ML20209B089
| ML20209B089 | |
| Person / Time | |
|---|---|
| Issue date: | 06/28/1999 |
| From: | Paperiello C NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | Thadani A NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES) |
| References | |
| NUDOCS 9907060264 | |
| Download: ML20209B089 (14) | |
Text
{{#Wiki_filter:-. _- _ _ _ _ _ _ - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - _ _ - _ - _ _ _ -. 1 Sa ns p UNITED STATES g j NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 300eM1001 ~% ***./ June 28, 1999 MEMORANDUM TO: Ashol: C. Thadani, Director Office of Nuclear Regulatory Research FROM: Carl J. Paperiello, Director Office of Nuclear Material Safety and Safeguards
SUBJECT:
REQUEST FOR RESEARCH ASSISTANCE ON CONSIDERJNG BURNUP EFFECTS IN THE CRITICALITY SAFETY EVALUATIONS FOR SPENT FUEL CASKS The Office of Nuclear Material Safety and Safeguards (NMSS) requests assistance from the Office of Nuclear Regulatory Research (RES) in investigating a range of technicalissues concerning the consideration of burnup reactivity effects in evaluating the nuclear criticality safety of irradiated fuel outside reactors. With RES's assistance in assembling, analyzing, and confirming the best available knowledge, NMSS will seek to incorporate the analysis of bumup-related criticality effects into the NMSS licensing processes for storage and transportation of spent fuel. Backaround - In evaluating criticality safety for spent fuel casks, the NRC's current practice is to rely on the simple and highly conservative " fresh fuel" assumption. The term burnup credit" refers to consideration of the net reduction of fuel reactivity that results from burning the fuelin a reactor. Such burnup reactivity effects are governed by the burnout of the fissile actinides and neutron absorbers originally present in the fuel and by the production, burnout, and decay of new fissile actinides, neutron-absorbing actinides, and neutron-absorbing fission products. Accounting for the reactivity effects of actinides only, while ignoring fission products, is referred to as " actinide-only burnup credit." Depending on the initial enrichment and burnup history, actinides are estimated to account for 50 to 70 percent of the net reactivity losses in typic:J pressurized-water reactor (PWR) fuels. Modem PWR fuels may lose over 40 percent of their initial in reactor reactivity in going from fresh to fully burned. Crediting the reactivity losses can therefore have major impacts on the criticality safety design of spent fuel storage and transport systems. Applied to PWR spent fuel packages, burnup credit would allow existing cask designs to load spent fuel with higher initial enrichments and could eventually lead to new cask designs with larger payloads. In addition to substantial cost savings, the need for fewer spent fuel packages and shipments may also result in small reductions in radiological and non-radiological risks. Because crediting burnup effects in fuel from boiling water reactors (BWRs) is inherently more complex and offers relatively small benefits in cask design, the anticipated uses of burnup credit in casks are presently limited to PWR spent fuel. 9@#I e5an(3 k 9907060264 990628 PDR ORO NOMA fsd-IQ--{~ PDR
o . The Office of Nuclear Reactor Regulation (NRR) routinely allows bumup credit in evaluating postulated boron dilution events in the borated spent fuel storage pools at PWR plants. NRR also uses burnup calculations to determine the maximum reactivity of the irradiated fuel stored in the spent-fuel pools at BWR plants [Ref.1] (references listed in Attachment 1). In two cases involving single-purpose dry storage casks [Refs. 2, 3], NMSS has acknowledged applicants' calculations of bumup reactivity effects as illustrating the existence of an additional, unquantified safety margin that goes beyond the criteria for double contingency protection under 10 CFR Part 72. Much like the bumup credit considered by NRR for in-pool storage racks, those bumup-credit calculations addressed postulated loss-of-soluble-boron events that are considered extremely unlikely or incredible, arising only as a third contingency during the wet loading or unloading of PWR fuel in dry storage casks. This is further discussed in. Several regulatory bodies outside the U.S. allow similar uses of burnup credit in wet storage and handling operations, and also in reprocessing [Ref. 4]. However, transportation uses of burnup credit have been granted to date only in France. The French reprocessing program has developed an extensive set of proprietary validation data to support the limited credit needed for shipping modern PWR fuels with higher initial enrichments in the existing fleet of casks [Refs. 4,5]. Safety authorities in the United Kingdom and Japan are now working toward similar uses ' of burnup credit in transport packages. As discussed below, NMSS is starting a phased process for introducing limited burnup credit in evaluating transport and storage casks for PWR spent fuel [Refs. 6 - 10) and is starting to examine the role of burnup credit and other burnup-related reactivity effects in criticality evaluations for the high-level waste (HLW) repository [Refs. 11,12). Areas of Needed Assistance The U.S. Department of Energy (DOE) has been working on the development of a topical report that proposes a method for taking credit for actinide-only burnup in casks for spent fuel from I pressurized water reactors (PWRs) [Refs. 6,7]. DOE's proposal has been submitted to the NRC and has gone through two cycles of revisions based on NRC's review and comment. Open technical issues with the proposed method and its validation basis [Refs. 8, 9,10] have i precluded staff approval of the method for the level of burnup credit proposed by DOE. Nevertheless, the Spent Fuel Project Office (SFPO) of NMSS has recently developed an Interim Staff Guidance (ISG-8, included as Attachment 2) approving the method for use in crediting actinide-only effects at only half of the fuel assembly's recorded burnup (e.g., analyzing an adjusted nominal 40 GWD/MTU burnup as 20 GWD/MTU). This interim method for partial actinide-only burnup credit enables near-term benefits while leaving ample residual margin, including fission-product margin, to readily bound the remaining uncertainties surrounding the method's intended use for full actinide-only credit. Industry reactions to the ISG-8 position on partial actinide-only credit have indicated a continuing interest in pursuing substantially higher levels of burnup credit for PWR spent fuel packages, eventually including credit for fission products [Ref.13). SFPO's greatest needs for RES assistance in this area are in investigating and confirming the adequacy of proposed methods for these extended levels of bumup credit. Analyses for the HLW repository must necessarily consider the burnup and decay compositions of irradiated fuel materials from essentially all types of commercial and noncommercial 1 [
L ,. reactors. Such information may be considered by NMSS's Division of Waste Management . (DWM) both in establishing the unlikelihood of criticality in waste-package configurations over the 10,000-year compliance period of the repository and in analyzing the progression and - consequences of any postulated criticality events that could arise from gross package degradation, radionuclide decay and transmutation, and relocation of fissile materials and absorbers during the compliance period and beyond [Refs. 11,12,14,15,16]. DWM's particular needs for evaluating HLW criticality will be affected by ongoing technical and regulatory developments and are therefore not defined at this time. Nevertheless, it is expected that the analytical methods, validation data, and verification techniques needed for bumup credit in spent-fuel storage and transportation will overlap significantly with those that might eventually be considered in criticality evaluations for the HLW repository. Scopo of W.uk The research efforts for SFPO should proceed substantially in the manner discussed in the meeting on January 19,1999, between NMSS and RES staff and an RES contractor [Ref.17), and in the follow-up meetings on March 22, April 15, and May 18,1999. As noted in the NMSS revision to the RES Task Action Plan (Attachment 3), the initial steps should include an assessment of the residual margins in SFPO's interim method for partial actinide-only burnup credit. Parallel and subsequent steps should then focus on assembling the data and i knowledge bases needed for considering greater levels of burnup credit in spent-fuel packages. These steps should culminate in the development of a Standard Review Plan for bumup-credit analysis in spent-fuel storage and transport casks. High priority should be given to the phenomena-based identification and ranking of burnup credit data needs and, wherever possible and warranted, timely participation in the acquisition of existing and new experimental data and benchmarks for use in methods validation and confirmatory analysis (e.g., from the existing French and British proprietary data [Ref.18) and the proposed REBUS program [Ref.19]). Evaluations of existing or proposed experimental data for application to burnup-credit validation may make use of the sensitivity techniques that RES has been developing for NMSS at Oak Ridge National Laboratory [Refs. 20,21]. Investigation of modeling issues should build wherever possible on ongoing analytical benchmarking efforts for burnup credit [Refs. 22,23,24). As needed for utilization of new and existing reactivity worth data, consideration should also be given to the use of advanced differentiation techniques (e.g., perturbation, correlated sampling) within existing Monte Carlo criticality codes. Evolving methods for reactivity-related subcritical measurements and their interpretation (e.g., Refs. 24 - 29) should be evalucted for their potential operational or validation uses in spent-fuel burnup credit. As appropriate, the requested work should be conducted in coordination with the related efforts of the Nuclear Energy Institute (NEI), the Electric Power Research Institute (EPRI), DOE, and others, including foreign entities. The statements of work should be developed based on the updated Task Action Plan in close consultation with NMSS. After analyzing residual margins and uncertainties in the initial ISG 8 method for partial burnup credit (Task 1), the long-term focus should be on assembling a technical basis to support the development of a standard review plan for extended burnup credit in PWR spent fuel packages. Results of the requested work should also be considered in the contexts of spent fuel storage pools at reactors to ensure that NRR's existing applications of burnup credit can be reconciled with the new information and
June 28, 1999 A. Thadani 4 updated as appropriate. As part of the RES effort, administrative consideration should be given to: 1) quarterly contractor presentations to NRC staff,2) project coordination and oversight through an interoffice steering committee composed of key technical staff in RES, NMSS/SFPO, NMSS/DWM, and NRR,3) as appropriate, periodic public technical meetings, and 4) formal peer review. In a closely related area, by memorandum dated March 27,1998, (Paperiello to Knapp) NMSS also requested assistance concerning "Nuclide inventories and Source Terms for Storage and Transportation of High-Burnup Spent Fuel." We indicated at that time that our needs for the results of that requested assistance, which involved evaluation and recommendation of methods for predicting high-burnup isotopics in modern PWR and BWR spent fuels, would start becoming urgent in approximately four years. Since then, however, we have received cask applications showing the need to load high-burnup fuels in the nearer term. We therefore ask that that request now be given higher priority as our staffs work together to develop a combined statement of work for addressing the nuclear physics issues of spent fuel outside reactors. To that end, we suggest that work pursuant to both requests be coordinated within a single research program. As discussed in recent meetings between staff in RES and NMSS, the interoffice steering committee will advise on detailed scheduling priorities for individual tasks within the combined program. Attachments: 1. References
- 2. SFP0 Interim Staff Guidance 8
- 3. Research Task Action Plan DISTRIBUTION:
NRC File Center Public SFPO r/f NMSS r/f NES Dir. Office r/f A:\\Bucneed3.wpd
- see previous concurrence JPH 5/24/99 OFC:
SFPO SFPO SFPO DD:SF 'O, D:SFJ'Q D;NMSS NAME:
- DCarlson:jh
- CWithee
- EEaston MWNohs E
ach CJPakello DATE: 5/24/99 5/24/99 5/25/99 5)(,/99 St((99 [ /99 OFFICIAL RECORD COPY
L; A. Thadani- . -4 updated as appropriate. As part of the RES effort, administrative consideration should be given to; 1) quarterly contractor presentations to NRC staff,2) project coordination and oversight through an interoffice steering committee composed of key technical staff in RES, ' NMSS/SFPO, NMSS/DWM, and NRR,3) as appropriate, periodic public technical meetings,,/ and 4) formal peer review. / In a closely related area, by' memorandum dated March 27,1998, (Paperiello to Knapp MSS also requested assistance conceming "Nuclide inventories and Source Terms for orage and Transportation of High-Burnup Spent Fuel." We indicated at that time that our eds for the results of that requested assistance, which involved evaluation and recomm dation of methods for predicting high-burnup isotopics in modern PWR and BWR spent fuels, would start ' becoming urgent in approximately four years. Since then, however, we ave received cask applications showing the need to load high-burnup fuels in the nearer rm. We therefore ask that that request now be given higher priority as our staffs work tog er to develop a combined statement of work for addressing the nuclear physics issues of s nt fuel outside reactors. To that end, we suggest that work pursuant to both requests be c rdinated within a single research program. As discussed in recent meetings betwee staff in RES and NMSS, the interoffice steering committee will advise on detailed sche ling priorities for individual tasks within the combined program. Through this memorandum, we are also committing,to provide appropriate funding to help . support early portions of the requested research pr'ogram that have not received adequate consideration in the RES budget processes for scal Years 1999 and 2000. Attachments: 1~ Referonces 2.- SFPO interim Staff Gui nce 8
- 3. Research Task Actio Ian
/ ,/ / DISTRIBUTION: NRC File Center. - Public SFPO r/f NMSSrM A:\\Bucneed3.wpd 4 JPH E/24/99 OFC: , SFPO, [ k SFPh SFPO DD:SFPO D:SFPO D:NMSS EEah MWHodges EWBrach CJPaperiello NAME: DCarlson:jh CWithee DATE: 5/M/99 5/#/99 M9 -- 5/ /99 5/ /99' 5/ /99 OFFICIAL RECORD COPY l
References for NRC Burnup Credit Research i 1. U.S. Nuclear Regulatory Commission," Guidance on the Regulatory Requirements for Criticality Analysis of Fuel Storage at Light-Water Reactor Power Plants," Memorandum j from Laurence Kopp to Timothy Collins, Division of Systems Safety and Analysis, Office of Nuclear Reactor Regulation, Washington, D.C., August 19,1998. 4 2. VECTRA Technologies Inc., Safety Analysis Report for the Standardized NUHOMS HorizontalModular Storage System for Irradiated Nuclear Fuel, NRC Docket No. 72-1004. 3. Sierra Nuclear Corporation, Safety Analysis Report for the Ventilated Storage Cask System (VSC-24), NRC Docket No. 72-1007. 4. H.P. Dyck, IAEA, " Summary," pp 1-24 in Implementation of Bumup Credit in Spent Fuel Management Systems, Proceedings of an Advisory Group meeting heldin Vienna,20-24 October 1997, international Atomic Energy Agency, IAEA-TECDOC-1013, April 1998. 5. M. Zachar and P. Pretesacque,"Burnup Credit in Spent Fuel Transport to COGEMA La Hague Reprocessing Plant," pp. 1407-1414 in Proceedings of PATRAM'95, the f f* Intemational Conference on the Packaging and Transportation of Radioactive Materials, Las Vegas, Nevada, December 3-81995. 6. No authors, Topical Report on Actinide-Only Bumup Credit for PWR Spent Nuclear Fuel Packages, U.S. Department of Energy, Office of Civilian Radioactive Waste Management, DOE /RW-0472, Rev.0, May 1995; Rev.1, May 1997; Rev.2, September 1998. 7. M.D. DeHart, Oak Ridge National Laboratory, Sensitivity and Parametric Evaluations of Significant Aspects of Burnup Credit for PWR Spent Nuclear Fuel Packages, sponsored by DOE /OCRWM, ORNL/TM-12973, May 1996. 8. Meraj Rahimi, JAl Corporation, E. Fuentes and D. Lancaster, TRW inc., Isotopic and Criticality Validation for PWR Actinide-Only Burnup Credit, U.S. Department of Energy, Office of Civilian Radicactive Waste Management, DOE /RW-0497, May 1997. 9. M.D. DeHart and O.W. Herman, Oak Ridge National Laboratory, An Extension of the Validation of SCALE (SAS2H) Isotopic Predictions for PWR Spent Fuel, work sponsored by DOE /OCRWM, ORNt/TM-13317, September 1996. 10. M.D. DeHart and S.M. Bowman, Oak Ridge National Laboratory, Analysis of Fresh Fuel CriticalExperiments Appropriate for Bumup Credit Validation, work sponsored by DOE /OCRWM, ORNllTM-12959, October 1995. 11. U.S. Department of Energy, Office of Civilian Radioactive Waste Management, Yucca Mountain Site Characterization Project, Disposal Criticality Analysis Methodology Topical Report, YMP/TR-0040, Rev.011/1998. I 1 L_ e
12. T. Pollog, U.S. DOE, and S. Vance, JAl Corporation, " Office of Civilian Radioactive Waste Management Spent Nuclear Fuel Verificcion Plan,"in Proceedings of the 3# Annual Meeting of the Institute for Nuclear Materials Management, Phoenix, AZ, July 20-24, 1997.-
- 13. Lynette Hendricks, Nuclear Energy Institute, Letter to E. William Brach, U.S. NRC, dated March 11,1999.
14. W.E. Kastenberg, P.F. Peterson, J. Ahn, B. Bessinger, J. Burch, C. Casher, P. Chambre, N.G.W. Cook, F.M. Doyle, E. Greenspan, B. Hilbert, D.R. Olander, and J. Vujic, Mechanisms for Autocatalytic Criticality of Fissile Materials in Geologic Repositories, University of Califomia - Berkeley, Nuclear Engineering Department Final Report, UCB- . NE-4214, October 1996, summarized in Nuclear Technology,115, pp. 298-310 (1996). 15. E. Greenspan, S. Armel, J. Ahn, and J. Vujic, UC-Berkeley, " Critical Depositions of Uranium in Rock Having Positive Reactivity Feedback," pp. 91-98 in Poc. ANS Topical ' Meeting on Criticality Safety Challenges in the Next Decade, Chelan, WA, September 7-11,1997. 16. N.B. McLeod, JAl Corporation, " Evaluation of Long Term Criticality in the High-Level Waste Repository," in Proceedings of the 39 AnnualMeeting of the Institute for Nuclear . Materials Management, Phoenix, AZ, July 20-24,1997.
- 17. Cecil V. Parks, Oak Ridge National Laboratory, " Technical and Licensing issues for Using Burnup Credit in Transportation Packages," informal whitepaper requested by RES and NMSS staff, e-mail dated 10/8/98.
- 18. Phillip J. Finck, Argonne National Laboratory, and N.T. (Jim) Gulliford, AEA Technology (U.K.), Provision of Technical Assistance to the U.S. NRC in the Application of Burnup Credit for Spent Fuel Transportation, Interim Storage, and Disposal, a Proposal to the U.S. NBC, Rev.1, March 1999.
- 19. Belgonucleaire, REBUS IntemationalProgramme TechnicalProposal, RE 98/04, rev.
Oct.98; included in presentation handouts for a meeting at NRC, January 13,1999. 20. B.L. Broadhead and B.T. Rearden, Oak Ridge National Laboratory, Exploratory Studies for Three-Dimensional Sensitivity Methods, Letter Report for NRCIRES Contract JCN W6479, August 1998. 21.- B.T. Rearden, J.J. Lichtenwalter, and C.M. Hopper, Oak Ridge National Laboratory, . Evaluation of Critical Experiment Parameters and Uncertainties With First-Order Sensitivity Techniques, NUREG/CR-5624, ORNL/TM-13718, Draft -- December 3,1998. 22. M.C. Brady, H. Okuno, M.D. DeHart, A. Nouri, E. Sartori, "Intemational Studies on Bumup Credit Criticality Safety by an OECDINEA Working Group," Proceedings of the 1998 Intemational Conference on the Physics of Nuclear Science and Technology, American Nuclear Society; Islandia, New York, October 6-8,1998 2 1
77 r 23. A. Nouri, iPSN (France), OECD/NEA Bumup Credit Benchmark - Analysis of Phase II-B Results: Conceptual PWR Spent Fuel Transportation Cask, Rapport 1PSN/98-05, NEA/NSC/ DOC (98)1, March 1998. 24. N.T. (Jim) Gulliford et al, " Proposal to Form a WPNCS Task Force on Suberitical Measurements," presented at Second Meeting of the OECD/NEA Working Party on Nuclear Criticality Safety, Chateau de Cadarache, France,20 May 1998. 25. A.J. Luptak and K.D. Bulmahn, Evaluation of Nondestructive Assay / Nondestructive Examination Capabilities for Department of Energy Spent Nuclear Fuel, DOEISNFIREP-030, Rev.0, September 1998. I 26. T.E. Valentine, J.T. Mihalczo, and J.K. Mattingly, Oak Ridge National Laboratory, " Feasibility of Subcriticality and NDA Measurements for Spent Fuel by Frequency Analysis i Techniques with Cf," pp. 883-891 in Proceedings of the TopicalMeeting on Nuclear Plant Instrumentation, Control, and Human-Machine Interface Technologies, American Nuclear Society (1996). 27. J.D. Cole, M.W. Drigert, R. Aryaeinejad, E.L. Rober, and J.K. Jewell, Idaho National Engineering and Environmental Laboratory," Multi-Detector Analysis System for Spent Nuclear Fuel Characterization," a draft paper provided as private communication, 3/30/99. 28. H.O. Menlove and D.H. Beddingfield, Los Alamos National Laboratory," Passive Neutron Reactivity Measurement Technique,"in Proceedings of the 38 AnnualMeeting of the institute for Nuclear Materials Management, Phoenix, AZ, July 20-24,1997. 29. U.S. NRC, Regulatory Guide 3.71, Nuclear Criticality Safety Standards for Fuels and ' MaterialFacilities, August 1998. 3 h
Spent Fuel Project Office interim Staff Guidance - 8
- ssue:
Limited Burnup Credit in the Criticality Safety Analyses of PWR Spent Fuel in Transport and Storage Cesks Discussion: When fuel is irradiated in a reactor, the reactivity of the fuel decreases. This reduction of . reactivity with burnup is caused by the change in fissile content of the fuel (i.e., bumup of U 235 and production of Pu-239 and other fissile actinides), the production of actinide neutron absorbers, and the production of fission-product neutron absorbers. Until now, criticality safety analyses for spent fuel casks, including storage, transport, and dual-purpose casks, were performed under the assumption that the fuel was unirradiated. This " fresh fuel" assumption ,l was used as a bounding condition because of unresolved issues over the technical basis and methods for including credit for fuel burnup in the criticality analysis of spent-fuel casks. The U.S. Department of Energy (DOE) has been working on the development of a topical report that proposes a method for taking bumup credit in casks for transporting and storing spent fuel from pressurized water reactors (PWRs). DOE's proposal has been submitted to the U.S. Nudear Regulatory Commission (NRC) and has gone through two cycles of revisions based on NRC's review and comment. Based on the technical information provided in DOE's topical report, with its supporting technical reports, and information available from other sources, both foreign and domestic, the staff has now found sufficient basis to approve a limited level and scope of bumup credit while it pursues the development of a more complete basis for more comprehensive burnup credit. l l As justified through the review of additional supporting data and analysis, the staff will issue i revised interim guidance to reflect the evolution of approved methods for greater levels of burnup credit. Future revisions of the Standard Review Plans (NUREG-1617, NUREG-1536, and NUREG-1537) will incorporate or reference the current guidance, as appropriate.
Background:
Existina NRC Uses of Burnuo Credit in Soent Fuel Storace The' Office of Nuclear Reactor Regulation (NRR) has long allowed the use of burnup credit in the borated spent fuel storage pools at PWR plants. This is based in part on the established ability of licensees to predict the core burnup behavior over hundreds of reactor years of operation. Additional safety assurance is based on application of the double contingency . principio as defined in ANSI /ANS-8.1-1983, and in Title 10, Code of Federal Regulations (10 1 CFR), Section 72.124(a), which requires two unlikely, independent, concurrent events to produce a criticality accident. For example, if soluble boron is normally present in the spent fuel pool water, the loss of soluble boron is considered as one accident condition and a second concurrent accident need not be assumed. Alternatively, credit for the presence of soluble boron in PWR pools may be assumed in evaluating other accident conditions such as the j misloading of fresh fuel assemblies into racks restricted to irradiated fuel. Typically, there is sufficient soluble boron in PWR pools to maintain at least a 5% suberiticality margin even if an entire burnup-dependent storage rack were mistoaded with fresh fuel assemblies. t l I
ISG-8 2 As noted by DOE and others, bumup credit calculations can also be found in the applicants' aafety analysis reports (SARs) for two approved single-purpose dry storage casks for PWR spent fuel (i.e., NUHOMS 24P and VSC 24). There, the applicants performed bumup credit calculations in evaluating hypothetical underboration events during wet loading or unloading of the dry storage casks. However, the staff's safety evaluation reports for those cases used the " fresh fuel" analysis assumption in combination with credit for boron in the water. Boron credit was made possible by creating in the license or certificate a Technical Specification requiring two independent verification controls on soluble boron concentration during wet loading and unloading operations..This satisfied the double-contingency criterion of 10 CFR 72.124(a) while obviating consideration of loss-of boron events in the review under 10 CFR Part 72. Although triple contingencies are not directly considered in the staff's evaluations for 10 CFR Part 72 reviews, applicants have chosen to retain the bumup credit calculations in their SARs in order to help address plant specific requirements under 10 CFR 50.5g for use of the casks at reactor spent-fuel pools. Those bumup credit calculations are acknowledged by NRC's Office . of Nuclear Material Ssafety and Safeguards as litustrating an additional safety margin, of uncertain magnitude, that goes beyond the regulatory requirements of 10 CFR Part 72. Therefore, the bumup credit analyses for wet loading and unloading of dry storage casks can be viewed as technically consistent with NRR's applications of bumup credit at PWR spent-fuel ~ storage pools in that both are used, on a risk informed basis, only in addressing the criticality safety margins for extreme hypothetical events that are considered entremely unlikely or incredible. The need for considering burnup credit after drying and closure of casks has generally been avoided in 10 CFR Part 72 storage applications by showing that fresh-water ingress into sealed dry storage casks is not credible. Specifically, the double-contingency criterion is satisfied by showing that water ingress into a storage cask would require both a flooding event and an accident that would cause seal failure. On the other hand, transportation regulations under 10 CFR Part 71 include explicit requirements for assuming fresh-water inlaakage in the criticality analysis of transport packages for fissile materials. Sections 6.5.4 and 6.5.5 in NUREG 1617," Standard Review Plan for Transportation Packages for Spent Nuclear Fuel," further discuss the water-inleakage considerations for spent fue! evaluations under 10 CFR Part 71. Bumuo Credit in Other Countries Several regulatory bodies outside the U.S. have allowed various uses of bumup credit in wet storage and handling operations, and also in reprocessing. However, transportation uses of burnup credit have been granted to date only in France. The French reprocessing program has developed an extensive set of proprietary validation data to support the limited credit needed for shipping modem PWR fuels with higher initial enrichments in the existing fleet of casks. Safety authorities in the United Kingdom and Japan are now working toward similar uses of burnup credit in transport packages. As noted above, validation benchmarks provided from French and - other foreign or proprietary sources may be considered as part of the expanded technical basis needed for future NRC approval of greater levels of bumup credit. 4
P IM4 3 i Recommendation: When performing criticality safety analyses for spent fuel casks, hmited Dadal. credit for the reactivity effects of fuel bumup may be taken as follows: Method: Use the method described in DOE's report entitled, " Topical RepcA on Actinide-Only Bumup Credit for PWR Spent Nuclear Fuel Packages," DOE /RW-t 472, Rev. 2, except i as otherwise specified below. Assume fuel bumup is 50% of the verified and adjusted bumup level frWn plant records. EG928: Applies to intact commercial PWR fuel only. Includes actinide effects only (change in fissile content and actinide neutron absorbers). Ranae of Anotication: Covers UO, fuel with nom!nal Initial enrichments up to 4.0 weight percent U-235. Covers assembly average bumup levels up to 45 GWD/MTU. Establishina the Bumun Value: Use the reactor record assembly bumup, as adjusted, when confirmed by a direct assembly measurement, performed in the storage pool or loading facility, that is calibrated to the reactor records for a representative set of measured assemblies. Measurement confirmation must be within a 95% confidcnce interval based on the measurement uncertainty. Adjust the bumup-record value by reducing the record value by the combined uncertainties in the records and the measurement. Bumup measurement may be based on gamma emissions of the Cs 137 isotope. The requirement for bumup verification measuremciits is consistent with Regulatory ) Guide 3.71 and is justified in part by numerous licensee reported events invoMng failure of administrative controls on plant spent fuel records and the selection, handling, and placement of spent fuelin the storage pools at reactors. -[ Ih6 Approved dw E. William BracW Date' O n
j TASK ACTION PLAN FOR RESEARCH ASSISTANCE ON CONS. 5 RING BURNUP EFFECTS IN THE CRITICALITY EVALUATIONS OF SPENT FUEL SYSTEMS Revised May 10,1999 9 This action plan has been developed through discussions between staff in RES and NMSS/SFPO and NMSS/DWM. Regarding certain elements of the planned program, staff in NMSS and RES have also had contact with Oak Ridge National Laboratory, Argonne National Laboratory, Idaho National Engineering and Environmental Laboratory, Los Alamos National Laboratory, AEA Technology (U.K.), French safety authorities, Belgonucleaire, and the Paul Scherer Institute (Switzerland). l e The objective of the proposed plan is to assemble the best available knowledge for incorporating the analysis of burnup-related criticality effects into the NMSS licensing processes for transportation and storage of spent fuel. O "Burnup credit" refers to consideration of the net reduction of fuel reactivity that results from burning fuelin a reactor. Burnup credit has applicability to spent fuelin reactor pool storage, dry storage, transportation, and repository disposal. e The negative reactivity effects of burnup arise from (1) the net decrease in the fissile content, (2) the production of neutron-absorbing actinides, and (3) the production of neutron-absorbing fission products. 4 The governing transmutation phenomena are: (a) the burn-out of fissile actinides (U 235) originally present in the fuel; (b) the bum-out of neutron absorbers, including absorber actinides (U 238), originally present in the fuel; (c) the burn-in, burn-out, decay-in, and decay-out of new fissile actinides and new absorber actinides; and (d) the burn-in, burn-out, decay-in, and decay-out of neutron-absorbing fission products. O PROPOSED APPROACH: The proposed approach will start with an evaluation of residual margins and uncertainties in the method described in SFPO's interim staff guidance on limited credit for actinide-only burnup in PWR spent fuel packages. Parallel and subsequent steps will focus on assembling and assessing the data and knowledge bases needed for considering greater levels of burnup credit. These efforts willinclude considered participation in the acquisition of new and existing validation data and reference-code benchmarks, and will proceed in a phased program to the development of a standard review plan for extended burnup credit for PWR spent fuel transport. Research tasks on repository burnup credit and the related aspects of analyzing postulated HLW criticality events may be defined as more specific needs emerge in those areas. The work should be conducted in coordination with NEl, EPRI, DOE, and others as needed to access existing and new benchmarking data, if any. We will solicit collaboration with foreign entities as appropriate to leverage the NRC's funds in gathering information. 1 O
j RESEARCH TASK OUTLINE FOR NMSS BURNUP CREDIT Revised May 10,1999 l I&SK_1-Assessment of Interim Partial Actinide-Only Burnup Credit (Staff FTEs 0.1 RES/0.1 NMSS; start in FY1999, end in FY1999) PURPOSE: To support the assessment of residual burnup credit margins under SFPO's initial Interim Staff Guidance on limited actinide-only burnup credit for PWR spent fuel packages. PLAN: Assist the staff in reviewing the technical basis for SFPO's initial interim methodology for limited burnup credit and estimating the residual margins and uncertainties. l TASK 2 - Evaluation of issues and Uncertainties in Burnup Credit for PWR S,;ent Fuel Transport (Staff FTEs 0.2 RES/0.2 NMSS; start in FY1999, end in FY2001) PURPOSE: To elucidate and evaluate current staff technical positions on the major issues, and possible approaches to their reschtion, including: (a) uncertainty in determining the isotopic concentration, (b) uncertainty in the representation of the neutron cross sections, (c) benchmark validation of the criticality calculations, (d) analysis of fuel element axial burnup "end effects," and (e) measurements to verify the identity and burnup records of spent fuel assemblies. PLAN: Assist the staff with the following subtasks: 1) Identify and rank major phenomena for burnup credit in PWR spent fuel transport packages. 2) Collect and characterize relevant available technicalinformation and data. 3) Identify major analysis issues and uncertainties. 4) Conduct sensitivity analyses for a phased approach to resolve issues and bound uncertainties. 5) Perform code-to-code analytical benchmarks to evaluate modeling issues and help expose any unusual code and data dependences or errors. 6) As needed to make use of validation data, evaluate the use of correlated sampling, perturbation, or similar differentiation techniques in evieting Monte Carlo criticality codes. TASK 3 - Assessment and Acquisition of Existing Foreign / Proprietary Data for Benchmarking (Staff FTEs 0.2 RES/0.2 NMSS; start in FY1999, end in FY2001) PURPO_SE: To assess the potential usefulness of existing proprietary data from foreign burnup-credit programs (e.g., French and British), and, as warranted, participate in the acquisition of such data and/or the development of nonproprietary reference-code benchmarks correlated to such data. PLAN: Consistent with data needs and priorities established in Task 2, assist the staff with: 1) evaluating the potential usefulness of existing foreign / proprietary data and associated benchmarks, and, as warranted,2) coordinating NRC's participation with other stakeholders in acquiring such data and Denchmarks, and 3) negotiating with the holders of proprietary data the price, conditions, and terms for acquiring data and/or reference-code analytical benchmarks derived from such data. 2
. TASK 4 - Assessment and Acquisition of New Critical and Subcritical Measurements for Benchmarking (Staff FTEs 0.2 RES/0.2 NMSS; start in FY1999, end in FY2002) PURPOSE: To assist the staff in (a) assessing experiment proposals and their value toward resolving prioritized uncertainties, (b) clarifying NRC's participation in the proposed measurement and benchmark programs, (c) as appropriate, overseeing the planning and conduct of the experiments and their documentation, and (d) evaluating the resulting data and benchmarks. PLAN: Assist the staff in the following subtasks: 1) Conduct a technical review of the proposals with NRC staff and contractors. 2) Accept, reject, or propose technical modifications to, the NRC portions of the programs. 3) As appropriate, monitor the planning and conduct of measurements. 4) Review the draft documentation of the experiments, results, and their evaluation for specific benchmarking applications.
- 5) Finalize documentation.
TASK 5 - Analysis of the Technical Bases for Extending Burnup Credit (Staff FTEs 0.1 RES/0.2 NMSS; start in FY2000, end in FY2002) PURPOSE: To assist in assembling, analyzing, and summarizing the latest technical bases for updating the NRC's technical positions on the review and acceptance of extended burnup credit in PWR spent-fuel casks. PLAN: Considering new and pre-existing information, assist the staff in assembling, analyzing, and summariz!ng the latest technical bases for updating or refining the staff's positions on major issues, including: 1) uncertainties in spent-fuel characterization,
- 2) uncertainties in the codes and data for criticality safety analysis,3) design basis analysis assumptions and models (e.g., end effects),4) spent-fuel verification measurements and reactor records, and 5) any proposed reactivity-related suberitical measurement methods.
TASK 6 - Preparation of a Standard Review Plan (SRP) for Burnup Credit in PWR Spent Fuel Casks (Staff FTEs 0.1 RES/0.1 NMSS; start in FY2001, end in FY2002) PURPOJE: To assist in clarifying and documenting the staff's technical positions and guidance on the criteria and processes needed for review and approval of burnup credit in PWR spent-fuel casks and document the criteria and processes in a standard review plan (SRP). ELAN: Assist the staff with the following subtasks: 1) Assemble supporting documentation and references. 2) Outline and prepare a draft SRP. 3) Solicit comments from staff and public. 4) Address comments. 5) Finalize and issue the SRP. C 3 Q..}}