Regulatory Guide 5.38

Nondestructive Assay of High-Enrichment Uranium Fuel Plates by Gamma Ray Spectrometry

ML13064A076
Person / Time
Issue date:	09/30/1974
From:	US Atomic Energy Commission (AEC)
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RG-5.038
Download: ML13064A076 (7)

September 1974 U.S. ATOMIC ENERGY COMMISSION

REGULATORY

DIRECTORATE OF REGULATORY STANDARDS

GI JIDE

REGULATORY GUIDE 5.38 NONDESTRUCTIVE ASSAY OF HIGH-ENRICHMENT URANIUM

FUEL PLATES BY GAMMA RAY SPECTROMETRY

A. INTRODUCTION

nondestructive assay of high-enrichment fuel plates by Part 70 of Title 10 of the Code of Federal Regula- gamma ray spectrometry (Ref. 1). The 185.7-keV

tions requires each licensee authorized to possess more gamma ray is the most useful U-235 gamma ray for this than 350 grams of contained U-235 to conduct a application; it is emitted at the rate of 4.25 x 104 gamma rays per second per gram of U-235. Lower- physical inventory of all special nuclear material in his possession at intervals not to exceed 12 months. Each energy gamma rays emitted by U-235 are less pene- trating and more sensitive to errors due to fluctuations licensee authorized to possess more than one effective kilogram of high-enrichment uranium is required to in clad and core thickness. In general, more accurate fuel conduct measured physical inventories of his special plate assays may be made by measuring only the activity nuclear materials at bimonthly intervals. Further, these attributable to the 185.7-keV U-235 gamma ray.

licensees are required to conduct their nuclear material Assay measurements are made by integrating the physical inventories in compliance with specific require- response observed during the scanning of single fuel ments set forth in Part 70. Inventory procedures plates and comparing each response to a calibration acceptable to the Regulatory +,afffor complying with based on the response to known calibration standards.

thesc pi 'wisions of Part 70 are detailed in Regulatory Guide 5.13. "Conduct of Nuclear Material Physical 1. GAMMA RAY MEASUREMENT SYSTEM

Invcntories."

For certain nuclear reactors, the fuel consists of 1.1 GAMMA RAY DETECTION SYSTEM

highly enriched uranium fabricated into flat or bowed plates. Typically, these plates are relatively thin so that a 1.1.1 Gamma Ray Detector significant percentage of the U-235 gamma rays ptle- trate the fuel cladding. When the measurement condi- High-resolution gamma ray detectors, i.e., intrinsic or tions are properly controlled and corrections are made lithium-drifted germanium, provide resolution beyond for variations in the attenuation of the gamma rays, a that required for this assay application. While the measurement of the U-235 gamma rays can be used as an performance of such detectors is more than adequate, acceptable measurement of the distribution and the total their low intrinsic detection efficiency, extensive opera- U-235 content of each fuel plate. In lieu of assaying the tional and maintenance requirements, and high cost product fuel plates, fuel plate core compacts may be make them unattractive for this application.

assayed through the procedures detailed in this guide, provided steps are taken to ensure the traceability and Most ,sodium iodide [Nal (TI)] scintillation detectors are capable of sufficient energy resolution to be used for integrity of encapsulation of each assayed fuel plate core the measurement of the 185.7-keV gamma rays. The compact. This guide describes features of a gamma ray detector diameter is determined by the fuel plate width spectrometry system acceptable to the Regulatory staff and the scanning method selected (see Section B.I.2 of for nondestructive assay of high-enrichment uranium this guide). The thickness of the Nal crystal is selected fuel plates or fuel plate core compacts.

to avoid unnecessary sensitivity to gamma rays above the

B. DISCUSSION

185-keV region which produce a background in the

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

The number, energy, and intensity of gamma rays For measurements to be reproducible, it is necessary associated with the decay of U-235 provide the basis for 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 SorveceSection.

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 in the following ten broaeddivisions:

,ssued t. guides well beaasipgtobl if they piroind a basis for the findingl requisits to the iiufnt*ior oiminuenOt of a permit or licnese by the Commtissson. 1. Powr Reectorn

6. Products

2. Research and Test Ractors

7. Tranpomrtation

3. isid Materi- Fac;itisi Fuels a. O=upetiona! Meeilh Pubfb*ied guides will be revised periodicafly. as approprate. 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 1.2 SCANNING TECIINIQUES

measurements. Internally "seeded" Nal crystals which contain a radioactive source (typically Am-241) to It is critical that the scanningapparatds for ,'oviogt:,

produce a reference energy pulse art commercially plates relative to the detector provide a uniforn;.

available. The detection system is stabilized on the reproducible scan. The importance of a well-constructed, reference, and the amplifier gain is automatically cor- mechanically stable conveyor cannot be overemphasized.

rected to assure that that energy and the rest of the Either the detector can be moved and the plate held spectrum remain fixed in position. stationary, or the plate can be moved past a fixed detector. Care must be exercised to maintain fl

.1.2 Gamma Ray Collimator detector-to-plate spacing within close tolerance, minimize errors caused by the inverse-square dcpen(!

of detection on distance. This is especially important in To ensure that the only gamma ray activity detected the case of close spacing, which is sometimes desirable to originates from a well-defined segment of the fuel plate, maximize the count rate. Various commercial conveying the detector is shielded from extraneous background systems have been used and found to be adequate. Such.

radiations and collimated to define the area "seen" by systems may significantly reduce the cost of de..,7!i, g the detector crystal. The collimator consists of a disk of and building new scanning mechanisms. High-precision appropriate shielding material. A slit is machined tool equipment such as milling machines, lathes, and x-y through the center of the disk which will allow only scanning tables can be investigated. Numerically con- those gamma rays emitted within the slit opening to trolled units offer additional advantages when they can strike the detector. The disk thickness is a minimum of be incorporated into a scanning system. This is particu- six mean free path lengths to effectively stop all gamma larly true when an automated scanning system is being rays emitted from outside the view area. To prevent developed.

gamma rays from striking the crystal around the edges of Fuel plate core compacts may be sufficiently small the collimator disk, the disk diameter exceeds the crystal permit total assay without scanning in a fixed-geometry diameter by at least twice the crystal depth.

counting system. The scanning techniques for fuel plates The probability of detection for gamma rays emitted discussed in the following subsections can also be used at the center of the collimator slit is greater than that for for core compacts when total fixed compact counting is gamma rays emitted near the ends of the slit. This effect becomes increasingly important at small detector-to- not possible.

plate spacing, especially when scanning near the edge of a plate. To minimize this detection nonuniformity and

1.2.1 Linear Total Scan to minimize the sensitivity to jitter, the detector-to-plate The detector collimation consists of a rectangular distance can be made large, especially with respect to the opening which extends across the width of the fuel dimensions of the slit opening. As an alternative means plates beyond the edges of the uranium core contained of reducing the detection nonuniformity-across the slit, within the plate cladding. Scanning the total plate is the slit opening can be divided into channels by inserting accomplished by starting the count sequence on the end a honeycomb baffle into the slit or by fabricating the of a -plate and continuing to count until the entire length collimator by drilling holes through the disk in a pattern of plate has been scanned.

which ensures that each hole is surrounded by a To ensure that gamma rays emitted anywhere across minimum wall thickness of 0.2 mean free path length. A

the face of the fuel plate have an equal probability of

7:0-cm-thick iron disk with holes less than 0.5 cm in being detected, it is necesary that the diameter of the diameter drilled in a pattern having 0.2 cm of wall detector crystal exceed the plate width or that the between adjacent holes is one example of a collimator detector that would perform satisfactorily. A large number of Use ofbethe positioned away from the plate.

spot oi cizcalar collimator scan technique small-diameter holes is preferable to a few large-diameter eliminates or reduces to insignificance most of these holes.

edge effects.

1.1.3 Multiple Detectors 1.2.2 Sweeping Spot Scan (Ref. 2)

Several detectors may be used to shorten the mea- If the collimator channel width is smaller than the surement time. The detectors can be positioned to fuel plate width, the viewing area (spot) can be swept simultaneously measure different segments of a single across the plate as the detector scans along the length of fuel plate or to simultaneously measure additional fuel the plate. This scanning technique can be readily plates. In some cases it may be useful to sum the adapted to scanning bowed plates through the use of a response from two detectors positioned on opposite cam which is designed to maintain the detector-to-plate sides of a plate to increase counting efficiency. In such distance constant over the entire geometry of the fuel cases it is essential that the response of such detectors be plate. The collimator channel dimensions can be selected balanced-and checked at frequent intervals. to provide compatible information on the uniformity of

5.38-2

the fuel plate which is frequently obtained by comparing Uranium-238 decays by alpha-particle .emission to fixed (static) spot counts at a variety of locations to Th-234. Thorium-234 then decays by beta-particle emis- reference counts. sion with a half-life of 24.1 days to Pa-234 which, in turn, decays by beta-particle emission to U-234. Ap-

1.2.3 Sampled Increment Assay proximately 1% of the Pa-234 decays are followed by high-energy (e.g., 1001 keV, 766 keV) gamma rays.

When used in conjunction with radiographic dimen- These gamma rays frequently lose energy through sional measurements performed on all fuel plates, the Compton scattering and may appear in the 185-keV

U-235 content of a fuel plate can be measured by spectral region. It is important to note that activity from scanning the ends of each fuel plate and sampling the Pa-234 may be altered by disturbing the equilibrium balance of the plate. It is necessary to measure the between U-235 and Th-234, as frequently occurs in dimensions of the fuel core loading radiographically, uranium chemical conversion processes. The interference through gamma ray scanning along the length of the due to variations in U-238 daughter activity becomes less plate, or by spot scanning the fuel plate ends and important as the enrichment of U-235 increases. At measuring the distance between end spots where the fuel enrichment levels above 90%, this problem can essen- loading stops. The U-235 content of the plate is then tially be ignored.

determined by averaging the results of sample spot measurements of the U-235 content per unit area at a 2.2 RADIATION ATTENUATION

number of sites along the plate and multiplying this average value by the measured area of the fuel core. The The number of U-235 gamma rays which escape from radiograph of each plate is examined to ensure that the the fuel plate (and are thus available for detection)

core filter is uniform. without losing energy depends on the characteristics of The collimator shape and dimensions can be selected the fuel plate core and cladding. Gamma rays from to provide compatible information on the uniformity of U-235 are attenuated in the uranium, in the cladding, the fuel plate. and in the inert material that may be added with the uranium to form the core of the fuel plat

e. Through

1.3 COMPUTER CONTROL well-controlled product tolerance limits, each of these potential sources of signal variability can be controlled The reproducibility of measurements can be im- to permit accurate accountability assays.

proved and the measurement time per fuel plate can be reduced by using a computer to control the fuel plate 2.2.1 Self-Attenuation scanning operation. The computer can be used to control data acquisition by accumulating counts ac- The uranium photon attenuation coefficient for cording to a predetermined scheme. Also, the computer gamma ray energies corresponding to U-235 emissions is can be used for data analysis, including background quite large (Ref. 3). Small changes in uranium density corrections and intermachine normalization, calibration, resulting from increased fuel loading or from variations error analysis, and diagnostic test measurements and in the manufacturing process can significantly change analyses. Report preparation and data recording for the number of gamma rays which escape from the fuel subsequent analysis are also readily accomplished plate.

through an appropriately designed computer-controlled system. 2.2.2 Gadding Attentuation

2. INTERPRETATION OF MEASUREMENT DATA Small variations in cladding thickness may cause significant attenuation variations. Variations in cladding attenuation can be measured by a simple gamma ray The three factors discussed below may give rise to absorption test using thin sheets of cladding material as significant errors in interpreting measurement data. absorbers and varying the clad thickness over the range of thicknesses to be encountered in normal product

2.1 ENRICHMENT vARIATIONS variability.

Licensees authorized to possess highly enriched 2.2.3 Core Friler Attenuation uranium are required to account for element and isotope as prescribed in §70.51. Under the conditions detailed Radiation intensity measurements may be made of in this guide, the U-235 content of individual plates is plates fabricated with different ratios of uranium to measured. To determine the total uranium content of filler to show the effects of this type of attenuation. If each plate, the U-235 enrichment must be known from significant effects are noted, plates can be categorized by separate measurements. core composition characteristics and the assay system Enrichment variations may alter the radiation back- can be independently calibrated for each category of ground in the gamma ray energy region of interest. fuel plates.

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2.2.4 Attenuation Corrections uranium, U-235, inert matrix, and cladding are accu- rately measured and that these parameters bracket the When the thickness of the core and cladding of each nominal range of product plates anticipated to fall plate is known, an attenuation correction can be applied within manufacturing tolerances.

to improve the accuracy of the assay. Ultrasonic gauging may provide such a measure, provided the metallo- 3.2 ROUTINE OPERATIONS

graphic zones within the plate are sufficiently defined to provide a detectable interface. The performance of the assay system is periodically The alternative attenuation- correction is based on a monitored to ensure that the performance of the assay micrometer measurement of the total thickness of each system has not shifted since its last calibration. Control plate. The clad thickness of a plate is estimated by limits for acceptable performance can be established for subtracting the mean core thickness of the product the response to an appropriate working standard. The plates, which is determined by periodically sampling control chart of the responses to the working standard product plates and cutting a cross section to permit can be checked for indications of short-term instrument visual measurement of clad and core thickness. drift or malfunction. The control chart can also be analyzed to detect long-term shifts within the

2.3 INTERFERING RADIATIONS measurement-to-measurement control limits that may be corrected by recalibrating the system. Severe changes in As noted in Section B.2.1 of this guide, an internal instrument performance are investigated promptly and background variation may arise from changes in the their causes remedied.

amount of U.238 present in a fuel plate or from changes To ensure that the calibration remains valid during in the ratio of Th-234 to U-238 resulting from fuel normal operations and that accuracy estimates are manufacturing processes. Fluctuations in the internal rigorously justified, assay predictions are periodically background cause the response of the unknown items to compared with more accurate measurements of the be different from the calibration standards, thereby content of typical fuel plates (see Section C.4 of this creating a measurement bias. Such interferences can be guide). Guidance on methods to relate this assay to the compensated by measuring additional regions of the national measurement system and to reconcile verifi- gamma ray spectrum. cation measurements will be addressed in separate Other interfering radiations may come from external regulatory guides.*

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

C. REGULATORY POSITION

controlled through (1) removing radiation sources,

(2) shielding the detectors, and (3) monitoring the back- The content and distribution of U-235 in high- ground at frequent intervals. enrichment uranium plates can be measured through the gamma ray assay methods described in this guide.

3. CALIBRATION AND VERIFICATION Combining this measurement with the results of an independent measurement of the U-235 enrichment

3.1 INITIAL OPERATIONS 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 Calibration and the verification of assay predictions is an ongoing effort where performance is periodically acceptable to the Regulatory staff.

monitored and the calibration relationship is modified to improve the accuracy of assay predictions. During initial I. MEASUREMENT SYSTEM

operations, two means of basing preliminary calibrations are appropriate. 1.1 GAMMA RAY MEASUREMENT SYSTEM

3.1.1 Foil Calibration Technique 1.1.1 Gamma Ray Detector Methods for calibrating scanning systems for high- A thallium-activated sodium iodide scintillation enrichment uranium fuel plates through the assay of detector or series of detectors is recommended for this prepared uranium and clad foils are described in Refer- assay application. When more than one detector is to be ence 2. This method may be used in place of or in incorporated into the scan system, the performance characteristics of the detectors should be matched. The addition to the -technique described in the following subsection. diameter of the crystal should be larger than the projected view onto the crystal face through the

3.1.2 Fabricated Calibration Plates

• For example, regulatory guides related to measurement quality Calibration standard fuel plates can be fabricated amurance and calibration of nondestructive a&say systems are using special precautions to ensure that the amounts of being developed.

5.38-4

collimator channel. The thickness of the crystal should 2. MEASUREMENT INTERPRETATION

hc noi more than onc inch. The crystal should contain an internal cesium iodide seed which is doped with a 2.1 ENRICHMENT VARIATIONS

suilable alpha-emittcr for spectral stabilization. The seed should produce approximately 1,000 counts per second Procedures should be developed to ensure that the at the reference energy. enrichment of the plates being scanned is known through separate measurements. Fuel plates generally

1.1.2 Collimator satisfy the gamma ray penetrability criteria for quantita- tive U-235 assay; they do not satisfy the criteria for A collimator should be fabricated of appropriate nondestructive enrichment measurement through gamma gamma ray shielding material such as iron, lead, or ray spectrometry.* Facilities processing more than one uranium enrichment should maintain strict isotopic tungsten. The shielding should completely surround the detector and photomultiplier assembly and should be control and characterize the enrichment through appri.,-

sufficiently thick to completely block extraneous radi- priate measurement methods.

ations from the detector. The response variation from the center of the collimator opening to its edge should 2.2 ATTENUATION CORRECTIONS

be less than 1%.

Attenuation variations arising from plate-to-plate changes in core thickness, composition,-and clad thick-

1.1.3 Electronic Apparatus ness should be determined over the range of product tolerance specifications. When such variations cause the All electronic systems should be powered by filtered, assay error to exceed the error realized without the highly regulated power supplies. The ambient tempera- variations by 50% or more, procedures should be ture and humidity in the vicinity of the scanning system implemented to measure and apply a correction to the should be controlled so that permitted fluctuations do assay of each plate.

not significantly affect the assay measurements. All electronic circuitry in signal-processing components 2.3 RADIATION INTERFERENCES

should feature temperature compensation. Residual sen- sitivity to fluctuations in the, ambient environment A clear plastic template which shows an acceptable should be tested and monitored periodically. spectrum display should be prepared. When radioactive The capability for multichannel gamma ray pulse interference may be encountered, the assay spectrum height analysis with cathode ray tube spectral display should be compared at appropriate intervals to the should be provided. Signal-processing electronics capable reference spectrum for indications of interference. Bick- of stabilizing on the alpha radiations emitted within the ground radiation should be measured periodically during doped cesium iodide seed should be provided to stabilize each operating shift.

the energy spectrum.

3. MEASUREMENT CALIBRATION

1.2 SCANNING SYSTEM

During initial operations, the assay system should m, A mechanically sound, highly reproducible scanning calibrated either by the foil calibration method or with system should be employed. Scanning should be accom- specially prepared sample fuel plates as described mn plished by one of the three techniques discussed in Section B.3.1 of this guide.

Section B. 1.2 of this guide.

4. RANDOM AND SYSTEMATIC ASSAY ERRORS

1.3 COMPUTER CONTROL

4.1 RANDOM ERROR ESTIMATION

A dedicated minicomputer to control data acquisi- tion, analysis, calibration, diagnostic testing, and report A replicate assay program should be established to generate data for the evaluation of random assay errOrs preparation -should be employed for this assay appli- during each material balance period. During each bi- cation.

monthly interval, a minimum of fifteen plates should be selected for replicate assay. The second assay of each

1.4 MULTIPLE SCANNING ASSAY SYSTEMS plate selected for replicate assay should be made at least four hours after the first assay. Replicate assay diffet- When more than ornc scanning system is employed, ences should be collected and analyzed at the end (if the assay response should be normalized so that each instrument provides consistent results. Verification data *Criteria for uranium gamma ray enrichment measurenicTi*r. arff to establish the systeniatic assay error for each assay given in Regulatory Guide 5.21, "Nondestructrvr ltiranwurn.235 system should be obtained with the same plate. Enrichment Assay by Gamma Ray Spectromelr\* "

5.38-5

material balance period. The single-measurement stan- A minimum of one new calibration standard fuel plate dard deviation of the relative replicate assay differences should be introduced during each six-month period.

should be computed as described in Reference 4.

4.2.2 Comparative Evaluation

4.2 SYSTEMATIC ERROR ESTIMATION

When two measurements are made on each of a series The systematic error associated with the assay of all of items and the accuracy of ono of the methods used is fuel plates fabricated during a material balance period considerably greater than the other, the corresponding should be determined through one of the procedures* predictions can be compared to establish an estimate of presented below. bias between the measurement methods and to estimate the error. associated with the lens-accurate measurement

4.2.1 Propagation through the Calibration Function method. To precisely determine the systematic error in the nondestructive assay, the fuel plates selected for To estimate the systematic assay error through the comparative measurements should be randomly selected calibration function, the calibration should be based on but should span the range of U-235 contents en- the regression analysis of an appropriate function to the countered in normal production. The selected fuel plates calibration data. Uncertainties in the reference values of may be rejected from the process stream for failing to the calibration standards should be factored into the fit, meet quality assurance requirements. Each plate should and the errors propagated as demonstrated in Reference be repeatedly assayed to reduce the random asay error

5. to less than 10% of the estimated or previously To ensure the validity of the predictions, the stable established systematic error. To determine its U-235 and performance of the instrument should be monitored and total uranium content, the plate should be completely normalized through the response to appropriate working dissolved and the resulting solution should be analyzed standards which are assayed at frequent intervals. The by high-accuracy chemical and mass spectrometric pro- frequency for assaying working standards should be cedures.

determined through testing, but should not be lower For one material balance period during the initial than one test during each two-hour assay interval for implementation of this guide; a product fuel plate spot response stability and one full scan test during each should be randomly selected twice each week for an operating shift. Indications of shifting instrument perfor- accuracy verification measurement. Following this initial mance should be investigated and remedied, and the implementation period, facilities manufacturing 100 or instrument should be recalibrated to ensure the validity more fuel plates per week may reduce the verification of subsequent measurements. frequency to one plate per week and pool the verifi- In order to ensure that the calibration standards cation data for two consecutive material balance periods.

continue to adequately represent the unknown fuel LoW-throughput facilities manufacturing lesa than 100

plates, key production parameters which affect the plates per week should verify at least 4 plates per observed response should be monitored through separate material balance period through the procedures de- tests. Data should be compiled and analyzed at the close scribed above. At the close of each material balance of each material balance period. When a production period, data should be pooled to include only the 15 parameter shifts from previously established values, the most current data points.

impact of the shift on the response of the assay When the U-235 contents of the plates assayed using a instrument should be determined through an appropriate common calibration relationship varies over a ranpg of experiment or. calculation (Ref. 6). A bias correction +/-5% or more about the average of all plate loadings, the should be determined and applied to all items assayed systematic error should be estimated as described in from the point of the parameter change. The uncertainty paragraph 1. below; when plate loadings are tightly in the bias should be combined with the systematic error clustered about a nominal value, the systematic error predicted through the calibration function. When the should be estimated as described in paragraph 2.

bias exceeds 3% of the plate contents in a single material balance period, when a trend of 1.5% or more is 1. At the close of the reporting period, the assay observed in three consecutive material balance periods, value for each plate is plotted against the verified

,)r when the uncertainty in the observed bias is sufficient quantity. The verification data plot is examined for to increase the limit of error of the assay above 0.5%, indications of nonlinearity or obvious outlier data.

new calibration standards should be obtained, and the Anomalous indications should be investigated and scanning system should be recalibrated. remedied.

As a further check on the continued validity of the A linear regression analysis should be performed on calibration standards, a program to periodically intro- the comparison data. The intercept should be tested duce new calibration standards should be implemented. against zero for an indication of a constant measurement bias. The slope shouid be tested against unity for an

• These methods will be discussed in detail in a regulatory guide indication of a proportional bias. When bias is indicated,

,n preparation entitled "Calibration and Error Estimation assays performed during the preceding operating period Pr:ccduir"s fcr Nondestructi-e Asay." should be compensated. The systematic error should be

5.38-6

estimated as the standard error associated with the 1. Each core compact should carry a unique identifi- verification line. cation. Accountability records should be created for each compact. The fuel plate should carry an identifica-

2. When all plates contain essentially the same U-235 tion corresponding to the compact identification.

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 2. Each fuel plate should be radiographically exam- plates should be quoted as the standard deviation of the ined to ensure that the entire compact has been mean difference. For individual plates, the systematic encapsulated.

error should be quoted as the standard deviation of the difference distribution.

3. Each fuel plate should be checked with a gamma

5. CORE COMPACT ASSAY ray probe to qualitatively ensure that the plate core is uranium of the nominal product enrichment.

Final product assay in high-enrichment fuel plate manufacturing can also be accomplished through assay- ing each core compact following the procedures detailed 4. Calibration and error evaluation should follow the in this guide and the following supplemental criteria: procedures for fuel plate assay.

REFERENCES

I. J.E. Cline, R.J. Gehrke, and L.D. Mclsaac, "Gamma 4. John L. Jaech, "Statistical Methods in Nuclear Rays Emitted by the Fissionable Nuclides and Asso- Materials Control," TID-26298 (1973).

ciated Isotopes," ANCR-1029 (July 1972). 5. American National Standard N15.20, "Guide to

2. N.S. Beyer, "Assay of U-235 in Nuclear Reactor Fuel Calibrating Nondestructive Assay Systems," in Elements by Gamma Ray Scintillation Spec- preparation. Copies of the draft standard may be trometry," Proc. 4th Intl. Conf. on Nondestructive obtained from Institute of Nuclear Materials Manage- Testing, London, 1963. ment, 505 King Avenue, Columbus, Ohio 43201

3. J.H. Hubbell, "Photon Cross Sections, Attenuation (Attention: H.L. Toy).

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

from 10 keV to 100 GeV," Nat. Bur. Stds. Menlove, "Error Analysis of a Cf-252 Fuel Rod Assay NSRDS-NBS 29 (1969). System,'" LA-5317 (1974).

5.38-7