Regulatory Guide 5.34

Nondestructive Assay for Plutonium in Scrap Material by Spontaneous Fission Detection

ML13064A073
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Issue date:	06/30/1974
From:	US Atomic Energy Commission (AEC)
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RG-5.034
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1974.r U.S. ATOMIC ENERGY COMMISSION

UE17 R E*REGmULATORY

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 nondestruc- tive 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 USAEC REGULATORY

GUIDES Copies of published guides may be obtained by request indicating the divisions desired 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 Secretary tire 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 complinae w-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 to the issuance or Wcninuance of a permlt or license by the Cornrmission.

1. Power Reactors 6. Product 2. Resetrch ancd Test RI.tors

7. Trarnportation

3. Fuels and Materials Facilities

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to accommodate

4. Environmental and Siting 9. Antitrust Review comirnts and to reflect new informttion or experience.

5. Materials and Plant Protuction

10. Generel 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)hy weight. However, the recovery of combustible waste by incineration may produce 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 the passive methods of gamma ray spectrometry, calorimetry, and spontaneous fission detection.

Regulatory Guide 5.111 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 radio-nuclidic 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 5.34-2 is reasonably homogeneous.

When the radionuclidic abundances can be accurately

2 measured or controlled, calorimetry can be applied to scrap assay. 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 LEKUF 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 ,-f 500 grams of plutonium or more, give a spontaneous fission response that is iifficult to interpret due to high countina rates and noscihlP neutron multi-Dlication.

A large auantitv of plutonium can he a.sayed hv SF1) by subdividing the scrap into smaller amounts, or the items may be more amenable to nondestructive assay bv 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-2 4 0. Since only the even-numbered isotopes have significint spontaneous fission rates, the effective Pu-240 mass is given approximately by: M(2 4 0)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%.5.34-3 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 104 neutrons/sec.

The( dead time inherent in these slow coincidence systems can be reduced by employing a shift-register coincidence circuit. If the gross neutron detection rate is due primarily to random background and exceeds 2 x 104 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 7 be used for making matrix corrections, using the source addition technique.

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).

5.34-4 TABLE I MATRIX MATERIAL EFFECTS ON NEUTRON ASSAY Neutron Detection Efficiency

8 Lo1'4-Matrix Material (in "ý4 liter can)Empty Can Carbon Pellets Metal Slag-Crucible Concrete String Filters CH 2 (p=0.65 g/cc)CH 2 (p=O. 12 g/cc)CH 2 (P=0.2 7 g/cc)H 20 (p=l.O0 g/cc)Mass (kg)1.89 3.60 1.80 3.24 0.60 0.27 0.43 0.97 3.62 1.03 1.04 1.03 1.05 1.07 1.06 1.09 1.19 0.98 3 He Detector;Thermal 1.00 4 He Detector, Fast 1.00 0.83 0.94 0.84 0.95 0.96 0.92 0.71 0.36 ZnS Detector, 1.00 0.75 0.91 0.79 0.86 0.92 0.90 0.67 0.35 Coincidence ifficiency, He Detector, Thermal 1.00 1.05 1.09 1.08 1.10 1.17 1.11 1.19 1.36 0.98 Correcteda,7 Coincidence Efficiency He Detector, Thermal 1.00 0.97 1.02 1.01 1.02 1.05 1.00 0.98 1.04 0.96 aCorrected using the source addition technique.

(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 the composition of the matrix material surrounding the point source should be obtained.

Included in the matrix should be materials considered representative of commmon 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 per-formance 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.

qategorization 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 5.34-6 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/Plutionium 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 guideto 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*When copies become available, they may be obtained from the American National Standards Institute, Inc., 1430 Broadway, New York, New York 10018.5.34-7 TABLE II EFFECTIVE

PLUTONIUM-240

ABUNDANCE

AND UNCERTAINTY

CORRESPONDING

TO DIFFERENT

BURNUP CATEGORIESa Approximate Abundance

(%)BUXl IUP (N-dt) Pu-238 Pu-239 Pu-240 Pu-241 Pu-242 Pu-240eff 8,000-10,000 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 the equation 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-240 abundances in the table are from tie reportea standard deviations in the most reliable data available.-

5.34-8 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.

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 literature4,9 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 instrumentperformance should be made based on 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 the inalor source of bias and systematic error in NDA. If proper calibration standards aiid a proper calibration relationship are used, the calibrating error should be a reliableestimate 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 mf1ajority 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 .s 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 yea

r. 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 1972.5.34-10

6. K. Bbhnel, "Neutron Coincidence Counting with Overlapping Cycles," October 1972, Gesellschaft fUr Kernforschung mbH. 75 Karlsruhe, P. 0. Box 3640, Germany, or L. V. East and J. E. Foley, "An Improved Thermal-Neutron Coincidence Technique," LA-5197, 1972.7. H. 0. Menlove and R. B. Walton, "47r Coincidence Unit for One-Gallon Cans and Smaller Samples," LA-4457-MS, 1970.8. 11. 0. Menlove, "Matrix Material Effects on Fission-Neutron 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, 1974.5.34-11