% or more about the average of all plate loadings, the systematic error should be estimated as described in paragraph 1. below; when plate loadings are tightly clustered about a nominal value, the systematic error should be estimated as described in paragraph 2.
    In this case, an average attenuation correction is deter            The transmission of the 235U gamma ray can be inferred mined by measuring T for the entire plate using a 235U              from measured transmission just above and just below source behind the plate. An extended transmission source is          185.7 keV in energy. In one application using high-resolution recommended (ideally another fuel plate) in order to                gamma ray spectrometers (Reference 10), a 1 6 9 Yb trans observe an average transmission over as much of the                 mission source is used. Two of the gamma rays emitted by unknown plate as possible. The transmission source must            this isotope are at 177.2 and 198.0 keV, conveniently not extend beyond or radiate around the edges of the                bracketing the 185.7-keV energy regio


====n. Measurement of T====
1. At the close of the reporting period, the assay value for each plate is plotted against the verified quantity. The verification data plot is examined for indications of nonlinearity or obvious outlier data.
fuel plate being assayed. In this case, the assay involves          at these two energies and interpolating to 185.7 keV results three counts: (1) fuel plate plus shielded transmission            in a determination of the attenuation correction factor C at source I, (2) plate plus unshielded source IT, and (3)              the 235U gamma ray energy. A high-resolution detector unshielded source with no plate Io. The average plate              system must be used in order to resolve the 177.2-, 185.7-,
transmission T is then defined as:                                  and 198.0-keV gamma ray peaks. In this way, the assay and transmission correction data are acquired simultaneously T = (IT- I)/Io                        (7)        and multiple scans or multiple counts are not necessary. As a practical matter, 169yb has the short half-life of 32 days, so this source must be replaced frequently (or reirradiated A single attenuation correction from Equation 5 is then            in a reactor) in order to provide sufficient counts for a applied to the passive count of the plate I.                        precise measurement of the attenuation corrections.


5.38-10
Anomalous indications should be investigated and remedied.


SOURCE                                          SORE                TRANSMISSION          SOURCE
A linear regression analysis should be performed on the comparison data. The intercept should be tested against zero for an indication of a constant measurement bias. The slope shouid be tested against unity for an indication of a proportional bias. When bias is indicated, assays performed during the preceding operating period should be compensated. The systematic error should be
          COLLIMATED          DETECTOR                                "'1?>        oUNKNOWN FUEL            PLATE
5.38-6
                                                              U  (A)
                          *                                                                        *
                        11.&#xfd;c                                                                    &#xfd;,&#xfd;c
                          (8)                                                                      (C)
                                                              Figure 1 A schematic of the measurement arrangements for MTR fuel plate gamma ray assay with measured attenuation correction.


The close geometry of the scanning technique is used as an example. (A) Configuration for measuring 235 radiation coming only from the unknown fuel plate (I in the text). (B) Configuration for determining the sum of the fuel plate gamma intensity and the source intensity passing through the fuel plate (IT in the text). (C) Configuration for measuring
estimated as the standard error associated with the verification line.
___ the incident transmission source gamma intensity (10 in the text).
                                                              5.38-11


REFERENCES
2. When all plates contain essentially the same U-235 content, the difference in the mean content values should be tested against zero as an indication of bias, and the systematic error associated with an inventory of plates should be quoted as the standard deviation of the mean difference. For individual plates, the systematic error should be quoted as the standard deviation of the difference distribution.
1. J. E. Cline, R. J. Gehrke, and L. D. McIssac, "Gamma            ment," Proceedingsof the ERDA X- and Gamma-Ray Rays Emitted by the Fissionable Nuclides and Asso                Symposium, Ann Arbor, Michigan, Conf. 760639, ciated Isotopes," ANCR-1029, 1972.                              p. 219, May 1976.


2. N. S. Beyer, "Assay of 235U in Nuclear Reactor Fuel          7.  T. D.    Reilly, "Gamma-Ray      Measurements for Elements by Gamma-Ray Scintillation Spectrometry,"              Uranium Enrichment Standards," Proceedings of the Proceedings of the 4th International Conference on              American Nuclear Society Topical Meeting on Nondestructive Testing, London, 1963.                            "Measurement      Technology for Safeguards and Material Control," Kiawah Island, South Carolina,
===5. CORE COMPACT ASSAY===
3. R. Sher and S. Untermeyer, The Detection of Fission              November 1979; National Bureau of Standards able Materialby Nondestructive Means, American Nuclear          Special Publication No. 582, p. 103, June 1980.
Final product assay in high-enrichment fuel plate manufacturing can also be accomplished through assay- ing each core compact following the procedures detailed in this guide and the following supplemental criteria:
1. Each core compact should carry a unique identifi- cation. Accountability records should be created for each compact. The fuel plate should carry an identifica- tion corresponding to the compact identification.


Society Monograph, 1980.
2. Each fuel plate should be radiographically exam- ined to ensure that the entire compact has been encapsulated.


8. J. L. Jaech, "Statistical Methods in Nuclear Materials
3. Each fuel plate should be checked with a gamma ray probe to qualitatively ensure that the plate core is uranium of the nominal product enrichment.
4.  R. H. Augustson and T. D. Reilly, "Fundamentals of              Control," Atomic Energy Commission, Report Passive Nondestructive Assay of Fissionable Material,"          No. TID-26298, 1973.


Los Alamos Scientific Laboratory, LA-5651-M, 1974.
4. Calibration and error evaluation should follow the procedures for fuel plate assay.


9. R. A. Forster, D. B. Smith, and H. 0. Menlove,
REFERENCES
5.  E. Storm and H. Israel, "Photon Cross Sections from              "Error Analysis of a Cf-252 Fuel Rod Assay System,"
I. J.E. Cline, R.J. Gehrke, and L.D. Mclsaac, "Gamma Rays Emitted by the Fissionable Nuclides and Asso- ciated Isotopes," ANCR-1029 (July 1972).
    0.001 to 100 MeV for Elements 1 Through 100,"                    Los Alamos Scientific Laboratory, LA-5317, 1974.
2. N.S. Beyer, "Assay of U-235 in Nuclear Reactor Fuel Elements by Gamma Ray Scintillation Spec- trometry," Proc. 4th Intl. Conf. on Nondestructive Testing, London, 1963.
 
Los Alamos Scientific Laboratory, LA-3753, 1967.
 
10. E. R. Martin, D. F. Jones, and J. L. Parker, "Gamma
6.  J. L. Parker and T. D. Reilly, "Bulk Sample Self                Ray Measurements with the Segmented Gamma Scan,"
    Attenuation Correction by Transmission Measure-                 Los Alamos Scientific Laboratory, LA-7059-M, 1977.
 
5.38-12
 
VALUE/IMPACT STATEMENT
1. PROPOSED ACTION                                             
 
===2. TECHNICAL APPROACH===
1.1  Description and Need                                          Not applicable.
 
Regulatory Guide 5.38 was published in September           
 
===3. PROCEDURAL APPROACH===
1974. The proposed action, a revision to this guide, is needed to bringthe guide upto date with respect to advances          Of the procedural alternatives considered, revision of in measurement methods and changes in terminology.              the existing regulatory guide was selected as the most advantageous and cost effective.
 
1.2  Value Impact of Proposed Action                          4. STATUTORY CONSIDERATIONS
    1.2.1 NRC Operations                                        4.1    NRC Authority The regulatory positions will be brought up to date.            Authority for the proposed action is derived from the Atomic Energy Act of 1954, as amended, and the Energy
    1.2.2 Other Government Agencies                              Reorganization Act of 1974, as amended, and implemented through the Commission's regulations.
 
Not applicable.
 
4.2    Need for NEPA Assessment
    1.2.3 Industry The proposed action is not a major action that may Since industry is already applying the methods and          significantly affect the quality of the human environment procedures discussed in the guide, updating the guide            and does not require an environmental impact statement.
 
should have no adverse impact.


5. RELATIONSHIP TO OTHER EXISTING OR
3. J.H. Hubbell, "Photon Cross Sections, Attenuation Coefficients, and Energy Absorption Coefficients from
    1.2.4 Public                                                    PROPOSED REGULATIONS OR POLICIES
10
    No adverse impact on the public can be foreseen.                The proposed action is one of a series of revisions of existing regulatory guides on nondestructive assay techniques.
keV to
100 GeV,"
Nat.


1.3    Decision on Proposed Action
Bur. Stds.
                                                                6. SUMMARY AND CONCLUSIONS
    The regulatory guide should be revised to reflect the improvement in measurement techniques and to bring the              Regulatory Guide 5.38 should be revised to bring it up language of the guide into conformity with current usage.        to date.


5.38-13
NSRDS-NBS 29 (1969).
4. John L. Jaech, "Statistical Methods in Nuclear Materials Control," TID-26298 (1973).
5. American National Standard N15.20, "Guide to Calibrating Nondestructive Assay Systems,"
in preparation. Copies of the draft standard may be obtained from Institute of Nuclear Materials Manage- ment, 505 King Avenue, Columbus, Ohio 43201 (Attention: H.L. Toy).
6. See, for example, R.A. Forster, D.B. Smith, and H.O.


FIRST CLASS MAIL
Menlove, "Error Analysis of a Cf-252 Fuel Rod Assay System,'" LA-5317 (1974).
          UNITED STATES            POSTAGE & FEES PAID
5.38-7}}
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Nondestructive Assay of High-Enrichment Uranium Fuel Plates by Gamma Ray Spectrometry
ML13064A076
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Issue date: 09/30/1974
From:
US Atomic Energy Commission (AEC)
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References
RG-5.038
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U.S. ATOMIC ENERGY COMMISSION

REGULATORY GI

DIRECTORATE OF REGULATORY STANDARDS

REGULATORY GUIDE 5.38 NONDESTRUCTIVE ASSAY OF HIGH-ENRICHMENT URANIUM

FUEL PLATES BY GAMMA RAY SPECTROMETRY

September 1974 JIDE

A. INTRODUCTION

Part 70 of Title 10 of the Code of Federal Regula- tions requires each licensee authorized to possess more than 350 grams of contained U-235 to conduct a physical inventory of all special nuclear material in his possession at intervals not to exceed 12 months. Each licensee authorized to possess more than one effective kilogram of high-enrichment uranium is required to conduct measured physical inventories of his special nuclear materials at bimonthly intervals. Further, these licensees are required to conduct their nuclear material physical inventories in compliance with specific require- ments set forth in Part 70. Inventory procedures acceptable to the Regulatory +,aff for complying with thesc pi 'wisions of Part 70 are detailed in Regulatory Guide 5.13. "Conduct of Nuclear Material Physical Invcntories."

For certain nuclear reactors, the fuel consists of highly enriched uranium fabricated into flat or bowed plates. Typically, these plates are relatively thin so that a significant percentage of the U-235 gamma rays ptle- trate the fuel cladding. When the measurement condi- tions are properly controlled and corrections are made for variations in the attenuation of the gamma rays, a measurement of the U-235 gamma rays can be used as an acceptable measurement of the distribution and the total U-235 content of each fuel plate. In lieu of assaying the product fuel plates, fuel plate core compacts may be assayed through the procedures detailed in this guide, provided steps are taken to ensure the traceability and integrity of encapsulation of each assayed fuel plate core compact. This guide describes features of a gamma ray spectrometry system acceptable to the Regulatory staff for nondestructive assay of high-enrichment uranium fuel plates or fuel plate core compacts.

B. DISCUSSION

The number, energy, and intensity of gamma rays associated with the decay of U-235 provide the basis for nondestructive assay of high-enrichment fuel plates by gamma ray spectrometry (Ref. 1). The 185.7-keV

gamma ray is the most useful U-235 gamma ray for this application; it is emitted at the rate of 4.25 x 104 gamma rays per second per gram of U-235. Lower- energy gamma rays emitted by U-235 are less pene- trating and more sensitive to errors due to fluctuations in clad and core thickness. In general, more accurate fuel plate assays may be made by measuring only the activity attributable to the 185.7-keV U-235 gamma ray.

Assay measurements are made by integrating the response observed during the scanning of single fuel plates and comparing each response to a calibration based on the response to known calibration standards.

1. GAMMA RAY MEASUREMENT SYSTEM

1.1 GAMMA RAY DETECTION SYSTEM

1.1.1 Gamma Ray Detector High-resolution gamma ray detectors, i.e., intrinsic or lithium-drifted germanium, provide resolution beyond that required for this assay application. While the performance of such detectors is more than adequate, their low intrinsic detection efficiency, extensive opera- tional and maintenance requirements, and high cost make them unattractive for this application.

Most ,sodium iodide [Nal (TI)] scintillation detectors are capable of sufficient energy resolution to be used for the measurement of the 185.7-keV gamma rays. The detector diameter is determined by the fuel plate width and the scanning method selected (see Section B.I.2 of this guide). The thickness of the Nal crystal is selected to avoid unnecessary sensitivity to gamma rays above the

185-keV region which produce a background in the

185-keV energy region as a result of Compton scattering.

For measurements to be reproducible, it is necessary to assure that the detection system is stabilized on the USAEC REGULATORY GUIDES

Cop.- of published guids may be obtained by request indicintng the dinirsom deired to the US. Atomic Erw Commismson. Vlhington. D.C. 20545.

RegulatmrV Guide are ssued to iescribe and make availabte to the puublic Attlntion: Director of Regulatory Standstills. Comments wed suggestions for nalhods c.acptable to tht AEC Regulatory st&ff of imiplemnting specific parts of ifl3hmtflts in thes guides we o fiurqlpd and should be sent to the Secretary the Commnisson* s regulations, to delineate technqlus

.sead by the staff in of the Comnssmion, US. Atomic EnoqW Commission. Washington, D.C. 20545.

oealuat-0n specific problems or postulated accidents, or to provide guidenws to Attention: Do-kteting and Sorvece Section.

aml)icents. Regulatory Guides wit not substitutes for regulations and comNpflanio with them is not re*uired. Methods and sofutions diffrt from tfeor lit out in The guides are

,ssued in the following ten broaed divisions:

t.

guides well bea asipgtobl if they piroind a basis for the findingl requisits to the iiufnt*i or oiminuenOt of a permit or licnese by the Commtissson.

1. Powr Reectorn

6. Products

2. Research and Test Ractors

7. Tranpomrtation

3. Fuels isid Materi- Fac;itisi a. O=upetiona! Meeilh Pubfb*ied guides will be revised periodicafly. as approprat

e. to accomn oal

4. Environmenetal end Siting

9. Antitrust Review comments and to reflect new ,ntormition or expoerine.

5. Mot'earits and Plant Protection

1

0. Generaf

intended portion of the gamma ray spectrum during measurements. Internally "seeded" Nal crystals which contain a radioactive source (typically Am-241) to produce a reference energy pulse art commercially available. The detection system is stabilized on the reference, and the amplifier gain is automatically cor- rected to assure that that energy and the rest of the spectrum remain fixed in position.

.1.2 Gamma Ray Collimator To ensure that the only gamma ray activity detected originates from a well-defined segment of the fuel plate, the detector is shielded from extraneous background radiations and collimated to define the area "seen" by the detector crystal. The collimator consists of a disk of appropriate shielding material.

A slit is machined through the center of the disk which will allow only those gamma rays emitted within the slit opening to strike the detector. The disk thickness is a minimum of six mean free path lengths to effectively stop all gamma rays emitted from outside the view area. To prevent gamma rays from striking the crystal around the edges of the collimator disk, the disk diameter exceeds the crystal diameter by at least twice the crystal depth.

The probability of detection for gamma rays emitted at the center of the collimator slit is greater than that for gamma rays emitted near the ends of the slit. This effect becomes increasingly important at small detector-to- plate spacing, especially when scanning near the edge of a plate. To minimize this detection nonuniformity and to minimize the sensitivity to jitter, the detector-to-plate distance can be made large, especially with respect to the dimensions of the slit opening. As an alternative means of reducing the detection nonuniformity-across the slit, the slit opening can be divided into channels by inserting a honeycomb baffle into the slit or by fabricating the collimator by drilling holes through the disk in a pattern which ensures that each hole is surrounded by a minimum wall thickness of 0.2 mean free path length. A

7:0-cm-thick iron disk with holes less than 0.5 cm in diameter drilled in a pattern having 0.2 cm of wall between adjacent holes is one example of a collimator that would perform satisfactorily. A large number of small-diameter holes is preferable to a few large-diameter holes.

1.1.3 Multiple Detectors Several detectors may be used to shorten the mea- surement time. The detectors can be positioned to simultaneously measure different segments of a single fuel plate or to simultaneously measure additional fuel plates. In some cases it may be useful to sum the response from two detectors positioned on opposite sides of a plate to increase counting efficiency. In such cases it is essential that the response of such detectors be balanced-and checked at frequent intervals.

1.2 SCANNING TECIINIQUES

It is critical that the scanningapparatds for ,'oviog t:,

plates relative to the detector provide a uniforn;.

reproducible scan. The importance of a well-constructed, mechanically stable conveyor cannot be overemphasized.

Either the detector can be moved and the plate held stationary, or the plate can be moved past a fixed detector. Care must be exercised to maintain fl detector-to-plate spacing within close tolerance, minimize errors caused by the inverse-square dcpen(!

of detection on distance. This is especially important in the case of close spacing, which is sometimes desirable to maximize the count rate. Various commercial conveying systems have been used and found to be adequate. Such.

systems may significantly reduce the cost of de..,7!i, g and building new scanning mechanisms. High-precision tool equipment such as milling machines, lathes, and x-y scanning tables can be investigated. Numerically con- trolled units offer additional advantages when they can be incorporated into a scanning system. This is particu- larly true when an automated scanning system is being developed.

Fuel plate core compacts may be sufficiently small permit total assay without scanning in a fixed-geometry counting system. The scanning techniques for fuel plates discussed in the following subsections can also be used for core compacts when total fixed compact counting is not possible.

1.2.1 Linear Total Scan The detector collimation consists of a rectangular opening which extends across the width of the fuel plates beyond the edges of the uranium core contained within the plate cladding. Scanning the total plate is accomplished by starting the count sequence on the end of a -plate and continuing to count until the entire length of plate has been scanned.

To ensure that gamma rays emitted anywhere across the face of the fuel plate have an equal probability of being detected, it is necesary that the diameter of the detector crystal exceed the plate width or that the detector be positioned away from the plate.

Use of the spot oi cizcalar collimator scan technique eliminates or reduces to insignificance most of these edge effects.

1.2.2 Sweeping Spot Scan (Ref. 2)

If the collimator channel width is smaller than the fuel plate width, the viewing area (spot) can be swept across the plate as the detector scans along the length of the plate. This scanning technique can be readily adapted to scanning bowed plates through the use of a cam which is designed to maintain the detector-to-plate distance constant over the entire geometry of the fuel plate. The collimator channel dimensions can be selected to provide compatible information on the uniformity of

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the fuel plate which is frequently obtained by comparing fixed (static) spot counts at a variety of locations to reference counts.

1.2.3 Sampled Increment Assay When used in conjunction with radiographic dimen- sional measurements performed on all fuel plates, the U-235 content of a fuel plate can be measured by scanning the ends of each fuel plate and sampling the balance of the plate. It is necessary to measure the dimensions of the fuel core loading radiographically, through gamma ray scanning along the length of the plate, or by spot scanning the fuel plate ends and measuring the distance between end spots where the fuel loading stops. The U-235 content of the plate is then determined by averaging the results of sample spot measurements of the U-235 content per unit area at a number of sites along the plate and multiplying this average value by the measured area of the fuel core. The radiograph of each plate is examined to ensure that the core filter is uniform.

The collimator shape and dimensions can be selected to provide compatible information on the uniformity of the fuel plate.

1.3 COMPUTER CONTROL

The reproducibility of measurements can be im- proved and the measurement time per fuel plate can be reduced by using a computer to control the fuel plate scanning operation. The computer can be used to control data acquisition by accumulating counts ac- cording to a predetermined scheme. Also, the computer can be used for data analysis, including background corrections and intermachine normalization, calibration, error analysis, and diagnostic test measurements and analyses. Report preparation and data recording for subsequent analysis are also readily accomplished through an appropriately designed computer-controlled system.

2. INTERPRETATION OF MEASUREMENT DATA

The three factors discussed below may give rise to significant errors in interpreting measurement data.

2.1 ENRICHMENT vARIATIONS

Licensees authorized to possess highly enriched uranium are required to account for element and isotope as prescribed in §70.51. Under the conditions detailed in this guide, the U-235 content of individual plates is measured. To determine the total uranium content of each plate, the U-235 enrichment must be known from separate measurements.

Enrichment variations may alter the radiation back- ground in the gamma ray energy region of interest.

Uranium-238 decays by alpha-particle .emission to Th-234. Thorium-234 then decays by beta-particle emis- sion with a half-life of 24.1 days to Pa-234 which, in turn, decays by beta-particle emission to U-234. Ap- proximately 1% of the Pa-234 decays are followed by high-energy (e.g., 1001 keV, 766 keV) gamma rays.

These gamma rays frequently lose energy through Compton scattering and may appear in the 185-keV

spectral region. It is important to note that activity from Pa-234 may be altered by disturbing the equilibrium between U-235 and Th-234, as frequently occurs in uranium chemical conversion processes. The interference due to variations in U-238 daughter activity becomes less important as the enrichment of U-235 increases. At enrichment levels above 90%, this problem can essen- tially be ignored.

2.2 RADIATION ATTENUATION

The number of U-235 gamma rays which escape from the fuel plate (and are thus available for detection)

without losing energy depends on the characteristics of the fuel plate core and cladding. Gamma rays from U-235 are attenuated in the uranium, in the cladding, and in the inert material that may be added with the uranium to form the core of the fuel plate. Through well-controlled product tolerance limits, each of these potential sources of signal variability can be controlled to permit accurate accountability assays.

2.2.1 Self-Attenuation The uranium photon attenuation coefficient for gamma ray energies corresponding to U-235 emissions is quite large (Ref. 3). Small changes in uranium density resulting from increased fuel loading or from variations in the manufacturing process can significantly change the number of gamma rays which escape from the fuel plate.

2.2.2 Gadding Attentuation Small variations in cladding thickness may cause significant attenuation variations. Variations in cladding attenuation can be measured by a simple gamma ray absorption test using thin sheets of cladding material as absorbers and varying the clad thickness over the range of thicknesses to be encountered in normal product variability.

2.2.3 Core Friler Attenuation Radiation intensity measurements may be made of plates fabricated with different ratios of uranium to filler to show the effects of this type of attenuation. If significant effects are noted, plates can be categorized by core composition characteristics and the assay system can be independently calibrated for each category of fuel plates.

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2.2.4 Attenuation Corrections When the thickness of the core and cladding of each plate is known, an attenuation correction can be applied to improve the accuracy of the assay. Ultrasonic gauging may provide such a measure, provided the metallo- graphic zones within the plate are sufficiently defined to provide a detectable interface.

The alternative attenuation- correction is based on a micrometer measurement of the total thickness of each plate. The clad thickness of a plate is estimated by subtracting the mean core thickness of the product plates, which is determined by periodically sampling product plates and cutting a cross section to permit visual measurement of clad and core thickness.

2.3 INTERFERING RADIATIONS

As noted in Section B.2.1 of this guide, an internal background variation may arise from changes in the amount of U.238 present in a fuel plate or from changes in the ratio of Th-234 to U-238 resulting from fuel manufacturing processes. Fluctuations in the internal background cause the response of the unknown items to be different from the calibration standards, thereby creating a measurement bias. Such interferences can be compensated by measuring additional regions of the gamma ray spectrum.

Other interfering radiations may come from external sources, from fuel plates awaiting assay, or from nearby radiation sources used for other measurements. This is not expected to be a major problem and can be controlled through

(1) removing radiation sources,

(2) shielding the detectors, and (3) monitoring the back- ground at frequent intervals.

3. CALIBRATION AND VERIFICATION

3.1 INITIAL OPERATIONS

Calibration and the verification of assay predictions is an ongoing effort where performance is periodically monitored and the calibration relationship is modified to improve the accuracy of assay predictions. During initial operations, two means of basing preliminary calibrations are appropriate.

3.1.1 Foil Calibration Technique Methods for calibrating scanning systems for high- enrichment uranium fuel plates through the assay of prepared uranium and clad foils are described in Refer- ence 2. This method may be used in place of or in addition to the -technique described in the following subsection.

uranium, U-235, inert matrix, and cladding are accu- rately measured and that these parameters bracket the nominal range of product plates anticipated to fall within manufacturing tolerances.

3.2 ROUTINE OPERATIONS

The performance of the assay system is periodically monitored to ensure that the performance of the assay system has not shifted since its last calibration. Control limits for acceptable performance can be established for the response to an appropriate working standard. The control chart of the responses to the working standard can be checked for indications of short-term instrument drift or malfunction. The control chart can also be analyzed to detect long-term shifts within the measurement-to-measurement control limits that may be corrected by recalibrating the system. Severe changes in instrument performance are investigated promptly and their causes remedied.

To ensure that the calibration remains valid during normal operations and that accuracy estimates are rigorously justified, assay predictions are periodically compared with more accurate measurements of the content of typical fuel plates (see Section C.4 of this guide). Guidance on methods to relate this assay to the national measurement system and to reconcile verifi- cation measurements will be addressed in separate regulatory guides.*

C. REGULATORY POSITION

The content and distribution of U-235 in high- enrichment uranium plates can be measured through the gamma ray assay methods described in this guide.

Combining this measurement with the results of an independent measurement of the U-235 enrichment enables the total uranium content of the fuel plates to be measured. The factors presented below should be taken into consideration for this assay method to be acceptable to the Regulatory staff.

I. MEASUREMENT SYSTEM

1.1 GAMMA RAY MEASUREMENT SYSTEM

1.1.1 Gamma Ray Detector A

thallium-activated sodium iodide scintillation detector or series of detectors is recommended for this assay application. When more than one detector is to be incorporated into the scan system, the performance characteristics of the detectors should be matched. The diameter of the crystal should be larger than the projected view onto the crystal face through the

  • For example, regulatory guides related to measurement quality amurance and calibration of nondestructive a&say systems are being developed.

3.1.2 Fabricated Calibration Plates Calibration standard fuel plates can be fabricated using special precautions to ensure that the amounts of

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collimator channel. The thickness of the crystal should hc noi more than onc inch. The crystal should contain an internal cesium iodide seed which is doped with a suilable alpha-emittcr for spectral stabilization. The seed should produce approximately 1,000 counts per second at the reference energy.

1.1.2 Collimator A collimator should be fabricated of appropriate gamma ray shielding material such as iron, lead, or tungsten. The shielding should completely surround the detector and photomultiplier assembly and should be sufficiently thick to completely block extraneous radi- ations from the detector. The response variation from the center of the collimator opening to its edge should be less than 1%.

1.1.3 Electronic Apparatus All electronic systems should be powered by filtered, highly regulated power supplies. The ambient tempera- ture and humidity in the vicinity of the scanning system should be controlled so that permitted fluctuations do not significantly affect the assay measurements. All electronic circuitry in signal-processing components should feature temperature compensation. Residual sen- sitivity to fluctuations in the, ambient environment should be tested and monitored periodically.

The capability for multichannel gamma ray pulse height analysis with cathode ray tube spectral display should be provided. Signal-processing electronics capable of stabilizing on the alpha radiations emitted within the doped cesium iodide seed should be provided to stabilize the energy spectrum.

1.2 SCANNING SYSTEM

A mechanically sound, highly reproducible scanning system should be employed. Scanning should be accom- plished by one of the three techniques discussed in Section B. 1.2 of this guide.

1.3 COMPUTER CONTROL

A dedicated minicomputer to control data acquisi- tion, analysis, calibration, diagnostic testing, and report preparation -should be employed for this assay appli- cation.

1.4 MULTIPLE SCANNING ASSAY SYSTEMS

When more than ornc scanning system is employed, assay response should be normalized so that each instrument provides consistent results. Verification data to establish the systeniatic assay error for each assay system should be obtained with the same plate.

2. MEASUREMENT INTERPRETATION

2.1 ENRICHMENT VARIATIONS

Procedures should be developed to ensure that the enrichment of the plates being scanned is known through separate measurements. Fuel plates generally satisfy the gamma ray penetrability criteria for quantita- tive U-235 assay; they do not satisfy the criteria for nondestructive enrichment measurement through gamma ray spectrometry.* Facilities processing more than one uranium enrichment should maintain strict isotopic control and characterize the enrichment through appri.,-

priate measurement methods.

2.2 ATTENUATION CORRECTIONS

Attenuation variations arising from plate-to-plate changes in core thickness, composition,-and clad thick- ness should be determined over the range of product tolerance specifications. When such variations cause the assay error to exceed the error realized without the variations by 50% or more, procedures should be implemented to measure and apply a correction to the assay of each plate.

2.3 RADIATION INTERFERENCES

A clear plastic template which shows an acceptable spectrum display should be prepared. When radioactive interference may be encountered, the assay spectrum should be compared at appropriate intervals to the reference spectrum for indications of interference. Bick- ground radiation should be measured periodically during each operating shift.

3. MEASUREMENT CALIBRATION

During initial operations, the assay system should m, calibrated either by the foil calibration method or with specially prepared sample fuel plates as described mn Section B.3.1 of this guide.

4. RANDOM AND SYSTEMATIC ASSAY ERRORS

4.1 RANDOM ERROR ESTIMATION

A replicate assay program should be established to generate data for the evaluation of random assay errOrs during each material balance period. During each bi- monthly interval, a minimum of fifteen plates should be selected for replicate assay. The second assay of each plate selected for replicate assay should be made at least four hours after the first assay. Replicate assay diffet- ences should be collected and analyzed at the end (if the

  • Criteria for uranium gamma ray enrichment measurenicTi*r.

arff given in Regulatory Guide 5.21, "Nondestructrvr ltiranwurn.235 Enrichment Assay by Gamma Ray Spectromelr\\*

"

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material balance period. The single-measurement stan- dard deviation of the relative replicate assay differences should be computed as described in Reference 4.

4.2 SYSTEMATIC ERROR ESTIMATION

The systematic error associated with the assay of all fuel plates fabricated during a material balance period should be determined through one of the procedures*

presented below.

4.2.1 Propagation through the Calibration Function To estimate the systematic assay error through the calibration function, the calibration should be based on the regression analysis of an appropriate function to the calibration data. Uncertainties in the reference values of the calibration standards should be factored into the fit, and the errors propagated as demonstrated in Reference

5.

To ensure the validity of the predictions, the stable performance of the instrument should be monitored and normalized through the response to appropriate working standards which are assayed at frequent intervals. The frequency for assaying working standards should be determined through testing, but should not be lower than one test during each two-hour assay interval for spot response stability and one full scan test during each operating shift. Indications of shifting instrument perfor- mance should be investigated and remedied, and the instrument should be recalibrated to ensure the validity of subsequent measurements.

In order to ensure that the calibration standards continue to adequately represent the unknown fuel plates, key production parameters which affect the observed response should be monitored through separate tests. Data should be compiled and analyzed at the close of each material balance period. When a production parameter shifts from previously established values, the impact of the shift on the response of the assay instrument should be determined through an appropriate experiment or. calculation (Ref. 6). A bias correction should be determined and applied to all items assayed from the point of the parameter change. The uncertainty in the bias should be combined with the systematic error predicted through the calibration function. When the bias exceeds 3% of the plate contents in a single material balance period, when a trend of 1.5% or more is observed in three consecutive material balance periods,

,)r when the uncertainty in the observed bias is sufficient to increase the limit of error of the assay above 0.5%,

new calibration standards should be obtained, and the scanning system should be recalibrated.

As a further check on the continued validity of the calibration standards, a program to periodically intro- duce new calibration standards should be implemented.

  • These methods will be discussed in detail in a regulatory guide

,n preparation entitled "Calibration and Error Estimation Pr:ccduir"s fcr Nondestructi-e Asay."

A minimum of one new calibration standard fuel plate should be introduced during each six-month period.

4.2.2 Comparative Evaluation When two measurements are made on each of a series of items and the accuracy of ono of the methods used is considerably greater than the other, the corresponding predictions can be compared to establish an estimate of bias between the measurement methods and to estimate the error. associated with the lens-accurate measurement method. To precisely determine the systematic error in the nondestructive assay, the fuel plates selected for comparative measurements should be randomly selected but should span the range of U-235 contents en- countered in normal production. The selected fuel plates may be rejected from the process stream for failing to meet quality assurance requirements. Each plate should be repeatedly assayed to reduce the random asay error to less than

10% of the estimated or previously established systematic error. To determine its U-235 and total uranium content, the plate should be completely dissolved and the resulting solution should be analyzed by high-accuracy chemical and mass spectrometric pro- cedures.

For one material balance period during the initial implementation of this guide; a product fuel plate should be randomly selected twice each week for an accuracy verification measurement. Following this initial implementation period, facilities manufacturing 100 or more fuel plates per week may reduce the verification frequency to one plate per week and pool the verifi- cation data for two consecutive material balance periods.

LoW-throughput facilities manufacturing lesa than 100

plates per week should verify at least 4 plates per material balance period through the procedures de- scribed above. At the close of each material balance period, data should be pooled to include only the 15 most current data points.

When the U-235 contents of the plates assayed using a common calibration relationship varies over a ranpg of

+/-5% or more about the average of all plate loadings, the systematic error should be estimated as described in paragraph 1. below; when plate loadings are tightly clustered about a nominal value, the systematic error should be estimated as described in paragraph 2.

1. At the close of the reporting period, the assay value for each plate is plotted against the verified quantity. The verification data plot is examined for indications of nonlinearity or obvious outlier data.

Anomalous indications should be investigated and remedied.

A linear regression analysis should be performed on the comparison data. The intercept should be tested against zero for an indication of a constant measurement bias. The slope shouid be tested against unity for an indication of a proportional bias. When bias is indicated, assays performed during the preceding operating period should be compensated. The systematic error should be

5.38-6

estimated as the standard error associated with the verification line.

2. When all plates contain essentially the same U-235 content, the difference in the mean content values should be tested against zero as an indication of bias, and the systematic error associated with an inventory of plates should be quoted as the standard deviation of the mean difference. For individual plates, the systematic error should be quoted as the standard deviation of the difference distribution.

5. CORE COMPACT ASSAY

Final product assay in high-enrichment fuel plate manufacturing can also be accomplished through assay- ing each core compact following the procedures detailed in this guide and the following supplemental criteria:

1. Each core compact should carry a unique identifi- cation. Accountability records should be created for each compact. The fuel plate should carry an identifica- tion corresponding to the compact identification.

2. Each fuel plate should be radiographically exam- ined to ensure that the entire compact has been encapsulated.

3. Each fuel plate should be checked with a gamma ray probe to qualitatively ensure that the plate core is uranium of the nominal product enrichment.

4. Calibration and error evaluation should follow the procedures for fuel plate assay.

REFERENCES

I. J.E. Cline, R.J. Gehrke, and L.D. Mclsaac, "Gamma Rays Emitted by the Fissionable Nuclides and Asso- ciated Isotopes," ANCR-1029 (July 1972).

2. N.S. Beyer, "Assay of U-235 in Nuclear Reactor Fuel Elements by Gamma Ray Scintillation Spec- trometry," Proc. 4th Intl. Conf. on Nondestructive Testing, London, 1963.

3. J.H. Hubbell, "Photon Cross Sections, Attenuation Coefficients, and Energy Absorption Coefficients from

10

keV to

100 GeV,"

Nat.

Bur. Stds.

NSRDS-NBS 29 (1969).

4. John L. Jaech, "Statistical Methods in Nuclear Materials Control," TID-26298 (1973).

5. American National Standard N15.20, "Guide to Calibrating Nondestructive Assay Systems,"

in preparation. Copies of the draft standard may be obtained from Institute of Nuclear Materials Manage- ment, 505 King Avenue, Columbus, Ohio 43201 (Attention: H.L. Toy).

6. See, for example, R.A. Forster, D.B. Smith, and H.O.

Menlove, "Error Analysis of a Cf-252 Fuel Rod Assay System,'" LA-5317 (1974).

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