Publication Date: 6/1/74     Pages: 11     Date Entered: 1/5/93     Title: Nondestructive Assay for Plutonium in Scrap Material by Spontaneous Fission Detection     JUNE 1974     U.S. ATOMIC ENERGY COMMISSION     REGULATORY GUIDE     DIRECTORATE OF REGULATORY STANDARDS     REGULATORY GUIDE 5.34     NONDESTRUCTIVE ASSAY FOR PLUTONIUM IN SCRAP MATERIAL     BY SPONTANEOUS FISSION DETECTION A. INTRODUCTION     Section 70.51, "Material Balance, Inventory, and Records     Requirements," of 10 CFR Part 70, "Special Nuclear Material," requires     certain licensees authorized to possess at any one time more than one     effective kilogram of plutonium to establish and maintain a system of     control and accountability such that the limit of error (LE) associated     with the material unaccounted for (MUF), ascertained as a result of a     measured material balance, meets minimum standards.     Included in a typical material balance are containers of     inhomogeneous scrap material that are not amenable to assay by the     traditional method of sampling and chemical analysis. With proper     controls, the nondestructive assay (NDA) technique of spontaneous     fission detection (SFD) is an acceptable method for the assay of     plutonium in containers of bulk scrap material. The use of SFD thus     facilitates the preparation of a complete plant material balance whose     LEMUF meets established requirements.     This guide describes procedures acceptable to the Regulatory staff     for application of the technique of spontaneous fission detection for     the nondestructive assay of plutonium in scrap. B. DISCUSSION     Plutonium in scrap material can contribute significantly to the     material unaccounted for (MUF) and to its associated limit of error     (LEMUF). Unlike the major quantity of material flowing through a     process, scrap is typically inhomogeneous and difficult to sample.     Therefore, a separate assay of the entire content of each container of     scrap material is the only reliable method of scrap accountability.     Nondestructive assay (NDA) is a method for assaying the entire content     of every container of scrap.     The term "scrap" refers here to material that is generated     incident to the main process stream due to the inefficiency of the     process. Scrap material is generally economically recoverable. Scrap     therefore consists of reject or contaminated process material such as     pellet grinder sludge, sweepings from a glovebox, dried filter sludge,     and reject powder and pellets. Scrap is distinguished from "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 as it is in process material, i.e., 85-90% (U + Pu) by weight.     Plutonium in fast reactor scrap material is 15-20% by weight and in     thermal reactor recycle material 2-9% by weight. The main difference     between scrap and process material is that scrap is contaminated and     inhomogeneous. Waste, on the other hand, contains a low concentration     of plutonium and uranium, i.e., a few percent or less (U + Pu) by     weight. However, the recovery of combustible waste by incineration may     produce ash that is high in uranium and plutonium concentrations. Such     incinerator ash is also considered "scrap" in this guide.     Nondestructive assay for plutonium can be accomplished primarily     by the passive methods of gamma ray spectrometry, calorimetry, and     spontaneous fission detection. Regulatory Guide 5.11(1) provides a     framework for the utilization of these NDA methods.     Gamma ray spectrometry of scrap consisting of dense materials can     be unreliable because of the attenuation of gamma rays. Gamma ray     spectrometry is most applicable to waste assay.     Calorimetry is an accurate method of plutonium assay when there is     an accurate knowledge of the relative abundance of each plutonium     isotope and americium-241. Scrap may contain a mixture of materials of     different radionuclidic compositions, especially different americium-241     concentrations, necessitating the measurement of the average     radionuclidic composition. The average radionuclidic abundances can     only be accurately measured when the scrap is reasonably homogeneous.     When the radionuclidic abundances can be accurately measured or     controlled, calorimetry can be applied to scrap assay.(2) However,     calorimetry is time-consuming for heterogeneous materials of high heat     capacity and may not be a practical method for the routine assay of     large numbers of containers.     Spontaneous fission detection (SFD) is the most practicable and     generally applicable NDA technique for the assay of plutonium in scrap     material. Spontaneous fission radiations are sufficiently penetrating     to provide a representative signal from all the plutonium within a     container. The plutonium isotopic composition must be known for SFD     assay, but the accuracy of SFD is not as dependent on the accuracy of     analysis for the minor plutonium isotopes as is that of calorimetry. Nor     is SFD sensitive to americium-241 ingrowth. The quantity of scrap     material on inventory when a material balance is computed can be reduced     through good management, and the scrap remaining on inventory can be     assayed by SFD to meet the overall plant MUF and LEMUF constraints     required by paragraph (e)(5) of Section 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 few grams to a few hundred grams of plutonium or     approximately 50 grams of effective plutonium-240.(*) Containers with a     larger plutonium content, i.e., on the order of 500 grams of plutonium     or more, give a spontaneous fission response that is difficult to     interpret due to high counting rates and possible neutron     multiplication. A large quantity of plutonium can be assayed by SFD by     subdividing the scrap into smaller amounts, or the items may be more     amenable to nondestructive assay by calorimetry.     ----------     (*) The effective plutonium-240 mass is a weighted average of the     mass of each of the plutonium isotopes. The weighting is equal to the     spontaneous fission neutron yield of each isotope relative to that of     Pu-240. Since only the even-numbered isotopes have significant     spontaneous fission rates, the effective Pu-240 mass is given     approximately by:(3) 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 this equation are only known to approximately @@5%.     ---------- C. REGULATORY POSITION     The method of spontaneous fission detection (SFD) for the     nondestructive assay for plutonium in bulk inhomogeneous scrap material     should include: (1) discrimination of spontaneous fission radiations     from random background by coincidence techniques and (2) measurement of     the relative plutonium isotopic composition of the scrap by an     independent measurement technique. An acceptable SFD method of     plutonium assay is described below: 1. Spontaneous Fission Detection System     a. Detectors. Instruments based on moderated thermal neutron     detectors, i.e., neutron well coincidence counters,(4,5) are recommended     for applications in which the gross neutron detection rate does not     exceed 2 x 10(4) neutrons/sec. The dead time inherent in these slow     coincidence systems can be reduced by employing a shift-register     coincidence circuit.(6) If the gross neutron detection rate is due     primarily to random background and exceeds 2 x 10(4) neutrons/sec, then     a fast neutron detection, single coincidence system can be used,     provided that adequate corrections can be made for matrix effects.     Matrix effects are more severe in fast neutron detection systems, as     shown in Table I.     b. Detection Chamber. The chamber should permit reproducible     positioning of standard-sized containers in the location of maximum     spatial response uniformity.     c. Fission Source. A spontaneous fission source with a neutron     intensity comparable to the intensity of the largest plutonium mass to     be assayed should be used for making matrix corrections, using the     source addition technique.(7) A nanogram of Cf-252 is approximately     equivalent to a gram of effective Pu-240.     d. Readout. Readout should allow computation of the accidental     to real coincidence ratio in addition to the net real coincidence rate.     Live time readout or a means of computing the dead time should also be     provided.     e. Performance Specifications. The performance of a SFD     instrument should be evaluated according its stability, uniformity of     spatial response, and insensitivity to matrix effects. Therefore,     information should be obtained regarding:     (i) The precision of the coincidence response as a     function of the real coincidence counting rate and the accidental to     real coincidence ratio. Extremes in the background or accidental     coincidence rate can be simulated by using a source of random neutrons     (nonfission).     (Due to database constraints, Tables I and II are not included. Please     contact LIS to obtain a copy.) (ii) Uniformity of spatial response. Graphs should be     obtained on the relative coincidence response from a point source of     fission radiation as a function of position in the counting chamber.     (iii)Sensitivity of matrix interference. A table of the     relative coincidence response from a point source of fission radiation     as a function of rhe composition of the matrix material surrounding the     point source should be obtained. Included in the matrix should be     materials considered representative of common scrap materials. Table I     is an example of such a tabulation of the relative response for a wide     range of materials.     This information should be used for evaluating the expected     instrument performance and estimating errors. The above performance     information can be requested from the instrument suppliers during     instrument selection and should be acquired during preoperational     instrument testing. 2. Analyst     A highly trained individual should oversee SFD assay for plutonium     and should have primary responsibility for instrument specification,     preoperational instrument testing, standards and calibration, writing an     operation manual, measurement control, and error analysis. Experience     or training equivalent to a bachelors degree in science or engineering     from an accredited college or university and a laboratory course in     radiation measurement should be the minimum qualifications of the SFD     analyst. The SFD analyst should review SFD operation at least weekly     and should authorize all changes in SFD operation. 3. Containers and Packaging     A single type of container should be used for packaging all scrap     in each category, as discussed below. A recommended uniform container     that would facilitate accurate measurement and would standardize this     segment of instrument design is a thin-walled metal (steel) can with an     inside diameter of approximately 10 cm or less. 4. Reduction of Error Due to Material Variability     The SFD response variation due to material variability in scrap     should be reduced by: (1) segregation of scrap into categories that are     independently calibrated, (2) correcting for matrix effects using the     source addition technique,(7) or (3) applying both categorization and     the source addition technique. Categorization should be used if the SFD     method is more sensitive to the material variability from scrap type to     scrap type than to the material variability within a scrap type.     Application of the source addition technique reduces the sensitivity to     material variability and may allow the majority of scrap types to be     assayed under a single calibration. Material characteristics that     should be considered in selecting categories include:     a. Plutonium Isotopic Composition     b. Uranium/Plutonium Ratio     c. Containerization and Packaging     d. Abundance of High-Yield alpha-neutron Material, i.e.,     low-atomic-number impurities     e. Plutonium Content     f. Density (both average density and local density extremes     should be considered) g. Matrix Composition 5. Calibration     A guide to calibration for nondestructive assay is presently under     development by Task Force 8.3 of the N15 committee of the American     National Standards Institute(*) and will include details on calibration     standards, calibration procedures, curve fitting, and error analysis.     Guidelines relevant to SFD are given below.     a. A minimum of four calibration standards of the same isotopic     composition as the unknowns should be used for calibration. If     practicable, a calibration curve should be generated for each isotopic     blend of plutonium. When plutonium of different isotopic composition is     assayed using a single calibration, the effect on the SFD response of     isotopic composition should be determined over the operating ranges by     measuring standards of differing plutonium isotopic compositions. The     use of the effective Pu-240 concept can lead to error because of the     uncertainty in the spontaneous fission half-lives, as shown in Table II,     and the variation in response with isotopic composition.     b. Calibration standards should be fabricated from material     having a plutonium content determined by a technique traceable to or     calibrated with National Bureau of Standards standard reference     material. Well-characterized homogeneous material similar to the     process material from which the scrap is generated can be used to obtain     calibration standards.     c. Fabrication of calibration standards that are truly     representative of the unknowns is difficult for scrap assay. To measure     the reliability of the calibration based on the fabricated standards     discussed above, and to improve this calibration, unknowns that have     been assayed by SFD should periodically be selected for assay by an     independent more accurate technique. Calorimetry(2) can be used to     assay a random selection of scrap in containers and provide reliable     data that should be fed back into the calibration fitting procedure to     improve SFD calibration. The original calibration standards should be     retained as working standards.     ----------     (*) When copies become available, they may be obtained from the     American National Standards Institute, Inc., 1430 Broadway, New York,     New York 10018.     ---------- 6. Measurement Control     For proper measurement control, a "dummy" item should be assayed     on each day of scrap assay as a background measurement. Also, control     (or working) standards should be assayed each day scrap is assayed for     normalization and to assure reliable operation.     The source addition technique(7) is recommended for correcting the     SFD response for each assay. If not used routinely, the source addition     technique should be applied to a random selection of items but in no     case should be used less frequently than daily. The results of random     applications of the source addition technique can be used in two ways:     a. As an average correction factor to be applied to a group of     items, and     b. As a check on the item being assayed to verify that it is     similar to the standards used in calibration and that no additional     matrix effects are present, i.e., purely as a qualitative assurance that     the calibration is valid. 7. Error Analysis     The sources of error in SFD are discussed in Regulatory Guide     5.11.(1) Analysis of the error in the calibration is discussed in the     literature(4,9) and in the ANSI guide on calibration now under     development. In addition to the calibration error there are errors due     to the measurement process and due to variability in material     composition.     The error due to the measurement process, i.e., the     measurement-to-measurement error, accounts for most of the random error     in NDA. At least fifteen unknowns selected at random should be     repeatedly assayed to estimate the random error. Repeated measurements     should be made under as many different conditions as are experienced in     normal operation, e.g., different times of day, different operators,     different ambient conditions. The standard deviation in the     distribution of differences in replicate results should be used in     constructing a 95% confidence interval. The mean difference in     replicate results has an expected value of zero. Corrections for     significant drift in the instrument performance should be made based on     data from daily assay of control standards, i.e., the measurement     control program.     The error due to material variability, i.e., the item-to-item     error, is the major source of bias and systematic error in NDA. If     proper calibration standards and a proper calibration relationship are     used, the calibrating error should be a reliable estimate of the     systematic error. To test these assumptions, and to determine the bias,     SFD assay results on a random selection of unknowns should be compared     with assays on the same items by an independent more accurate technique,     as discussed in 4(c). Calorimetry is not sensitive to the majority of     interferences that cause error due to material variability in SFD and is     practical for this application because it is nondestructive. An     alternative method for verifying SFD assay is to sample the scrap     extensively and to perform chemical analyses for the plutonium     concentrations in these samples.     The mean difference in comparative assays should be used as the     bias for correcting SFD assay results. The bias correction should be     made if the mean difference 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 that should be used in     constructing a 95% confidence interval. (There will always be a     potential bias and systematic error in the technique used to verify SFD.     The systematic error should be known and should be insignificant     compared to systematic error 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 least two unknowns a week to determine bias and systematic     error. Data may be pooled and used to improve the calibration although     no data should be older than one year.     REFERENCES 1. Regulatory Guide 5.11, "Nondestructive Assay of Special Nuclear     Material Contained in Scrap and Waste." 2. Regulatory Guide 5.35, "Calorimetric Assay of Plutonium." 3. C. Weitkamp, "Nuclear Data for Safeguards: Can Better Data     Improve Present Techniques?" 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," IEEE Transactions Vol. NS-19, No. 3, pp. 453-456,     June 1972. 6. K. Bohnel, "Neutron Coincidence Counting with Overlapping Cycles,"     October 1972, Gesellschaft fur Kernforschung mbH. 75 Karlsruhe, P. O. Box 3640, Germany, or L. V. East and J. E. Foley, "An Improved     Thermal-Neutron Coincidence Technique," LA-5197, 1972. 7. H. O. Menlove and R. B. Walton, "4++ Coincidence Unit for     One-Gallon Cans and Smaller Samples," LA-4457-MS, 1970. 8. H. O. Menlove, "Matrix Material Effects on Fission-Nuetron     Counting Using Thermal- 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.     30