ML20151W635
| ML20151W635 | |
| Person / Time | |
|---|---|
| Issue date: | 10/31/1983 |
| From: | NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES) |
| To: | |
| References | |
| TASK-OS, TASK-SG-048-4, TASK-SG-48-4 REGGD-05.038, REGGD-5.038, NUDOCS 8808250063 | |
| Download: ML20151W635 (14) | |
Text
- - - - - - - -
s Revision 1 p#
U.S. NUCLEAR REGULATORY COMMISSION october 1683 4
4 l
, @,, - REGULATORY GUIDE OFFICE OF NUCLEAR REGULATORY RESEARCH REGULATORY GUIDE 5.38 (Task SG 048 4)
NONDESTRUCTIVE ASSAY OF HIGH ENRICHMENT URANIUM FUEL PLATES BY GAMMA RAY SPECTROMETRY A. INTRODUCTION B. DISCUSSION Part 70 of Title 10 of the Code of Federal Regulations The number, energy, and intensity of gamma rays 235 requires each licensee authosized to possess more than associated with the decay of U provide the basis for 350 grams of contained 23sU to conduct a physicat inven-nondestructive assay of high-enrichment fuel plates by tory of all special nuclear material in its possession at gamma ray spectrometr (Ref.1). The 185.7-kev gamma ray is the most usefuj 27s U gamma ray for this application; intervals not to exceed 12 months. Each licensee authorized 4
to possess more than one effective kilogram of high-it is emitted at the rate of 4.25 x 10 gamma rays per am of 235 enrichment uranium is required to conduct measured second per ps U are less penetrating and more sensitive to U. Lower energy gamma rays 2
physicalinventories of special nuclear materials at bimonthly emitted by intervals. Further, these licensees are required to conduct errors due to fluctuations in cladding and core thickness. In their nuclear material physical mventories in compliance general, rnore 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 complyhig with 235 U gamma ray, these provisions of Part 70 are detailed in Regulatory Guide 5.13, "Conduct of Nuclear hiaterial 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.
enriched uranium fabricated into flat or bowed plates.
Typically, these plates are relatively thin so that a signifi-
- 1. GAMM A R AY MEASUREMENT SYSTEM 23f cant percentage of the U 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 23sU l.1.1 Camma Ray Detector gamma rays can be used as an acceptable measurement of the distribution and the total 235 U content of each fuel liigh-resolution gamma ray detectors, i.e., high purity plate, in lieu of assaying the product fuel plates, fuel plate germanium, llPGe, also referred to as intrinsic germanium ccre compacts may be assayed through the procedures (IG), or lithium drifted germanium [Ge(L1)] detectors, detailed in this guide provided steps are taken to ertsure the provide resolution beyond that required for this assay traceability and integrity of encapsulathn 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 efficieney, 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.
Any guidance in this document related to information Most sodium iodide [Nal(T1)] scintillation detectors are p
collection activities has been c!:ared under OMB Clearance capable of sufficient energy resolution to be used for the No. 3150 0009.
8800250063 831031 easurement of the 185.7 kev gamma rays. For plate PDR REGGD 05.030 R PDR USNRC REGUL ArORY GUlOES Comments snould
- b. $.nt to tn. Secr.t.ry of in. Commission n.,s.i o,, c uio..... asu.a i o a.,c,io..n o m.s....i.. i. i o i n, nnMMoc",Tnf.ni aW27m"cn."*" " "".
f"?Jf,c "n',"f"J, fr."c"Ja1A"I, T/"""n?
==.n =,m=4"Sni.' a'an N
r n.,so...,. a,u.a, m. von o win, i.n o,..a.....> oe.,
==
oulo.,... nJr uo,ino..,o,...e.uoe...n. comon.n=u w in
)
- 2. n.., n...,0.,.. ~,cs.no r..
n..cio,.
e.
,.n,oo,,,.non
- i. 1
!"fe"U'tnN?o"Ei 180"f7.N?oINv"7".'y'[,'Ev'IllECs[f2ftn
I E"Oln"n.DI'd!O'OE'" I En'"t?$ [n'fr7n*En"ci.i nevi..
L"A"."oMu'm',a,'"in.'""""
5**"*'"""'""'*""2-Tnil guld. w.S IStu.d.f t., Con 11d...tlon of Com rn.nt s,SC eiv.d f,om
, f f C. O C..
ubsc lot on 5. v fo u,..
idesin SD.
'"?Xi's ?J"u"'t.".'J U".it",","','. L'.'E.!TX"A"','.L'o".*n iA's,Oi 0"!J'iu'ON,i"#"',".,'2.2/O"'*"i 3"'k",ff.'k'!;
fPtn'!,i'u'A,'a#t. "" d"
- """'"d' L'.tniit"!!. *dc*"A"id"*n.5 IM'sa!ri!,'?'IR'a0'ft a
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 sensitisity to vibration, the detector-to-plate method selected (see Section B.I.2 of this guide). For distance can be made large, especially with respect to the passiv : counting of the total fuel plate (see Section B.I.3 dimensions of the slit opening As an alternative means of 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 baftle into the slit or by fabricating the colli-cases, the thickness of the Nai crystalis 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.
"seeded" Nal crystals that contain a radioactive source (typically "3Am) 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.I.3), the collimator opening is on the reference, and the amplifier gain is automatically circular with a diamet:r less than that of the Nal crystal.
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 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 collimatoris intended to shie!d 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.)
measured. Thus the collimator shielding not only defines the front trea of the deferter cayStal to be exposed but it 1.1.3 Multiple Detectors also shields the sides and,if possible, the rear of the Jetec-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 positio.1cd to measure different technique to be employed. Once a measurement system is segments of a single fuel plate or several separate fuel plates calibrated with a particular collimator configuration, that simultaneously. In some cases it ir<y be useful to sum the configuration must be maintained for all subsequent assays.
response from two detectors posidoned 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 1.1. 2.1 Collimation for Scanning Techniques. lo enwre stabilhy.
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, 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 ISS.7 kev gamma rays emitted from outside the field of detector collimator ficld of view extends beyond the edges vie w.
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 minin.ize used. The linear dimensions of otner 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.I spacing, which is sometimes desirable to maximize the count rate. licwever, 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 enJs of the slit. This effect plate distanees. In this case, the measured material acts as becomes increasingly important at small de t e ct or-t o-an area source for which the counting rateis nearlyindepend-plate spacing, especially when scanning near the edge of a ent of the detector to-plate spacing. Therefore, in the
_I _
ror the IM.MeV gamma ray from U, a thickness equi.
"sweeping spot scan"technique discussed in Section B.I.2.2, ulent to si t mean fra path lengths in leaJ is arrroyimately the spacing is not as critical a measurement parameter.
rN yY m Various commercial conveying systems have been used and 5.362
l 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. liigh-precision tool equipment such is milling number of sites along the plate and multiplying this average machines, la thes, and x-y scanning tables can be investigated.
value by the measured area of the fuel core. The radiograph Numerically controHed 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 assa y.
being developed.
The collimator shape and dirnensions 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.
without scanning (see Section B.I.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 23s total U content of the plate. The detector response in a 1.2.1 Lincar Total Scan "wide-a ngle" counting geometry ca r. be converted to 235 grams of 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.
I plate cladding. Scanning the total plate is accomplished by starting the count sequence on the end of a plate and The detector colUmator and detector-to-plate distance continuing to count until the entire length of phte 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 transmicsion sources, detector, and collimator shieldingin a detected, it is necessary that the diameter of the detector standard measurement configuration.
crystal exceed the plate width or that the detector be 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.
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 I.2.2 Swceping 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 pbte 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 reproducibtuty. The computer can designed to maintain the detectoi-to-plate distance constant be used to control data acquisition by accumulating counts oser the entire fuel plate. The collimator channel dimensions according to a predetermined scheme. Also, the computer can be selected to proside compatible information on the can be used for data anabsis, 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 su bsequen t analysis are also readily accomplished 1.2.3 Sampled Increment Assay through an a ppropria tely designed computer-controued system. Use of a computer can be of great valuein many of When used in conjunction with raJiographic dimensional these functions for the total passive gamma counting measurements pctformed on all fuel plates, the 235 U technique as well.
l content of a fuel plate can be measured by scanning the ends of each fuel plate and sampling the balance of the
- 2. INTERPRET ATION 0F MEASUREMENT D ATA pbte. 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 p.
along the length of the pbte or by spot-scanning the fuct total passive counting technique can be distorted by several plate ends and measuring the distance between end spots effects for which corrections should tic made for accurate where the fuelloadins Mops. De 2n D content of the plate assas s. The three factors discussed below are the most 5.353
r
\\'
=
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.
2.1 Enrichment Variations 2.2.3 Core Filler Attenuation Ucensees 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 5 70.51. Under the conditions detailed in this show the effects of thie '.ype of attenuation. If significant I
l guide, the 235 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 i. depend-23s 0 enrichment of the core filler must be known from ently calibrated for each category of fuel plates.
separate measurements.
2.2.4 Attenuation Corrections Enrichment variations may also alter the radiation background in the gamma ray energy region ofinterest and When the thickness of the core and cladding and the cause fluctuations in the 235 U assay. The 23aU decays by composition of the core material are known, an attenuation 234 234 alpha particle emission to n The D 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 conections must also be 2 4 Pa which, in tu*n, decays by beta particle emission to applied to the assays of the standards in the calibration 234 234 U. Approximately I percent of the Pa decays are procedure. Ultrasonic gauging may provide such a measure followed by higlwnergy (e.g.,1001 kev,766 kev) gamma if the metallographic zones within the plate are sufficiently rays. These gamma rays frequently lose energy through define J to provide a detectable interface.
l 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 234 Pa may be altered by disturbing the equilibrium between micrometer measurement of the total thickness of each 338 U and 234 n, as frequently occurs in uranium chemical plate..he cir.d thickness of a plate is estimated by subtract-conversion precesses. De interference due to variations in ing the mean core tluckness of the product plates, which 338 U daughter activity becomes less important as the is determined by periodica!!y sampling product plates and enrichmer.t of 235 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.
I 2.2 Radiation Attenuation As long as the gamma ray attenaation corrections are computed on the basis of declared component thicknesses Attenuation of gamma radiation mayrange from complete and composition (or on the basis of ocersional me.sure-absorption of the radiatian by the intervening materia! to ments of thesc 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 processss. Both effects reduce the number of full energy the assays. A far more reliable approach to the application p35mma ray events that are detected. Gamma rays from of attenuation corrections is to measure the gamma ray 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. Brough well controlled complexity of the assay proceduds, but poses the further praduct tolerance limits, each of these potential sources of advantages (f {l) tendering the calibration dependent signal variability can be controlled to permit accurate only upon the U loading of the standard plates and 38 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 ne photon attenuation coefficient of uranium for accompanying passive assays are given in References 3,4, 235 gamma ray energies corresponding to U emissions is and 6. Specific details of a correctior. procedure for Materi-quite large (Ref. 5). Small changes in uranium density ots Testing Reactor (MTR) fuel t'ates are given in the resulting from increased fuel loading or frem variations in appendix to this guide, the manufnturing process can therefore significantly change the number of gamma rays that escape from the fuel 2.3 Interfering Radiations pla te.
As noted in Section B.2.1 of this guide, an internal 2.2.2 Cladding Attenuation background variation may arise from changesin the amount of 2 3sU present in a fuel plate or from changes in the ratio Small variations in cladding thickness may cause signif-of Th to 23sU resulting from fuel manufacturing 234 icant variations in attenuation. Rese variations in attenua-processes. Muctuations in the internal background cause l
tion can be measured by a simpic gamma ray absorption test the response of the unknown items to be different from the using thin sheets of cladJing material as absorbers and vary-calibratien standar is,therebycteating a fluctuating measure-5.38 4
l l
ment bias. In addition, some discrete gamma ray inter-more accurate measurements of the content of typical ferencea may oe present at energ es near 185.7 kev. For fuel plates (see Regulatory Postion 4 of this guide). Guid-further infoimation on these possible interferences, see ance on methods to relate this assay to the national measure-Reference 7. Bcth the back;;round 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 sp-etrum. Pertinent nuclear data for such Accounting Measurements."
measurements are available in Reference 1.
C. REGULATORY POSITIOfJ Other interfering radiations may come from external sources, from fuel pistes awaiting assay, or from nearby The content and distribution of235 U in high<nrichment 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 U enrichment enables the total urardum 235 intervals.
content of the fuel plates to be determined. The factors presented below should be taken into consideration for this
- 3. CALIDl!ATION AND VERIFICATION assay method to be acceptable to the NRC staff.
3.1 Initial Operations
- 1. MEASUREMENT SYSTEM 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 1.1.1 Camma Ray Detector accuracy of assay predictions. During initial operations, two means of basing preliminary calibrations are appropriate.
Thallium-activated sodium iodide [Nal(T1)] scintillation detectors are recommended for this assay application. When 1.1.1 Foi! Cahbration Technique more than one detector is to be incorporated into the measurement system, the perforrrance characteristics of Methods for calibrating scanning systerns for high-the detectors should be matched as closely as possible, and enrichment uranium fuel plates through the assay of the rela:ive response of the detectors should be checked prepared clad uranium fods are desc-ibed in Reference 2.
periodically to verify continued stability of the system.The These methods may be used in place of orin 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 colli nator channel.
A crystal thickness of 1/2 to I in. (13 to 25 mm)is recom-1.1.2 Fabricated Cohbration Pt.ites mended. The crystal should contain an internal seed that is doped w th a suitable alpha emitter (typically Am) to d
241 Cahbration standard fuel plates can be fabricated using produce a reference energy peak for spectral stabilization, special precautions to ensure that the amounts of uranium, The seed should produce approximately 1,000 ccrants per 235 U, inert matrix, and cladding are accurately measured second at the reference energy.
and that these parameters fall within manufacturing toler-ances for product plates.
1.1.2 Collimator and Detector Shielding 3.2 Routine Operations The collimator should be fabricated of appropriate ne performance of the assay system is periodically monitored to ensure that the response of tne assay system gamma ray shielding material such as iron, lead, or tungsten.
The shielding should completely surround the detector and has not shifted since its last cabbration. 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, 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.
1.1.3 Flectronic Apparatus lhe control chart can also be analyzed to detect long-term snifts 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 temperatu e system. In general, however, it is important that observed and humidity in the vicinity of the scanning system should instru nent drifts and performance changes be investigated be controlled so that permitted fluctuations do not signifi-and remedied rather than compensated for by recalibration.
cantly affect the assaymeasurements. Allelectronic circuitry To ensure that the calibration remains valid during in signal processing components should feature temperature compensation. Residual sensitivity to fluctuations in the normal operations and that accuracy estimatesare rigorously ambient environment should be tested and monitored justified, anay predictions are periodically compare 1 with periodically.
5.38 5
s.
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 wcll as the appendix on the reference energy peak produced by the alpha-emitter-to this guide, doped seed should be provided to stabilize the energy spect ru m.
2.3 Radiation Interferences l
1.2 Measurement System A graphic record of an acceptable (reference) gamma ray i
spectrum display (i.e., free of interferences and exhibiting f
plate scanning should be accomplished by one of the nominal background) should be prepared. When radioactive three techniques discussed in Section B.I.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 j
ducible, automated scanning system should be employed.
spectrum for indications of interference. Background l
When more than one scanning syste n is employed, the radiation should be measured periodically during each
)
assay responses of each system should be normalized so operating shift.
that eachinstrument provides consistent results. Verification l
data to estimate the bias for each assay system should be
- 3. 51EASURE3 TENT CALIBRATION AND CONTROL obtained with the same standard plate.
l>uring 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 desenbed in Sec-employed to ensure the achievement of a reproducible tion B.3.1 of this guide. Instrument resporse to appropriate rneasurement geometry.
working standards should also be checked periodically to verify the continued stabilit) of the assay system calibration.
l.3 Computer Control
- 4. SOURCES OF VARI ATION AND BIAS A dedicated minicomputer to control data acquisition, calibration, diagnostle testing, and report preparatior 4.1 Random Assay Standard Deviation Estimation should be employed for fuel plate assay operations.
A replicate assay program should be established to
- 2. SIE ASURFSIENT INTERPRETATION generate data for the evaluation of the random assay l
standard deviation during each material balance period.
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 c,f each plate selected for replicate assay should be ennchment 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 235 gamma ray penetrability criteria for quantitative U
the material balance period. The single measurement assay; they do not satisfy the criteria for nondestructive standard deviation of the replicate assay differences should enrichment measurement through gamma ray s'pectrometry.2 be computed as describedin Reference S. 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 characterite 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 frem 2.2 Attenuation Corrections previous inventory periods can innprove 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 Desiation 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 desiation realired 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, implemented to measure and apply a correction to the These methods are discussed in detailin ANSI 15 20 1975, assay of each plate. It should be noted that routine measure-
"Guide to Calibration of ND A Systems,"3 and in Regulatory ment of attenuation corrections for each plate is recom.
Guide 5.53, "Qualification, Calibration, and I tror 13tima-mended since such a procedure will remove the necessity of tion Methods for Nondestructive Assa y" (a proposed revision to this guide has been issued for comment as Task 2
SG 04 9-4)'
l Criteria for gamma ray uranium enrichment measurements are given in Regulatory Guide 5.21. "Nondestructive Uranium 235 3
I nri< hme n t Assay by Gamma Ray Spectrometry." A proposed MadaNe trom the American National Standards Institute
- reWion to this guiJe has been luued for comment as Task 56 0444.
14 3o Hroadw ay. New i ork, New York 1o014.
5.38-6
I l
i b
To estimate the standard deviation arising frorn 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 desiation platee selected for comparative measurements should be can be derived using the residual mean square. The standard rar.domly selected but should span the range of U
235 deviation for the calibration standards includes the standard con ten ts encountered in normal production. The fuel deviation of the reference values for the calibration stand.
plates could have b"n selected fram those rejected from ards. See ANSI 14.20-1975.
the process stream f or 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 of variation) to less than 10 percent. To determine its normalized through the response to appropriate working 238 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 operatmg shift.
For one material balance period during the initial l'or 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 beinvestigated 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.
lP ate 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 centinue two consecutive material balance periods. Low throughput to adequately represent the unknown fuel plates, k y facilities manufacturing less than ICO platis per week production parameters that affect the observed response should verify at least 4 plates per material balance period should be n onitored 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 9 the monitoring of plate parameters is less critical for assay pooled (if allowable) to include the 15 most cunent data accuracy.) Data should be compiled and analyzed at the points. Ilowever, if the data are demonstrably stable over close of each material balance period. When a production longer periods, using additional data points from previous 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 auay mstrument assay standard deviation estimate.
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, mined and applied to all items assayed from the point of When the 235 U content of the plates assayed using a common the parameter change. The variance of the bias estimate c.libration relationship varies over a range of 5 percent or should be combined with the variance due to the calibra-tion procedure. When the bias eAceeds 3 percent of the more of all plate loadings, the bias should be estimated by MethoJ No.1.
plate contents in a single material balance penod, when a When plate loadings are tightly clustered trend of 1.5 percent or more is observed in three consecutive about a nominal value, the bias d. avid be estimated by Method No..
material balance periods, or when the standard deviation in the estimated bias is sufficient to increase tue standard error Method No.1. At the close of the reporting renod, the (i.e., twice the standard deviation) of the assay above assay value for each plate is plotted against the verified 0.5 percent, new cabbration standards should be obtained ad Ihe scanning measurement s) stem should be recabbrated, quantity. The verification data plot is examined for indica-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 outber data are contained in Regulatory cahbration stanJarJs,a program tointroduce new calibranon Guidc 5.36, "Recommended Practice for Dealing with standards perinJically should be implemented. A nunimum Outlymg Observations." The comparison data should of one new cahbration standud fuel plate should he intro-duced during each sidmonth period.
be analyzed as descnbed in Regulatory position 7.3 of Regulatory Guide 5.53, "Quahfication, Calibration, and 4.3 Bias 1 stimation Error Dtimation Methods for Nondestructive Assay." A proposed revision to this guide has been issued for comment as Task SG 049-L When two sets of measurements are made on cach of a G is considerably better than the other, the corresponding wries of items and the accuracy of one of the methoJs used Method No. 2. When all plates contain essentially the 235 estimates can be comp.tred to estabbsh an estimate of bin 0 content, the thfference in the mean content ume bet w een the measurement methods and to estimate the values should be tested against tero as an indication of bias, and the standard deviation associated with an inventory of 5.3 L7
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.
tion should be estimated as the standard deviation of a single measurement.
- 2. 1:ach fuel plate should be radiographically examined
- 5. CORE COMP 4CT 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 ass.;ying each
- 3. Each fuel plate shoulJ 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.
- 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 assty.
8a u
.- m n
0 I
9 5.38 8
0 /
APPENDIX I
SAMPLE ATTENUATION CORRECTION BY TRANShilSSION MEASUREMENT FOR MATERIALS TESTING REACTOR FUEL PLATES a
1.DACKGROUND where p is the mass absorption coeff3.r,i if the cladding 185.8 kev (for aluminum, pc = 0.V ca '/g) and o is at 3
c Gamma ray assay data are subject to distortions due to the clidding density (for aluminum, q../ g/cm ). If the the attenuation of the gamma ray flus by the intervening cladding thickness varies by as much as 10 percent, the sample material and sample contaimr. The data must be corresponding variation in T will be only 0.2 percent. Thus c
corrected for this effect or the amount of nuclear material an assumption of invarisnt 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 Staterials assay variance when the constant cladding attenuation 23s Testing Reactor (SITR) fuel plate is related to the U
correction is applied. One then determines T for the fuel c
content A1 f the fuel plate by:
plates from careful measurement of the cladding thickness U
and application of Equation 4 SIU
- kI/C UI Under the abose assumption, one cari then determine where k is a calibration constant that Ndes effects such the transmission of the core material Tg rom the measured f
as detector efficiency, counting geom ry, and nuclear total plate transmission T, kr.awledge oT T,and Equation 3.
c properties of uranium. The factor C is the orrection 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):
Ik5.7-kev gamma ray by the plate cladding and core material in Tg C=
(5)
Deternunation of this attenuation correction factor can T (!-T I e
U be accomplished using an external gamma ray source.
235
' Ideally, this should be a U source. For detads on how 2.151PL L M E NT A TION
}
to use a transmission source with a gamma ray energy dif-ferent from that measured in the away, see Reference 4.)
2.1 The Scanning lechniques 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 l' is compared with the source intensity with no olate fuel plate as shown in Figure 1. The transmi sion cortection present l to obtain the gamma ray transmiuion T through must be measured and applied at each scan point so that g
the plate materials:
nonuniformities in core composition within a plate can be corrected for. The transmission of the plate T(i)is rocasured I = l'!!g (2) at each scan point i by ermining (1) the p12te count rate with the transmissien source shielded 1(i) (Figure I A),
b T his total plate transmission can be subdivided as (2)the total counting rate of the plate and unshielded f ollow s:
transmission source I (i)(Figure 1B), and (3) the transmis-T tion source count rate with no intervening plate I (i) g T=TI (3)
(Figure IC).
U T(i) = [1 (II'ICIIIII (i)
(6)
T o
w here T n the gamma ra) transmission through one thickness'of the plate cladding, and Tu is the transnnssion if the transmission source is a small locali7ed 235g through the core materut. (The same claJJmg thickness on source, a plate assay with the attenuation correction will both siJes of the plate is assumed.)
require two scans: one to get the I (i) values and one to p
gt t the f(i) values. (The quantity I (i) wdl be constant 2t g
fhe gamma ray transmission through a plate is dominated all scJn points and can be measured at a separate time.) If by the effect of the core meerial (i.e.. Ty T ). so it is the transmission source is anoth" 'el platt that remains c
convenient to treat the cluding transmission T as a stationar) with respect to the pla ing asst > ed, the I (i) c constant. Furthermore, variations in the core composition must be measured by scanning
...e transmission source g
udi cause more drasile fluctuations in the gamma ra) fuel llate. That is, an unattenuated transminion source attenuation than the snull variatio9s in the claJding thick.
plate intensity i (i) must be measured at the same scan g
ness (c. For esample, a 20-md (0.051 cm) aluminum romts i and associated with the corresponding 1 (i)and 1(i) 7 slaJJmg thukness attenuates the iM.7 kev camma intens-from the measurements with the unknown plate. The it) ncording to count arrays 1 (i),10), and I (i) must be stored in the 7
g
[
computer memory as they are measured. The counts 1(i) are
,c:
then currected by the factor in liquation 5 for each total T
=e
= 0. N (4) plate transmisdon T(i).
c 5.M
2.2 T'otal Passive Count Technique
- 3. MEASUREMENTS WITil filGil RESOLUTION SYSTEMS in this case, an average attenuation correction is deter.
The tnnsmission of the 235 U gamma ray can be inferred 238 mined by measuring T for the entire plate using a U
from mearured transmission just above and just below source behind the plate. An extended transmission svurce is 185.7 kev in energy. In one application using high reso!ution recommended (ideally another fuel plate) in order to gamma ray spectrometers (Reference 10), a 3 6'Yb trans.
observe an average transmission over as much of the anission source is u '. Two of the gamma rays emitted by unknown plate as possible. The transmission source must this hotope 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 region. Measurement of T fuel plate being tssayed. 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 cenection factor C at source l, (2) plate,lus unshielded source 1, and (3) the 235 7
U gamma ray ca.ergy. A high-resolution detector unshielded source with no plate 1. The average plate system must be used i t order to resolve the 177.2,185.7, 0
transmission 1 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 ' II/I (7) and multiple scans or multiple counts are not necessary. As o
a practical matter, '69Yb has the short half-life of 32 days, so this source must be replaced frequently (or reitradiated 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 1.
precise measurement of the attenuation cor+ections.
9 l
l l
9
$.3 8-10 L.
. +
e l
I (v
)
SOURCE SHIELDING TR ANSMISSION SOURCE l 7' / 7'l /1 s
s COL LIM ATED DETECTOR UNKNOWN FUEL PLATE x
(A) s s
Er///A E / / / / /1 1
I (B)
(C)
Figure i A schematic of the measu;ement arrangements for MTR fuel plate pmma ray assay wit',
., red attenuation correction.
The close geometry of the scanning technique is used as an example. (A)Configuratio -or measuring U radiation 235 9
coming only from the unknown fuel plate (1 in the text). (B) Configuration for deterrmning the sum of the fuel plate pmma intensity and the source intensity passmg through the fuel plate (IT n the text). (C) Configuratio.i for measuring i
the incident trans.nission source gamma intensity (1,in the text),
$.3811
REFERENCES 1.
J C. Clint, R. J. Geh:ke, and L. D. Mclssac, "Gamma ment." Proceedings of the ERDA X and Gamma Ray Rays Emitted by the Fissionable Nuclidas and Asso-Symposium. Ann Arbor, Michigan, Conf. 760639, ciated isotopes," ANCR 1029,1972.
- p. 219, Msy 1976, 2.
N. S. Beyer, "Assay of 23s U in Nuclear Reactor Fuel 7.
T. D.
Reilly, "Gamma Ray Measurements for IJements by Gamma Ray Scintillation Spectrometry,"
Uranium Enrichment Standards," hoceedings of the haceedings of the 4th International Conference on A merican Nuclear Society Topical Meeting on Nondesiructive Testing, Lcadon,1963.
"Measurement Technology for Safeguards and Material Control," Kiawah Island, South Carolina, 3.
R. Sher and S.Untermeyer,7he Detection of Fission-November 1979; National Bureau of Standards sbk MarialbyNnkstructive.Wns, American Ntwlear Special Publication No. 582, p.103, June 1980.
Society Monograph,1980.
8.
J. L. Jaech, "Statistical Methods in Nuclear Materials 4.
R.11. Augustson and T. D. Reilly, "Fundamentals of Control," Atomb Energ/ Comntinion, Report Passive Nondestructive Assay of Fissionable Material,"
No. TI D-26298,1973.
Los Alamos Scientific laboratory, IA 5651 M,1974.
9.
R. A.
- Forster, D. B.
Smid, and 11. O. Menlove, 5.
E. Storm and 11. Israel, "Photon Cross Sections from "Error Analysis of a Cf 252 Fuel Rod Assay System,"
0.001 to 100 MeV for Elements 1 Through 100,"
Los Alamos Scientific Laboratory, LA-5317,1974 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.
O l
l
(
O 5.3h 12
i l
e em
)
VALUE/ IMPACT STATEMENT
%_)
- 1. PROPOSED ACTION
- 2. TECllNICAL APPROACil 1.1 Description and Need Not applicable.
Regulatory Guide 5.38 was published in September
- 3. PROCEDUR AL APPROACil 1974. The proposed action, a revision to this guide, is needed to bring the guide upto date with respect to advances Of the procedural alternatises considered, revision of in measurement methods and changes in terrninology, 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 Authorlty 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 <tgencies Reorganization Act of 1974, as amended,and implemented through the Commission's regulations.
Not applicable.
4.2 Need for NEPA Assessment f.2.3 Industry The proposed action is not a major action that may Smee 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.
9 1.2.41%blic
- 5. RELATIONSillP TO OTIIER EXISTING OR PROPOSED REGULATIONS OR POLICIES No aJverse impact m the public can be foreseen.
The proposed action is one of a series of revisions of existing regulatory guides on nondestructive assay techniques 1.3 Decision on Proposed Actio
- 6. SU5thl ARY AND CONCLUSIONS The regulatory guide should be revised to retlect 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.
l l
5.38 13
UNITED STATES
,,ast et ass sait is ta<c NUCLEAR REGULATORY COMMISSION
'05'
- GM ','g WASHINGTON, O C. 20565 aasa o c es muit 6..ut of ficiAL suSINESS PEN ALTY FOR PmivAf f USE. s)4 l
l l
1 l
l t