Regulatory Guide 5.35
| ML13064A074 | |
| Person / Time | |
|---|---|
| Issue date: | 06/30/1974 |
| From: | US Atomic Energy Commission (AEC) |
| To: | |
| References | |
| RG-5.035 | |
| Download: ML13064A074 (5) | |
CSi JUNE 1974U.S. ATOMIC ENERGY COMMISSIOWREGULATe RY GUDE 0%i DIRECTORATE OF REGULATORY YTANDARDSREGULATORY GUIDE 5.35CALORIMETRIC ASSAY OF PLUTONIUMA. INTRODUCT_!'IONSection 70.51, "Material Balance, inventory, and Records Requirements," of10 CFR Part 70, "Special Nucl.,,!ar Material," requires certain licensees authorizedto possess at any one time more than one effective kilogram of plutonium toestablish and maintain a system of control and accountability such that thelimit of error (LE) of the material unaccounted for (MIUF), ascertained as aresult of a m-asured material balance. meets established minimum standards. Theselection and proper application of an adequate measurement method for eachmaterial form in the fuel cycle is essential in meeting these requirements.Calorimetry is a precise nondestructive method for plutonium assay that isapplicable primarily for material of high plutonium concentration such as oxidepowder and fuel pellets. CalorimnetrNy with proper calibration, is capable of anaccuracy sufficient to meet LEMUF requirements established by 10 CFR Part 70and is not dependent on representative sampling. Sitce calorimetry can also beapplied to sealed containers, it is an accurate and timely method for verifyingreceipt quantities of plutonium. Calorimetry is also useful as a method forimproving the calibration of other NDA techniques, as suggested in two Regula-tory Guides,1:.2 and thus improving the control and accountability of plutonium.This guide describes procedures acceptaoDle to the Regulatory staff for oper-ating and calibrating heat flow ca.ocIimeters in measuring the power associatedwith plutoniumi decay.USAEC REGULATORY GUIDES Copies n.f publis:ied quides may be obtained by request irsditating the divisionsdelsirej to the U.S. Atomic Energy Commission, Washington, D.C. 20545,Regulatory Guides are issued to describe rnd mrnke available to the public Attentio,.: Director of Rogulatory Standards. Comnent tsdn suggestiOns formethods acceptable to the AEC Regulatory staff of implementing spocific Parts of irtprovenrents in thies guid:s are encouraged and should be sent to the Secretarythe Commissiorn's regulations, to delineate techniques used by the stats in of the Commission, U.S. Atomic Energy Commission, Washington, D.C. 20545.evaluating specific problems or postulated accidents, or to provide j-djnce to Attc ntion: Chief. Public Proceedings Staff.applicants. Regulatory Guides are not substitut" for regulations end rompis.hTtwith them is not required. Methods and solutions different from thole -t out ;n he gsjdA are i=sued in the tolloming tan bred divisions:the guides will be acceptable if they provide e besis for the findings requisite tothe iruuance or continuance of a permit or license by the Commission. 1. Power Reac ctors 6. Products2. Research an Test Rir...ters 7. Tranrsportation3. Fuels end Materials Facilities S. occupationUl HealthPublished guides will be revised periodically, as ar.1mropriate. to accotymodiae 4. 7nvironmental end Siting 9. Anti.trtut Review-..... ...... ..5. Moterials end Plart Protectior 10. Generalcomments and to reflect niiniv infor t on or pe
B. DISCUSSION
Calorimetry is the measurement of heat or the rate of heat emission (power).Calorimetry can therefore be applied as a nondestructive method for determiningthe quantity of plutonium in material in bulk form by measuring the thermal powergenerated as a result of radionuclide decay. As a plutonium assay technique,calorimetry must be accompanied by supplemental measurements to determine theproper relationship for converting the thermal power to the quantity of plutonium.Although the actual power measurement is accurate on a wide range of materials, itis the ability to accurately determine the conversion relationship that limitsthe application of calorimetric plutonium assay.The conversion of a power measurement to a mass of plutonium is sometimesaccomplished by considering calorimetry an absolute plutonium assay method,i.e., an assay technique that does not depend on representative calibrationstandards for accurate assay. Theoretically, a measurement of the thermal powerfrom plutonium can be'converted directly to plutonium mass if the relativeabundance of each plutonium isotope and americium-241 is known. However, prac-tically, this general method for converting calorimetric measurements of powerto plutonium mass has two limitations: (1) the nuclear decay constants of the-plutonium isotopes used in the conversion are known only to within approximately+0.5% and (2) the present state of the art in measuring the relative abundanceof plutonium isotopes limits the accuracy of this approach. Even with theselimitations, the use of calorimetry as an absolute measurement whose interpretationis based on fundamental nuclear constants is adequate in many situations.Calorimetry is capable of higher accuracy and is applicable to more materialswhen the assay results are interpreted through a calibration based on representa-tive standards rather than conversion of the power measurement on the basis ofradionuclide abundances and decay constants. A representative standard forcalorimetry need not have all the same physical characteristics of the unknownsas it does for other NDA methods. For calorimetry, the standard need only besimilar to the unknowns in a few characteristics, the most important of which isthe relative abundance of each radionuclide. If material is processed in batchesand the batches are characterized by a single plutonium isotopic composition, asingle standard from each batch is usually sufficient to normalize the calorimetercalibration to allow accurate interpretation of the power measurement. Such a5.35-2 standard can be obtained from the batch at a point where the batc;i material isreasonably pure and homogeneous. Calibration based on this standard is thenapplicable, with corrections for decay of the plutonium isotopes, for the lifeof the material in the process. Nondestructive gamma ray spectrometry can beused to verify the plutonium isotopic composition and to assure thatthe cor-rection of the calibration relationship for decay is valid.*Under the sponsorship of the Institute of Nuclear Materials Management(INMM), Subcommittee N15.8 of the American National Standards Institute (ANSI)Standards Committee N15, Methods of Nuclear Control, has developed AmericanNational Standard N15.22, "Calibration Techniques for the Calorimetric Assay ofPlutonium-Bearing Solids Applied to Nuclear Material Control." This standardidentifies procedures for the calibration and operation of calorimeters to beapplied for measuring the thermal power generated from the radioactive decay ofplutonium and its daughters. The standard also describes, in detail, methodsfor interpreting the power measurement in terms of plutonium mass, either througha calculation using the measured radionuclidic abundances and their decay constants,or through a representative calibration. Methods for correcting the calibrationor relative radionuclidic abundances for Pu-241 decay and Am-241 ingrowth arealso discussed in this standard, as well as procedure, for computing an estimateof the total error in the plutonium assay to be used in calculating the limit oferror (LE) in the material unaccounted for (MUF).Calorimetry can be used not only for routine plutonium assay but also toimprove other NDA methods for plutonium assay and to ease the task of estimatingthe error in othner NDA methods. Radiometric plutonium assay methods of spontaneousfission detection and quantitative gamma ray spectrometry are susceptible tointerferences resulting from variation in the composition of the material beingassayed and rely on highly representative standards for accurate assay. Calorimetryis not susceptible to most of these interferences. Therefore, if the relativeabundance of the plutonium isotopes is constant for each batch, errors can bedetected and calibrations corrected by comparison of assay results from theseNDA methods with calorimetric results.*A regulatory guide is under development that identifies acceptable methods forverifying radionuclidic abundances. Reference 3 provides information for theselection of gamma rays to be used for verification.5.35-3
C. REGULATORY POSITION
The procedures on operation and calibration of heat flow calorimetersspecified in the June 1974, draft of American National Standard N15.22-1974,*"Calibration Techniques for Calorimetric Assay of Plutonium-Bearing SolidsApplied to Nuclear Material Control," should be used for measurements of thepower associated with plutonium decay. The conversion of the power measurementto plutonium mass should be made according to one of the-methods discussed inANSI N15.22-1974, i.e., either by a calculation using the measured abundances ofthe plutonium isotopes and americium-241 and the nuclear decay constants ofthese radionuclides given in N15.22-1974, or by reference to a calibration basedon representative standards.When material is processed in identifiable batches characterized by asingle blend of plutonium isotopes, the calibration or radionuclidic compositionused to convert power measurements to plutonium mass is the same for all materialin the batch. If calorimetry is applied for plutonium assay using a batchcalibration or batch radionuclidic composition, then the following proceduresshould be used to assure that calorimetric assay is accurate and reliable:1. Measurement of the effective specific power** of the plutonium should bemade on a sufficient number of samples to generate a reliable estimate ofthe sampling error. Preferably, samples should be taken at a point in theprocess.where the blend originates.2. The rate of increase of the effective specific power due to ingrowth ofAm-241 should be used to determine the current value of the effectivespecific power at the time of each measurement.3. The process should be studied to determine if separation of americium fromplutonium could occur in process steps between the point of sampling formeasuring the effective specific power and the point of measurement of thethermal power. If separation behavior is unpredictable, a redeterminationof the effective specific power at later steps of the process is requiredfor proper calorimetric plutonium assay.*Copies may be obtained from the Institute of Nuclear Materials Management,505 King Ave., Columbus, Ohio 43201, ATTN: Mr. H. C. Toy.**Effective specific power is defined in ANSI N15.22-1974 and is the quantity usedto convert power to mass of plutonium.5.35-4 4. If mixing of plutonium isotopic blends is suspected, either the effectivespecific power should be redetermined or the radionuclidic abundancesshould be monitored to detect such mixing and/or to allow the averageeffective specific power to be computed.5. For each container of material assayed, the ratio of the abundances of twoof the plutonium isotopes and americium-241 should be measured by gamia-rayspectrometry to verify the plutonium isotopic composition3 and to verifythe predicted americium-241 abundance.REFERENCES1. Regulatory Guide 5.11, "Nondestructive Assay of Special Nuclear MaterialContained in Scrap and Waste."2. Regulatory Guide 5.34, "Nondestructive Assay of Plutonium in Scrap bySpontaneous Fission Detection."3. R. Gunnink and R. J. Morrow, "Gammna Ray Energies and Absolute BranchingIntensities for 238, 239, 240, 241Pu and 241Am," UCRL-51087, July 1971.5.35-5