Regulatory Guide 5.34

Nondestructive Assay for Plutonium in Scrap Material by Spontaneous Fission Detection

ML13064A073
Person / Time
Issue date:	06/30/1974
From:	US Atomic Energy Commission (AEC)
To:
References
RG-5.034
Download: ML13064A073 (11)

1974.r U.S. ATOMIC ENERGY COMMISSION UE17R E*REGmULATORY GUIDE"DIRECTORATE OF REGULATORY STANDARDSREGULATORY GUIDE 5.34NONDESTRUCTIVE ASSAY FOR PLUTONIUM IN SCRAP MATERIALBY SPONTANEOUS FISSION DETECTION

A. INTRODUCTION

Section 70.51, "Material Balance, Inventory, and Records Requirements," of10 CFR Part 70, "Special Nuclear Material," requires certain licensees authorized topossess at any one time more than one effective kilogram of plutonium to establishand maintain a system of control and accountability such that the limit of error(LE) associated with the material unaccounted for (MUF), ascertained as a resultof a measured material balance, meets minimum standards.Included in a typical material balance are containers of inhomogeneousscrap material that are not amenable to assay by the traditional method ofsampling and chemical analysis. With proper controls, the nondestructive assay(NDA) technique of spontaneous fission detection (SFD) is an acceptable methodfor the assay of plutonium in containers of bulk scrap material. The use of SFDthus facilitates the preparation of a complete plant material balance whoseLEMUF meets established requirements.This guide describes procedures acceptable to the Regulatory staff forapplication of the technique of spontaneous fission detection for the nondestruc-tive assay of plutonium in scrap.

B. DISCUSSION

Plutonium in scrap material can contribute significantly to the materialunaccounted for (MUF) and to its associated limit of error (LEMUF). Unlike themajor quantity of material flowing through a process, scrap *is typicallyUSAEC REGULATORY GUIDES Copies of published guides may be obtained by request indicating the divisionsdesired to the US. Atomic Energy Commission. Wasshington, D.C. 20545,lioulato v Gu-des ... ,s.ed to describe end make availeble to the public Attention: Director of Regulatory Standards. Comiments and suggestions for,iethodt a sceptablit to thn AEC Reguletor,/ staff of implementing specific parts of irnprovem'antm in the guides ere encourad and should be sent to the Secretarytire C1nn nont's to delineate techniques used by the staff in of the Commission. US. Atomic Energy Comenission, Washington, D.C. 20545,-iuJt.01P specific problems or postulated accidents, or to provide guidance to Attention: Chief. Public ProcmedingStaff.lpolicans Regulatory Guides sm not sutStitutes for regulations and complinaew-th them i* not required. Methods ernd solutions different from those eat out in The guides are issued in the following ten broad divisions:the guldes will be acceptable if they prosvide a basis for the findings requisite tothe issuance or Wcninuance of a permlt or license by the Cornrmission. 1. Power Reactors 6. Product2. Resetrch ancd Test RI.tors 7. Trarnportation3. Fuels and Materials Facilities 8. Occupational HealthPublished guides will be revised periodically, as appropriate. to accommodate 4. Environmental and Siting 9. Antitrust Reviewcomirnts and to reflect new informttion or experience. 5. Materials and Plant Protuction 10. Generel inhomogeneous and difficult to sample. Therefore, a separate assay of theentire content of each container of scrap material is the only reliable methodof scrap accountability. Nondestructive assay (NDA) is a method for assayingthe entire content of every container of scrap.The term "scrap" refers here to material that is generated incident to themain process stream due to the inefficiency of the process. Scrap material isgenerally economically recoverable. Scrap therefore consists of reject orcontaminated process material such as pellet grinder sludge, sweepings from aglovebox, dried filter sludge, and reject powder and pellets. Scrap is distinguishedfrom "waste" by the density or concentration of heavy elements in the two materials.The concentration of uranium and plutonium in scrap is approximately the same asit is in process material, i.e., 85-90% (U + Pu) by weight. Plutonium in fastreactor scrap material is 15-20% by weight and in thermal reactor recycle material2-9% by weight. The main difference between scrap and process material is thatscrap is contaminated and inhomogeneous. Waste, on the other hand, contains alow concentration of plutonium and uranium, i.e., a few percent or less (U + Pu)hy weight. However, the recovery of combustible waste by incineration mayproduce ash that is high in uranium and plutonium concentrations. Such incin-erator ash is also considered "scrap" in this Ruide.Nondestructive assay for plutonium can be accomplished primarily by thepassive methods of gamma ray spectrometry, calorimetry, and spontaneous fissiondetection. Regulatory Guide 5.111 provides a framework for the utilization ofthese NDA methods.Gamma ray spectrometry of scrap consisting of dense materials can beunreliable because of the attenuation of gamma rays. Gamma ray spectrometry ismost applicable to waste assay.Calorimetry is an accurate method of plutonium assay when there is anaccurate knowledge of the relative abundance of each plutonium isotope andamericium-241. Scrap may contain a mixture of materials of different radio-nuclidic compositions, especially different americium-241 concentrations,necessitating the measurement of the average radionuclidic composition. Theaverage radionuclidic abundances can only be accurately measured when the scrap5.34-2 is reasonably homogeneous. When the radionuclidic abundances can be accurately2measured or controlled, calorimetry can be applied to scrap assay. However,calorimetry is time-consuming for heterogeneous materials of high heat capacityand may not be a practical method for the routine assay of large numbers ofcontainers.Spontaneous fission detection (SFD) is the most practicable and generallyapplicable NDA technique for the assay of plutonium in scrap material. Spontaneousfission radiations are sufficiently penetrating to provide a representativesignal from all the plutonium within a container. The plutonium isotopic compositionmust be known for SFD assay, but the accuracy of SFD is not as dependent on theaccuracy of analysis for the minor plutonium isotopes as is that of calorimetry.Nor is SFD sensitive to americium-241 ingrowth. The quantity of scrap materialon inventory when a material balance is computed can be reduced through goodmanagement, and the scrap remaining on inventory can be assayed by SFD to meetthe overall plant MUF and LEKUF constraints required by paragraph (e)(5) ofSection 70.51 of 10 CFR Part 70.This guide gives recommendations useful for the SFD assay of containers,each containing a few liters of scrap and having contents ranging from a fewgrams to a few hundred grams of plutonium or approximately 50 grams of effectiveplutonium-240.*: Containers with a larger plutonium content, i.e., on the order,-f 500 grams of plutonium or more, give a spontaneous fission response that isiifficult to interpret due to high countina rates and noscihlP neutron multi-Dlication. A large auantitv of plutonium can he a.sayed hv SF1) by subdividingthe scrap into smaller amounts, or the items may be more amenable to nondestructiveassay bv calorimetry.*The effective plutonium-240 mass is a weighted average of the mass of each ofthe plutonium isotopes. The weighting is equal to the spontaneous fissionneutron yield of each isotope relative to that of Pu-240. Since only the even-numbered isotopes have significint spontaneous fission rates, the effective Pu-240mass is given approximately by:M(240)eff = M(240) + 1.64M(242) + 2.66M(238)where M is the mass of the. isotope indicated in parentheses. The coefficients in thisequation are only known to approximately +5%.5.34-3

C. REGULATORY POSITION

The method of spontaneous fission detection (SFD) for the nondestructiveassay for plutonium in bulk inhomogeneous scrap material should include: (1)discrimination of spontaneous fission radiations from random background bycoincidence techniques and (2) measurement of the relative plutonium isotopiccomposition of the scrap by an independent measurement technique. An acceptableSFD method of plutonium assay is described below:1. Spontaneous Fission Detection Systema. Detectors. Instruments based on moderated thermal neutron detectors,i.e., neutron well coincidence counters,4,5 are recommended for applications inwhich the gross neutron detection rate does not exceed 2 x 104 neutrons/sec.The( dead time inherent in these slow coincidence systems can be reduced by employinga shift-register coincidence circuit. If the gross neutron detection rate isdue primarily to random background and exceeds 2 x 104 neutrons/sec, then afast neutron detection, single coincidence system can be used, provided thatadequate corrections can be made for matrix effects. Matrix effects are moresevere in fast neutron detection systems, as shown in Table I.b. Detection Chamber. The chamber should permit reproducible positioningof standard-sized containers in the location of maximum spatial response uniformity.c. Fission Source. A spontaneous fission source with a neutron intensitycomparable to the intensity of the largest plutonium mass to be assayed should7be used for making matrix corrections, using the source addition technique. Ananogram of Cf-252 is approximately equivalent to a gram of effective Pu-240.d. Readout. Readout should allow computation of the accidental to realcoincidence ratio in addition to the net real coincidence rate. Live timereadout or a means of computing the dead time should also be provided:e. Performance Specifications. The performance of a SFD instrumentshould be evaluated according its stability, uniformity of spatial response, andinsensitivity to matrix effects. Therefore, information should be obtainedregarding:(i) The precision of the coincidence response as a function of thereal coincidence counting rate and the accidental to real coincidence ratio.Extremes in the background or accidental coincidence rate can be simulated byusing a source of random neutrons (nonfission).5.34-4 TABLE IMATRIX MATERIAL EFFECTS ON NEUTRON ASSAYNeutron Detection Efficiency8Lo1'4-Matrix Material(in "ý4 liter can)Empty CanCarbon PelletsMetalSlag-CrucibleConcreteString FiltersCH2 (p=0.65 g/cc)CH2 (p=O. 12 g/cc)CH2 (P=0.27 g/cc)H 20 (p=l.O0 g/cc)Mass(kg)1.893.601.803.240.600.270.430.973.621.031.041.031.051.071.061.091.190.983He Detector;Thermal1.004He Detector,Fast1.000.830.940.840.950.960.920.710.36ZnS Detector,1.000.750.910.790.860.920.900.670.35Coincidenceifficiency,He Detector,Thermal1.001.051.091.081.101.171.111.191.360.98Correcteda,7CoincidenceEfficiencyHe Detector,Thermal1.000.971.021.011.021.051.000.981.040.96aCorrected using the sourceaddition techniqu (ii) Uniformity of spatial response. Graphs should be obtained onthe relative coincidence response from a point source of fission radiation as afunction of position in the counting chamber.(iii) Sensitivity of matrix interference. A table of the relativecoincidence response from a point source of fission radiation as a function ofthe composition of the matrix material surrounding the point source should beobtained. Included in the matrix should be materials considered representativeof commmon scrap materials. Table I is an example of such a tabulation of therelative response for a wide range of materials.This information should be used for evaluating the expected instrument per-formance and estimating errors. The above performance information can be requestedfrom the instrument suppliers during instrument selection and should be acquiredduring preoperational instrument testing.2. AnalystA highly trained individual should oversee SFD assay for plutonium and shouldhave primary responsibility for instrument specification, preoperational instrumenttesting, standards and calibration, writing an operation manual, measurementcontrol, and error analysis. Experience or training equivalent to a bachelorsdegree in science or engineering from an accredited college or university and alaboratory course in radiation measurement should be the minimum qualifications ofthe SFD analyst. The SFD analyst should review SFD operation at least weekly andshould authorize all changes in SFD operation.3. Containers and PackagingA single type of container should be used for packaging all scrap in eachcategory, as discussed below. A recommended uniform container that would facilitateaccurate measurement and would standardize this segment of instrument design isa thin-walled metal (steel) can with an inside diameter of approximately 10 cmor less.4. Reduction of Error Due to Material VariabilityThe SFD response variation due to material variability in scrap should bereduced by: (1) segregation-of scrap into categories that are independentlycalibrated, (2) correcting for matrix effects using the source addition technique,7or (3) applying both categorization and the source addition technique. qategorizationshould be used if the SFD method is more sensitive to the material variabilityfrom scrap type to scrap type than to the material variability within a scrap5.34-6 type. Application of the source addition technique reduces the sensitivityto material variability and may allow the majority of scrap types to be assayedunder a single calibration. Material characteristics that should be consideredin selecting categories include:a. Plutonium Isotopic Compositionb. Uranium/Plutionium Ratioc. Containerization and Packagingd. Abundance of High-Yield alpha-neutron Material, .i.e., low-atomic-number impuritiese. Plutonium Contentf. Density (both average density and local density extremes should beconsidered)g. Matrix Composition5. CalibrationA guideto calibration for nondestructive assay is presently under developmentby Task Force 8.3 of the N15 committee of the American National Standards Institute*and will include details on calibration standards, calibration procedures, curvefitting, and error analysis. Guidelines relevant to SFD are given below.a. A minimum of four calibration standards of the same isotopic compositionas the unknowns should be used for calibration. If practicable, a calibrationcurve should be generated for each isotopic blend of plutonium. When plutonium ofdifferent isotopic composition is assayed using a single calibration, the effecton the SFD response of isotopic composition should be determined over the operatingranges by measuring standards of differing plutonium isotopic compositions.The use of the effective Pu-240 concept can lead to error because of the uncertaintyin the spontaneous fission half-lives, as shown in Table II, and the variation inresponse with isotopic composition.b. Calibration standards should be fabricated from material having aplutonium content determined by a technique traceable to or calibrated withNational Bureau of Standards standard reference material. Well-characterizedhomogeneous material similar to the process material from which the scrap isgenerated can be used to obtain calibration standards.c. Fabrication of calibration standards that are truly representative ofthe unknowns is difficult for scrap assay. To measure the reliability of thecalibration based on. the fabricated standards discussed above, and to improvethis calibration, unknowns that have been assayed by SFD should periodically be*When copies become available, they may be obtained from the American NationalStandards Institute, Inc., 1430 Broadway, New York, New York 10018.5.34-7 TABLE IIEFFECTIVE PLUTONIUM-240 ABUNDANCE AND UNCERTAINTYCORRESPONDING TO DIFFERENT BURNUP CATEGORIESaApproximate Abundance (%)BUXl IUP(N-dt) Pu-238 Pu-239 Pu-240 Pu-241 Pu-242 Pu-240eff8,000-10,000 0.10 87 10 2.5 0.3 10.75+/-0.03(0.3%)16,000-18,000 0.25 75 18 4.5 1.0 20.30+/-0.08(0.4%)25,000-27,000 1.0 58 25 9.0 7.0 39.14+/-0.50(1.3%)38,000-40,000 2.0 45 27 15.0 12.0 52.00+/-0.87(1.7%)aComputed using the following coefficients for Pu-238 and Pu-242 in theequation for Pu-240 effective:M(240)eff = M(240) + 1.64+/-0.07 M(242) + 2.66+/-0.19 M(238)The uncertainties in the cn'.:icients and in the effective Pu-240abundances in the table are from tie reportea standard deviations inthe most reliable data available.-5.34-8 selected for assay by an independent more accurate technique. Calorimetry2 canbe used to assay a random selection of scrap in containers and provide reliable datathat should be fed back into the calibration fitting procedure to improve SFDcalibration. The original calibration standards should be retained as workingstandards.6. Measurement ControlFor proper measurement control, a "dummy" item should be assayed on eachday of scrap assay as a background measurement. Also, control (or working)standards should be assayed each day scrap is assayed for normalization and toassure reliable operation.The source addition technique7 is recommended for correcting the SFD responsefor each assay. If not, used routinely, the source addition technique should beapplied to a random selection of items but in no case should be used less frequentlythan daily. The results of random applications of the source addition techniquecan be used in two ways:a. As an average correction factor to be applied to a group of items, andb. As a check on the item being assayed to verify that it is similar tothe standards used in calibration and that no additional matrix effects arepresent, i.e., purely as a qualitative assurance that the calibration is valid.7. Error AnalysisThe sources of error in SFD are discussed in Regulatory Guide 5.11.1Analysis of the error in the calibration is discussed in the literature4,9and in the ANSI guide on calibration now under development. In addition to thecalibration error there are errors due to the measurement process and due tovariability in material composition.The error due to the measurement process, i.e., the measurement-to-measurementerror, accounts for most of the random error in NDA. At least fifteen unknownsselected at random should be repeatedly assayed to estimate the random error.Repeated measurements should be made under as many different conditions as areexperienced in normal operation, e.g., different times of day, different operators,different ambient conditions. The standard deviation in the distribution ofdifferences in replicate results should be used in constructing a 95% confidenceinterval. The mean difference in replicate results has an expected value of zero.Corrections for significant drift in the instrumentperformance should be made basedon data from daily assay of control standards, i.e., the measurement control program.5.34-9 The error due to material variability, i.e., the item-to-item error, is theinalor source of bias and systematic error in NDA. If proper calibration standardsaiid a proper calibration relationship are used, the calibrating error should be areliableestimate of the systematic error. To test these assumptions, and todetermine the bias, SFD assay results on a random selection of unknowns should becompared with assays on the same items by an independent more accurate technique,as discussed in 4(c). Calorimetry is not sensitive to the mf1ajority of interferencesthat cause error due to material variability in SFD and is practical for thisapplication because it is nondestructive. An alternative method for verifying SFDassay .s to sample the scrap extensively and to perform chemical analyses for theplutonium concentrations in these samples.The mean difference in comparative assays should be used as the bias forcorrecting SFD assay results. The bias correction should be made if the meandifference is greater than 0.1 times the standard deviation in the mean difference.The standard deviation in the bias (mean difference) is a systematic error thatshould be used in constructing a 95% confidence interval. (There will always bea potential bias and systematic error in the technique used to verify SFD. Thesystematic error should be known and should be insignificant compared to systematicerror in SFD for the technique to be viable for verifying SFD assay results.)Comparisons of SFD with a more accurate assay method should be made on at leasttwo unknowns a week to determine bias and systematic error. Data may be pooled andused to improve the calibration although no data should be older than one year.REFERENCES1. Regulatory Guide 5.11, "Nondestructive Assay of Special Nuclear MaterialContained in Scrap and Waste."2. Regulatory Guide 5.35, "Calorimetric Assay of Plutonium."3. C. Weitkamp, "Nuclear Data-for Safeguards: Can Better Data Improve PresentTechniques?" Symposium on Practical Applications of R&D in the Field of.Safeguards, Rome, March 1974, and J. D. Hastings and W. W. Strohm,J. Inorg. Nucl. Chem., Vol. 34, pp. 25-28, 1972.4. R. Sher, "Operating Characteristics of Neutron Well Coincidence Counters,"BNL- 50332, January 1972.5. J.' E. Foley, "Neutron Coincidence Counters in Nuclear Applications," IEEETransactions Vol. NS-19, No. 3, pp. 453-456, June 1972.5.34-10 6. K. Bbhnel, "Neutron Coincidence Counting with Overlapping Cycles," October1972, Gesellschaft fUr Kernforschung mbH. 75 Karlsruhe, P. 0. Box 3640,Germany, or L. V. East and J. E. Foley, "An Improved Thermal-NeutronCoincidence Technique," LA-5197, 1972.7. H. 0. Menlove and R. B. Walton, "47r Coincidence Unit for One-Gallon Cans andSmaller Samples," LA-4457-MS, 1970.8. 11. 0. Menlove, "Matrix Material Effects on Fission-Neutron Counting UsingThermal- and Fast-Neutron Detectors," LA-4994-PR, p. 4, 1972.9. J. Jaech, "Statistical Methods in Nuclear Material Control," TID-26298,Section 3.3.8, 1974.5.34-11