Regulatory Guide 5.34: Difference between revisions

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{{Adams
{{Adams
| number = ML003739949
| number = ML13064A073
| issue date = 05/31/1984
| issue date = 06/30/1974
| title = (Task SG 046-4), Nondestructive Assay for Plutonium in Scrap Material by Spontaneous Fission Detection
| title = Nondestructive Assay for Plutonium in Scrap Material by Spontaneous Fission Detection
| author name =  
| author name =  
| author affiliation = NRC/RES
| author affiliation = US Atomic Energy Commission (AEC)
| addressee name =  
| addressee name =  
| addressee affiliation =  
| addressee affiliation =  
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| license number =  
| license number =  
| contact person =  
| contact person =  
| document report number = RG-5.34, Rev 1
| document report number = RG-5.034
| document type = Regulatory Guide
| document type = Regulatory Guide
| page count = 9
| page count = 11
}}
}}
{{#Wiki_filter:Revision 1" May 1984 U.S. NUCLEAR REGULATORY
{{#Wiki_filter:1974.r U.S. ATOMIC ENERGY COMMISSION  
COMMISSION
UE17 R E*REGmULATORY
0 SREGULATORY
GUIDE"DIRECTORATE
GUIDE OFFICE OF NUCLEAR REGULATORY  
OF REGULATORY  
RESEARCH REGULATORY  
STANDARDS REGULATORY  
GUIDE 5.34 (Task SG 046-4) NONDESTRUCTIVE  
GUIDE 5.34 NONDESTRUCTIVE  
ASSAY FOR PLUTONIUM  
ASSAY FOR PLUTONIUM  
IN SCRAP MATERIAL BY SPONTANEOUS  
IN SCRAP MATERIAL BY SPONTANEOUS  
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==A. INTRODUCTION==
==A. INTRODUCTION==
Section 70.51, "Material Balance, Inventory, and Records Requirements," of 10 CFR Part 70, "Domestic Licensing of Special Nuclear Material," requires certain licensees authorized to possess at any one time more than one effective kilogram of special nuclear material to establish and maintain a system of control and accountability so that the standard error (estimator)  
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.
associated with the inventory difference (SEID), obtained as a result of a measured material balance, meets minimum standards.


This guide is intended for those licensees who possess plutonium scrap materials and who are also subjected to the requirements of § 70.51 of 10 CFR Part 70.  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.
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 non destructive assay (NDA) technique of spontaneous fission detection is one acceptable method for the assay of plutonium in containers of bulk scrap material.
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 spontaneous fission detection thus facilitates the preparation of a complete plant material balance whose SEID meets established requirements.
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 NRC staff for applying the NDA technique of spontaneous fission detection to plutonium in scrap.  Any guidance in this document related to informa tion collection activities has been cleared under OMB Clearance No. 3150-0009.
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==
==B. DISCUSSION==
Plutonium in scrap material can contribute signif icantly to the inventory difference and its associated standard error. Unlike the major quantity of material flowing through the process, scrap is typically inhomogeneous and difficult to sample. Therefore, a separate assay of the entire content of each container of scrap material is a more reliable method of scrap account ability. NDA is a method for assaying the entire content of every container of scrap.  The term "scrap" refers to material that is generated from the main process stream because of the ineffi ciency of the process. Scrap material is generally economically recoverable.
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.


Scrap, therefore, consists of rejected or contaminated process material such as pellet grinder sludge, sweepings from gloveboxes, dried filter sludge, and rejected powder and pellets. Scrap is generally distinguished from "waste" by the density or concentra tion of heavy elements in the two materials, but it is the recovery cost (per mass unit of special nuclear material)
Wasshington, D.C. 20545, lioulato v Gu-des ... ,s.ed to describe end make availeble to the public Attention:
that determines whether a material is "scrap" or "waste." The concentration of uranium and pluto nium in scrap is approximately the same as it is in process material, i.e., 85-90 percent (uranium + pluto nium) by weight. However, on occasion the fraction in both process and scrap material can be less than 25 percent. Plutonium in fast reactor scrap material is 15-20 percent by weight and in thermal reactor recycle material, 2-9 percent by weight. The main difference between scrap and process material is that scrap is contaminated and inhomogeneous.
Director of Regulatory Standards.


Waste, on the other hand, contains a low concentration of uranium and plutonium, ie., a few percent or less (uranium + pluto nium) by weight. However, the recovery of combustible waste by incineration may produce ash that is high in uranium and plutonium concentrations.
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.


Such incinerator ash is also considered "scrap" in this guide. However, it should be noted that ash may be more homogeneous in
US. Atomic Energy Comenission, Washington, D.C. 20545,-iuJt.01P
* The substantial number of changes in this revision has made it impractical to indicate the changes with lines in the margi
specific problems or postulated accidents, or to provide guidance to Attention:
Chief. Public ProcmedingStaff.


====n. USNRC REGULATORY ====
lpolicans Regulatory Guides sm not sutStitutes for regulations and complinae w-th them i* not required.
GUIDES Comments should be sent to the Secretary of the Commission, U.S. Nuclear Regulatory Commission, Washington, D.C. 20555, Regulatory Guides are issued to describe and make available to the Attention:
Docketing and Service Branch.  public methods acceptable to the NRC staff of implementing specific parts of the Commission's regulations, to delineate tech- The guides are issued in the following ten broad divisions:
niques used by the staff in evaluating specific problems or postu lated accidents or to provide guidance to applicants.


Regulatory
Methods ernd solutions different from those eat out in The guides are issued in the following ten broad divisions:
1. Power Reactors 6. Products Guides are no? substitutes for regulations, and compliance with 2. Research and Test Reactors 7. Transportation them is not required.
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.


Methods and solutions different from those set 3. Fuels and Materials Facilities
1. Power Reactors 6. Product 2. Resetrch ancd Test RI.tors
8. Occupational Health out in the guides will be acceptable if they provide a basis for the 4. Environmental and Siting 9. Antitrust and Financial Review findings requisite to the issuance or continuance of a permit or 5. Materials and Plant Protection
10. General license by the Commission.


Copies of issued guides may be purchased at the current Government This guide was issued after consideration of comments received from Printing Office price. A subscription service for future guides in spe the public. Comments and suggestions for improvements in these cific divisions is available through the Government Printing Office. guides are encouraged at all times, and guides will be revised, as Information on the subscription service and current GPO prices may appropriate, to accommodate comments and to reflect new Informa- be obtained by writing the U.S. Nuclear Regulatory Commission, tion or experience.
===7. Trarnportation===
3. Fuels and Materials Facilities
8. Occupational Health Published guides will be revised periodically, as appropriate.


Washington, D.C. 20555, Attention:
to accommodate
Publications Sales Manager.
4. Environmental and Siting 9. Antitrust Review comirnts and to reflect new informttion or experience.


its characteristics compared to most scrap and may, therefore, be accountable using sampling and chemical analysis methods.
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.


NDA of plutonium can be accomplished primarily by the passive methods of gamma ray spectrometry, calorimetry, and spontaneous fission detection.
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.


Active neutron methods using total count rates or delayed neutron detection can also be used in scrap assay measurements.
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.


Regulatory Guide 5.11, "Nondestructive Assay of Special Nuclear Material Contained in Scrap and Waste," provides a framework for the use of these NDA methods.1 The NDA of dense scrap materials using gamma ray spectroscopy can be unreliable because of severe gamma ray attenuation.
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.


However, the isotopic composition of plutonium in scrap materials, with the exception of 242pu, can be obtained quite reliably using high-resolution gamma ray spectrometry measurements (Ref. 1). Calorimetry is an accurate method of plutonium assay when there is an accurate knowledge of the relative abundance of each plutonium isotope and 2 4 tAm. Scrap may contain a mixture of materials of different radionuclide compositions, especially different
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.
241Am concentrations, thereby necessitating the measure ment of the average radionuclide composition.


The average radionuclide abundances can be accurately meas ured only when the scrap is reasonably homogeneous.
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.


When the radionuclide abundances can be accurately measured or controlled, calorimetry can be applied to scrap assay (Ref. 2). However, calorimetry is time con suming for materials of high heat capacity and may not be a practical method for the routine assay of large numbers of containers.
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.


Spontaneous fission detection is a practical NDA technique for the assay of plutonium in scrap material.
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 assay method involves the passive counting of spontaneous fission neutrons emitted primarily from the fission of 240 Pu. Neutron coincidence counters are used to detect these time-correlated neutrons.
The average radionuclidic abundances can only be accurately measured when the scrap 5.34-2 is reasonably homogeneous.


The theory and practice of neutron coincidence counting for plutonium assay are discussed thoroughly in References
When the radionuclidic abundances can be accurately
3 through 6. Spontaneous fission neutrons are sufficiently penetrating to provide a representative signal from all the plutonium within a container.
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.


Since the neutron coincidence signal is dependent on both the quantity and relative abundance of 2 3 8 Pu, 2 4 0 pu, and 242pu, the plutonium isotopic composition must be known for assay of total plutonium by spontaneous fission detection.
Spontaneous fission detection (SFD) is the most practicable and generally applicable NDA technique for the assay of plutonium in scrap material.


The quantity of scrap material on inventory when a material balance is com puted can be reduced through good management, and the scrap remaining on inventory can be assayed by spontaneous fission detection to meet the overall plant inventory difference (ID) and SEID constraints required by paragraph
Spontaneous fission radiations are sufficiently penetrating to provide a representative signal from all the plutonium within a container.
70.5 1(e)(5) of 10 CFR Part 70.  1 Revision I to this guide was issued in April 1984.This guide gives recommendations useful for the assay by spontaneous fission detection of containers, each containing a few liters of scrap and having contents ranging from a few grams to 10 kilograms of plutonium or up to approximately
2 kilograms of effective
2 4 0 Pu 2 (see Ref. 7). Containers with a significant plutonium content (i.e., 50 grams or more) give a spontaneous fission response that must be corrected for the effects of neutron multiplication (Refs. 8, 9). Scrap materials that have large loadings of plutonium in addition to fluorine, oxygen, or other alpha/neutron-producing elements are difficult to measure and correct for multi plication effects because of the large random neutron flux from the (ct,n) reactions in the matrix materials.


These samples should be segregated into smaller quanti ties for measurements.
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.


In general, a large quantity of plutonium can be assayed by spontaneous fission detec tion by subdividing the scrap into smaller amounts, or the items may be more amenable to assay by calorim etry.  C. REGULATORY
Nor is SFD sensitive to americium-241 ingrowth.
POSITION The spontaneous fission detection method for the NDA of 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 pluto nium isotopic composition of the scrap. An acceptable spontaneous fission detection method of plutonium assay is described below.


===1. SPONTANEOUS ===
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
FISSION DETECTION
50 grams of effective plutonium-240.*:
SYSTEM 1.1 Detectors Instruments based on moderated thermal neutron detectors, i.e., neutron well coincidence counters, are recommended for applications in which the gross neutron detection rate does not exceed 2 x 105 neutrons/sec.
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.


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 Lprimarily due to random background and exceeds 2 x 10 neutrons/sec, a fast-neutron-detection, single-coincidence system can be used, provided adequate corrections can be made for matrix effects. Matrix effects are more severe in fast-neutron-detection systems, as shown in Table 1.  2 The effective
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.
24 0 Pu 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 2 0 4Pu. Since only the even-numbered isotopes have significant spontaneous fission rates, the effective
*The effective plutonium-240
240 Pu mass is given approxi mately by: M(240)eff
mass is a weighted average of the mass of each of the plutonium isotopes.
= M(240) + (1.64 + 0.07)M(242)
+ (2.66 +/- 0.19)M(238)
where M is the mass of the isotope indicated in parentheses.


The uncertainties in the coefficients and in the effective  
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)
240 Pu abun dances in the table are from the reported standard deviations in the most reliable data available (Ref. 7). The mathematical procedure for converting from M(240)eff to M(total Pu) is presented in the appendix to this guide together with a sample calculation.
+ 2.66M(238)
where M is the mass of the. isotope indicated in parentheses.


5.34-2 TABLE 1 MATRIX MATERIAL EFFECTS ON NEUTRON ASSAY Correcteda (Ref. 10) Neutron Detection Efficiency (Ref. 11) Coincidence Coincidence Efficiency, Efficiency, Matrix Material Mass 3 He Detector, 4 He Detector, ZnS Detector, 3 He Detector, 3He Detector, (in %4-liter can) (kg) Thermal Fast Fast Thermal Thermal Empty Can -1.00 1.00 1.00 1.00 1.00 Carbon Pellets 1.89 1.03 --1.05 0.97 Metal 3.60 1.04 0.83 0.75 1.09 1.02 Slag-Crucible
The coefficients in this equation are only known to approximately
1.80 1.03 0.94 0.91 1.08 1.01 Concrete 3.24 1.05 0.84 0.79 1.10 1.02 String Filters 0.60 1.07 0.95 0.86 1.17 1.05 CH 2 (p=0.6 5 g/cc) 0.27 1.06 0.96 0.92 1.11 1.00 CH 2 (p=0.1 2 g/cc) 0.43 1.09 0.92 0.90 1.19 0.98 CH 2 (p=0.2 7 g/cc) 0.97 1.19 0.71 0.67 1.36 0.04 H120 (p=l.00 g/cc) 3.62 0.98 0.36 0.35 0.98 0.96 aCorrected using the source addition technique (see Ref. 7).1.2 Detection Chamber The chamber should permit reproducible positioning of standard-sized containers in the location of maximum spatial response uniformity.
+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.


1.3 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 (Ref. 10).  A nanogram of 252Ca is approximately equivalent to a gram of effective
An acceptable SFD method of plutonium assay is described below: 1. Spontaneous Fission Detection System a. Detectors.
2 4 0 Pu.  1.4 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.


1.5 Perfonnance Specifications The performance of a spontaneous fission detection instrument should be evaluated according to its stability, uniformity of spatial response, and insensitivity matrix effects. Therefore, information should obtained regarding:
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.
to be 1. 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 back ground or accidental-coincidence rate can be simulated by using a source of random neutrons (nonfission). 
2. The uniformity of spatial response.


Graphs should be obtained on the relative coincidence response to a small fission neutron source as a function of position in the counting chamber.
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.


3. The sensitivity of matrix interference.
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 table of the relative coincidence response to a small fission neutron source as a function of the composition of the matrix material surrounding the point source should be obtained.
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.


Included in the matrix should be materials considered representative of common scrap materials.
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.


Table 1 is an example of such a tabulation of the relative response for a wide range of materials.
(ii) Uniformity of spatial response.


This information should be used for evaluating the expected instrument performance and for estimating
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.
5.34-3 errors. The above performance information can be requested from the instrument suppliers during instru ment selection and should be verified during preopera tional instrument testing.2. ANALYST 6. Density (both average density and local density extremes should be considered), and 7. Matrix composition.


===5. CALIBRATION===
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.
A trained individual should oversee spontaneous fission detection assay of plutonium and should have primary responsibility for instrument specification, preoperational instrument testing, standards and calibra tion, an operation manual, measurement control, and error analysis.


Experience or training equivalent to a bachelor's 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 analyst. The spontaneous fission detection analyst should frequently review the sponta neous fission detection operation and should authorize any changes in the operation.
Included in the matrix should be materials considered representative of commmon scrap materials.


===3. CONTAINERS ===
Table I is an example of such a tabulation of the relative response for a wide range of materials.
AND PACKAGING
A single type of container should be used for packaging all scrap in each category.


A uniform con tainer that would facilitate accurate measurement and would standardize this segment of instrument design, e.g., a thin-walled metal (steel) can with an inside diameter between 10 and 35 cm, is recommended.
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.


For further guidance on container standardization in NDA measurements, see Reference
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.
12.  4. REDUCING ERROR DUE TO MATERIAL VARIABILITY
The variation in spontaneous fission detection response due to material variability in scrap should be reduced by (1) segregating scrap into categories that are independently calibrated, (2) correcting for matrix effects using the source addition technique (Ref. 10), or (3) applying both the categorization and the source addition technique.


Categorization should be used if the spontaneous fission detection 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.
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.


Material characteristics that should be considered in selecting categories include: 1. Plutonium isotopic composition and content, 2. Uranium/plutonium ratio, 3. Types of container and packaging, 4. Abundance of high-yield alpha/neutron material, i.e., low-atomic-number impurities, 5. Size and distribution of materials in packages, Guidelines for calibration and measurement control for NDA are available in Regulatory Guide 5.53, "Qualifi cation, Calibration, and Error Estimation Methods for Nondestructive Assay," which endorses ANSI N15.20 1975, "Guide to Calibrating Nondestructive Assay Systems." 3 The guide and standard include details on calibration standards, calibration procedures, curve fitting, and error analysis.
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.


Guidelines relevant to spontaneous fission detection are given below.  Calibration can be used for either a single isotopic composition or variable isotopic mixtures.
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)


In the former case, the resulting calibration curve will be used to convert "net real-coincidence count" to "grams pluto nium." In the latter case, the conversion is from "net real-coincidence count" to "effective grams 24°pu." The mathematical procedure for converting from effective grams 2 4 0 pu, M(2 4 0)eff, to total grams pluto nium, M(total Pu), is presented in the appendix to this guide together with a sample calculation.
====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.


A minimum of four calibration standards with isotopic compositions similar to those of the unknowns should be used for calibration.
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 calibra tion curve should be generated for each isotopic blend of plutonium.
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 of isotopic composition on the spontaneous fission detection response should be determined over the operating ranges by measuring standards of different plutonium isotopic compositions.
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.


This is necessary because the use of the effective  
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.
2 4 0 pu concept can lead to error owing to the uncertainty in the spontaneous fission half-lives and the variation in response with isotopic composition.


Table 2 illustrates the uncertainty in effective
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.
2 4 0 Pu abundance with different isotopic compositions (Ref. 13).  Calibration standards should be fabricated from material having a plutonium content determined by a technique traceable to or calibrated with the standard reference material of the National Bureau of Standards.


Well-characterized homogeneous material similar to the process material from which the scrap is generated can be used to obtain calibration standards.
Well-characterized homogeneous material similar to the process material from which the scrap is generated can be used to obtain calibration standards.


Fabrication of calibration standards that are truly representative of the unknowns is impossible 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 3 Copies of this standard may be obtained from the American National Standards Institute, Inc., 1430 Broadway, New York, New York 10018.5.34-4 TABLE 2 EFFECTIVE  
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  
2 4°pu ABUNDANCE  
PLUTONIUM-240
AND UNCERTAINTYa'b CORRESPONDING  
ABUNDANCE  
AND UNCERTAINTY
CORRESPONDING  
TO DIFFERENT  
TO DIFFERENT  
ISOTOPIC COMPOSITION
BURNUP CATEGORIESa Approximate Abundance  
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%)
(%) BURNUP (MWd/t) 23pu 2 3 9 pu 240pu 241pu 2 4 2 pu 24°PUeff 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%)
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%)
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%)
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:
aComputed using the equation given in footnote 2. bplutonium isotopic compositions were selected based on light-water-reactor fuel exposures.
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.-
assayed by spontaneous fission detection should periodically be selected for assay by an independent technique.
5.34-8 selected for assay by an independent more accurate technique.


Calorimetry (Ref. 2) can be used to assay a random selection of scrap in containers and to provide reliable data that should be fed back into the calibra tion fitting procedure to improve spontaneous fission detection calibration.
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.
The original calibration standards should be retained as working standards.


===6. MEASUREMENT ===
6. Measurement Control For proper measurement control, a "dummy" item should be assayed on each day of scrap assay as a background measurement.
CONTROL For proper measurement control, on each day that scrap is assayed, a secondary standard should be assayed as a background measurement.
 
Also, on each day that scrap is assayed, control (or working) standards should be assayed for normalization and for ensuring reliable operation.
 
The source addition technique (Ref. 10) is recom mended for correcting the spontaneous fission detection response for each assay. If not used routinely, the source addition technique should be applied to a random selection of items with a frequency comparable to the assay schedule.
 
The results of random applica tions of the source addition technique can be used in two ways: 1. As an average correction factor to be applied to a group of items, and 2. 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, ie., purely as a qualitative assurance that the calibration is valid.  7. ERROR ANALYSIS The sources of error in spontaneous fission detection are discussed in Regulatory Guide 5.11. Analysis of the error in the calibration is discussed in ANSIN15.20-1975 and in References
4 and 13.5.34-5 REFERENCES
1. J. F. Lemming and D. A. Rakel, "Guide to Pluto nium Isotopic Measurements Using Gamma-Ray Spectroscopy," MLM-2981, August 1982.  2. U.S. Nuclear Regulatory Commission, "Calorimetric Assay for Plutonium," NUREG-0228, 1977.  3. N. Ensslin et al., "Neutron Coincidence Counters for Plutonium Measurements," Nuclear Materials Management, Vol. VII, No. 2, p. 43, 1978.  4. R. Sher, "Operating Characteristics of Neutron Well Coincidence Counters," Brookhaven National Laboratory, BNL-50332, 1972.  5. K. Boehnel, "Determination of Plutonium in Nuclear Fuels Using the Neutron Coincidence Method," AWRE-Trans-70(54/4252) (English translation of KfK 2203), 1978.  6. M. S. Zucker, "Neutron Correlation Counting for the Nondestructive Analysis of Nuclear Materials," in Analytical Methods for Safeguards and Account ability Measurements of Special Nuclear Materials, NBS Special Publication
528, pp. 261-283, November 1978.  7. J. D. Hastings and W. W. Strohm, "Spontaneous Fission Half-Life of 2 3 8 Pu,' Journal of Inorganic and Nuclear Chemistry, Vol. 34, p. 25, 1972.8. N. Ensslin, J. Stewart, and J. Sapir, "Self Multiplication Correction Factors for Neutron Coincidence Counting," Nuclear Materials Manage ment, Vol. VIII, No. 2, p. 60, 1979.  9. M. S. Krick, "Neutron Multiplication Corrections for Passive Thermal Neutron Well Counters," Los Alamos Scientific Laboratory, LA-8460-MS, 1980.  10. H. 0. Menlove and R. B. Walton, "41r Coincidence Unit for One-Gallon Cans and Smaller Samples," Los Alamos Scientific Laboratory, LA-4457-MS, 1970.  11. H. 0. Menlove, "Matrix Material Effects on Fission Neutron Counting Using Thermal- and Fast-Neutron Detectors," Los Alamos Scientific Laboratory, LA-4994-PR, p. 4, 1972.  12. K. R. Alvar, H. R. Lukens, and N. A. Lurie, "Standard Containers for SNM Storage, Transfer, and Measurement," U.S. Nuclear Regulatory Commission, NUREG/CR-1847, 1980.  13. J. Jaech, "Statistical Methods in Nuclear Material Control," Atomic Energy Commission, TID-26298, Section 3.3.8, 197
 
===4. BIBLIOGRAPHY===
American National Standards Institute, "Standard Test Methods for Nondestructive Assay of Special Nuclear Materials Contained in Scrap and Waste," ANSI/ASTM
C 853-79, 1979.Brouns, R. J., F. P. Roberts, and U. L. Upson, "Considerations for Sampling Nuclear Materials for SNM Accounting Measurements," U.S. Nuclear Regulatory Commission, NUREG/CR-0087, 1978.5.34-6 APPENDIX Procedure for Converting M(2 4 0)eff to M(total Pu) and Sample Calculation When the measurement situation dictates the expres sion of the primary assay result as "effective grams of 2 4 0 pu," it is necessary to convert this result to total grams of plutonium using the relationship between these two quantities and the known isotopic composi tion of the plutonium sample. Let f 2 3 8 ,' f 2 3 9' f 2 4 0' f241' f242 represent the weight fractions of the pluto nium isotopes in the unknown sampl
 
====e. The effective ====
24°pu mass from coincidence counting, M(2 4 0)eff, and the individual masses of the spontaneously fissioning plutonium isotopes are related by: M(2 4 0)eff = M(240) + 1.64M(242)
+ 2.66M(238)
(1) The masses of the 2 4 2 Pu and 2 3 8 pU isotopes can be "expressed in terms of M(240), using the isotopic weight fractions, so that: M(2 4 0)eff = M(240)[f 2 4 0+ 1.64f 2 4 2+ 2.66f 2 3 8 1/f 2 4 0 (2)f242 = (2.0 +/- 0.2)% = 0.020 +/- 0.002 Using these results in Equation 3, we have: M(total Pu) = 10.0/[0.20
+ 1.64 x 0.02 + 2.66 x 0.01] = 10.0/0.259
= 38.6 grams To obtain the value of the variance of the M(total Pu) result, we must propagate the variances of the M(2 4 0)eff and the isotopic weight fractions.
 
Let the variance in M(2 4 0)eff = cieff, and let the variances in the relevant plutonium weight fractions be G238' 2 and G:42. The variance of the total plutonium
0240'2j 4 mass, apu, is given by: 2 = [M(total Pu)] 2 {[ Oeff/M(2 4 0)eff] 2 + [ 242 + (1.6 4 2 4)2 + (2.660238)]/
= M(total Pu), we have the final M(total Pu) = M(2 4 0)eff/[f 2 4 o + 1.64f 2 4 2 + 2.66f 2 3 8]The quantity in the denominator of Equation 3 is called the " 2 4 0 Pu effective weight fraction, f 2 4 0 (effect ive)." Thus the total plutonium mass can be expressed as the 2 4 0 Pu effective mass divided by the 2 4 0 pu effective weight fraction: M(total Pu) = M(240)eff/f
2 4 o(effective)
(4)As an example, suppose that the net coincidence count from an unknown sample indicates
10.0 +/- 0.5 effective grams of 2 4 0 Pu. Furthermore, suppose that the plutonium isotopic composition of the unknown sample was previously established to be: f238 = (1.0 +/- 0.5)% = 0.010 +/- 0.005 f239 = (73.0 +/- 0.5)% f240 = (20.0 +/- 0.4)% = 0.200 +/- 0.004 f241 = (4.0 +/- 0.2)%[f240 + 1.64f 2 4 2+ 2.66f 2 3 8] 2}(5)In our example calculation, 0 eff = 0.5 gram, 02ý8 = 0.005, 0240 = 0.004, and 0242 = 0.002. The variance in the total plutonium mass is therefore given by: 2 = IM(total Pu)]2 [(0.5/10.0)2
+ 0.000204/(0.259)2
] 0 Pu = M(total Pu) [(0.5/10.0)2
+ 0.000204/(0.259)2]
1/2 = 38.6 x 0.074 = 2.9 grams Thus the final assay result from this coincidence count is quoted as: M(total Pu) = 38.6 +/- 2.9 grams.  For most plutonium samples, the dominant measure ment uncertainties will be in the 2 4°pu effective mass and the 2 4 0 pU isotopic weight fraction, f24 0.Thus good precision in M(total Pu) is achieved primarily through minimizing the uncertainties in these quantities.
 
5.34-7 S Since M(240)/f 2 4 0 results: (3)
VALUE/IMPACT
STATEMENT 1. PROPOSED ACTION 1.1 Description Licensees authorized to possess at any one time more than one effective kilogram of plutonium are required in § 70.51 of 10 CFR Part 70, "Domestic Licensing of Special Nuclear Material," to establish and maintain a system of control and accountability so that the standard error (estimator)
associated with the inventory difference (SEID) 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 nondestruc tive assay (NDA) technique of spontaneous fission detection is one acceptable method for the assay of plutonium in containers of bulk scrap material.
 
The use of spontaneous fission detection thus facilitates the preparation of a complete plant material balance whose SEID meets established requirements.


Regulatory Guide 5.34 was issued in June 1974 to describe procedures acceptable to the NRC staff for applying the NDA technique of spontaneous fission detection to plutonium in scrap.  1.2 Need for Proposed Action Improvements in technology have occurred since Regulatory Guide 5.34 was issued, and the proposed action is needed to bring it up to date.  1.3 Value/Impact of Proposed Action 1.3.1 NRC Operations The improvements in technology that have occurred since the guide was issued will be made available for the regulatory procedure.
Also, control (or working)standards should be assayed each day scrap is assayed for normalization and to assure reliable operation.


Using these updated tech niques should have no adverse impact. 1.3.2 Other Government Agencies Not applicable.
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.


1.3.3 Industry Since industry is already applying the techniques discussed in the guide, updating these techniques should have no adverse impact.  1.3.4 Public No impact on the public can be foreseen.
In addition to the calibration error there are errors due to the measurement process and due to variability in material composition.


1.4 Decision on Proposed Action The guide should be revised to reflect improvements in the technique and to bring the language of the guide into conformity with current usage.
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.


===2. TECHNICAL ===
The standard deviation in the distribution of differences in replicate results should be used in constructing a 95% confidence interval.
APPROACH Not applicable.


===3. PROCEDURAL ===
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.
APPROACH Of the procedural alternatives considered, revision of the existing regulatory guide was selected as the most advantageous and cost effective.


===4. STATUTORY ===
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.
CONSIDERATIONS
4.1 NRC Authority Authority for this guide is derived from the safety requirements of the Atomic Energy Act through the Commission's regulations, in particular, § 70.51 of 10 CFR Part 70. 4.2 Need for NEPA Assessment The proposed action is not a major action that may significantly affect the quality of the human environ ment and does not require an environmental impact statement.


===5. RELATIONSHIP ===
The standard deviation in the bias (mean difference)
TO OTHER EXISTING OR PROPOSED REGULATIONS
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
OR POLICIES The proposed action is one of a series of revisions of existing regulatory guides on NDA techniques.


6. SUMMARY AND CONCLUSIONS
====r. REFERENCES====
Regulatory Guide 5.34 should be updated.5.34-8-V
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:
UNITED STATES NUCLEAR REGULATORY
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
COMMISSION
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}}
WASHINGTON, D.C. 20555$I RST CLASS MAIL POSTAGE & FEESPAID USSNAC WASH D C PERMIT No -9k OFFICIAL BUSINESS PENALTY FOR PRIVATE USE. $300}}


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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)
v • d • e


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

8. Occupational Health Published guides will be revised periodically, as appropriate.

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


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