ML101380059
| ML101380059 | |
| Person / Time | |
|---|---|
| Issue date: | 06/24/2010 |
| From: | Ruland W NRC/NRR/DSS |
| To: | Redmond E Nuclear Energy Institute |
| Cunanan, Davida, NRR,DSS,SRXB 415-3573 | |
| Shared Package | |
| ML101380160 | List: |
| References | |
| TAC ME3698, Y020100074 | |
| Download: ML101380059 (8) | |
Text
June 24, 2010 Everett L. Redmond Senior Project Manager Used Fuel Storage and Transportation Nuclear Energy Institute 1776 I Street NW, Suite 400 Washington, D.C. 20006-3708
SUBJECT:
THE UNITED STATES NUCLEAR REGULATORY COMMISSION RESPONSE TO THE NUCLEAR ENERGY INSTITUTE PERSPECTIVE ON THE UNITED STATES NUCLEAR REGULATORY COMMISSION SPENT FUEL POOL CRITICALITY LICENSING PRECEDENT
Dear Mr. Redmond:
In your letter dated March 24, 2010, the Nuclear Energy Institute (NEI) expressed that the U. S.
Nuclear Regulatory Commission (NRC) has introduced a degree of regulatory uncertainty in the area of spent fuel pool (SFP) criticality. The NRC staff acknowledges that there has been a higher level of NRC scrutiny and additional staff questions related to SFP criticality applications in the past several years. This uncertainty has been driven by the reduction in margin and increased complexity in the submitted applications. The staff has re-examined its guidance and technical bases given the changes in the SFP amendment applications. The NRC has developed a comprehensive SFP criticality action plan (ADAMS Accession number ML1015204630) to address SFP criticality analysis issues and to strengthen the technical basis used in licensee and staff evaluations. The overall goal of this plan is to establish appropriate safety criteria through improved guidance to the industry. Until the NRC can issue the final guidance the staff plans to issue additional guidance in the form of an Interim Staff Guidance.
The current schedule has the plan being issued for public comment by July 30, 2010.
A key outcome of the current NRC Strategic Plan is to prevent the occurrence of any inadvertent criticality events. To this end, the NRC conducts thorough reviews to verify compliance with regulations. In your letter, you specifically identified four areas where you believe the NRC did not apply precedent established in previously approved license amendments. The NRC staff responses to your issues are provided as an attachment to this letter.
The NRC staff uses precedent to make our reviews more efficient and is not necessarily controlled by precedent when licensees propose to change their licensing basis by amendment.
In the case of our review of spent fuel pool criticality amendments, staff has sought additional information to assure compliance with the regulations as margins have decreased. We have sought additional rigor in analyses where using engineering judgment was no longer appropriate. The attachment also clarifies that, in some cases, the staff has re-confirmed our
E. Redmond guidance (the Kopp letter) to ensure licensees understand how we are applying our current guidance. In closing, the NRC values your assistance in the coordination of public interactions between the staff and industry. We look forward to your continued support as future public meetings will likely be needed as we continue to work towards sustainable guidance regarding spent fuel criticality licensing.
Sincerely,
/RA/
William H. Ruland, Division Director Division of Safety System Office of Nuclear Reactor Regulation
Enclosure:
As stated
E. Redmond guidance (the Kopp letter) to ensure licensees understand how we are applying our current guidance. In closing, the NRC values your assistance in the coordination of public interactions between the staff and industry. We look forward to your continued support as future public meetings will likely be needed as we continue to work towards sustainable guidance regarding spent fuel criticality licensing.
Sincerely,
/RA/
William H. Ruland, Division Director Division of Safety System Office of Nuclear Reactor Regulation
Enclosure:
As stated DISTRIBUTION: Y20100074 RidsNrrDSS RidsNrrOD RidsNrrMailCenter SRXB r/f Package Number: Pkg ML101380160 Letter ML101380059 OFFICE NRR/DSS/SRXB NRR/DSS/SRXB:BC NRR/DSS NAME DCunanan AUlses WRuland DATE 06/17/10 06/17/10 06/24/10 OFFICIAL RECORD COPY
U.S. NRC Response to NEI Concerns NEI Comment #1: Margins are decreasing The U.S. Nuclear Regulatory Commission (NRC) staff and management have stated that the margins in industrys spent fuel pool criticality analyses are being reduced.
However, Nuclear Energy Institute (NEI) does not concur with the view that margins are decreasing. In support of their view, the NRC identifies recently submitted licensing actions with calculated k-effective values that are very close to the regulatory limit of 1.0 and as a result have a calculational margin that is less than 0.005 (References 3 and 4).
Previous NRC staff, however, approved licensing actions with calculated k-effective values as high as 0.999 and 0.99923 (calculated margins of 0.001 and 0.00077, respectively) (References 5 and 6).
Staff Response:
The NRC staff finds that conservatisms/margins in spent fuel criticality (SFP) analyses are decreasing and therefore requires more detailed review to make the regulatory determination and these trends have raised the NRC staffs level of concern regarding SFP criticality safety.
The licensees are increasingly utilizing high-density storage racks with less spacing between fuel assemblies. Certain neutron absorbers materials (e.g., Boraflex and Carborundum) used in storage racks are degrading and in some cases the level of degradation is not well characterized due to the lack of physical measurements and/or poorly validated prediction models. Furthermore, power uprates and increased capacity factors are leading to higher fuel enrichment and increased reactivity of discharged assemblies.
To address evolving design requirements, licensees are submitting revised analyses that credit increasing amounts of soluble boron, increasing amounts of burnup, more aggressive storage configurations, cooling times, and other less conservative assumptions. As a result, the SFP criticality analysis and storage controls are becoming increasingly complex. Recent license amendment requests (LAR) have not demonstrated the increased level of rigor in the analysis that these more complex storage requirements warrant. NRC questioning of old assumptions has lead the staff to believe that several old assumptions are suspect in the new SFP criticality environment, including some contained in staff guidance. Commensurate with the current state of the SFPs, the NRC staff finds it necessary that the licensees provide increased level of technical justification to support their application.
As the analytical k-eff gets closer to the regulatory limit, the staff has; appropriately, increased the level of review. In the case of an example you reference in the NEI letter (Reference 4), the analysis showed a reference k-eff of 0.99951 due to issues identified during NRC staff review.
The proximity of the analytical k-eff to criticality essentially required that there were no other uncertainties and that the analysis was without issues. With available margin so small, issues that may be otherwise resolved by engineering judgment become more important to consider and quantify. Consequently, an issue that was relatively insignificant in the past can now be significant enough to cause the regulatory limits to be exceeded. The licensee retains the option to justify margin by identifying and quantifying the conservatisms in their analysis.
ENCLOSURE
However, this approach usually results in a more lengthy review with additional requests for additional information (RAIs) to substantiate the identified conservatisms.
NEI Comment #2: Inclusion of New Critical Experiment Data The NRC has stated that the effect of the lack of critical experiments that resemble spent fuel (i.e. contain actinides and fission products) must be assessed in the benchmarking of the computer codes (Reference 3). Historically this has not been required. The staff is now requiring, through requests for additional information and acceptance reviews, that critical experiments that contain actinides be incorporated into license amendments even though this data only became available in October of 2008 and at that time the NRC did not state that its expectation was that industry would use this data. Since the development of a license amendment is a multi-year effort, it is potentially a significant burden to incorporate this data into license amendments that are currently being finalized and submitted.
Staff Response:
The NRC agrees that it is a burden to incorporate additional data into license amendments.
However, consistent with the Kopp Letter (Reference 2), the NRC staffs position is that the, critical experiments used for benchmarking should include, to the extent possible, configurations having neutronic and geometric characteristics as nearly comparable to those of the proposed storage facility as possible. Additional benchmarking information became available with the issuance of NUREG/CR 6979, Evaluation of the French Haut Taux de Combustion Critical Experiment Data, dated September 2008 (Reference 7). The expectation that criticality codes should be validated with best available data was reemphasized during the Spent Fuel Pool Criticality Industry Meeting, held in February 2009 (Reference 8). The NRC staff expects that submittals to the NRC after February 2009 should contain expanded validation, if applicable, consistent with the Kopp Letter. The submittals to the NRC prior to February 2009 were not required to provide the expanded validation.
NEI Comment #3: Code to Code Comparison The NRC has stated that depletion codes (e.g. CASMO) that are used for performing differential reactivity calculations should be benchmarked against critical experiments rather than another computer code (Reference 8). Since many of these codes are two-dimensional, benchmarking against three-dimensional critical experiments may not be possible. Historically this type of benchmarking has not been required and the NRC staff has previously accepted code to code comparisons. For example, NRC staff has stated in at least four documents (References 9 through 12) that The intercomparison between different analytical methods is an acceptable technique for validating calculational methods for nuclear criticality safety.
Staff Response:
The Kopp letter mentions that it is preferable to benchmark depletion codes such as CASMO to critical experiments when performing code validation. The Kopp letter states that The proposed analysis methods and neutron cross-section data should be benchmarked, by the
analyst or organization performing the analysis, by comparison with critical experiments. ANS 8.1-1998 (Reference 13), ANS 8.24-2007 (Reference 14), and NUREG/CR-6698 (Reference
- 15) reinforce this statement. The licensees in references 10, 11, and 12 all use two different codes in each of their criticality analyses. In each case, however, these codes were also benchmarked against critical experiments used in each in the respective criticality analyses.
The licensee in each reference simply stated that the two codes agree with the critical experiments performed for each criticality analysis, as well as to each other. In other words, the codes were not used exclusively because the critical experiments confirmed the validation.
As with any guidance, applicants can use alternate methods, provided those methods are technically sound. The NRC has not definitively told applicants that they could not attempt a code to code comparison for validating criticality codes; the staff has indicated that there is not an accepted standard by which a code to code comparison for validating a criticality code may be performed and judged. Without an accepted standard, any applicant attempting a code to code comparison for validating a criticality code should expect a higher level of staff scrutiny associated with that application.
NEI Comment #4: Misloaded Fuel Assemblies The NRC staff did not accept a licensees statements that it is not credible that a fuel assembly can be misloaded into a physically blocked storage cell (Reference 16).
However, many licensees utilize blocked cells and some have considered the accidental misloading of a fuel assembly into these locations as non-credible in their licensing actions. For example, Reference 17 states: The fuel mishandling events assumed for this analysis did not consider the possibility of locating a fresh fuel assembly in one of the blocked Region 0 locations. The approval for Reference 17 is documented in Reference 18.
Staff Response:
The NRC did not arbitrarily reject the licensees assertion that it was not credible that a fuel assembly can be misloaded into a physically blocked storage cell. Rather, the NRC could not make a reasonable assurance finding that given the description of the items to be used as blocking devices and the controls on those items provided by the licensee when coupled with industrys history of misloadings that a misloading event would be non-credible. To come to closure on the issue, the NRC requested justification of the statement made by the licensee. In Reference 16, the NRC asked the licensee to perform a Human Reliability Analysis that quantitatively evaluates the probability of occurrence of a fuel misloading event to support that the event is not credible. The licensee attempted the analysis, and then withdrew their LAR.
The references cited as precedent in your letter are not appropriate as that was for a pressurized water reactor with soluble boron in the SFP and the licensee in question is a boiling water reactor without soluble boron.
References
- 1. Letter from Nuclear Energy Institute to NRC dated March 24, 2010, ADAMS Accession Number ML101050243
- 2. NRC Memorandum from L. Kopp to T. Collins, Guidance on the Regulatory Requirements for Criticality Analysis of Fuel Storage at Light-Water Reactor Power Plants, August 19, 1998.
- 3. NRC Presentation to NEI Licensing Forum, October 7, 2009, available at http://www.nei.org/newsandevents/conferencesandmeetings/lf/archives/
- 4. Point Beach Nuclear Plant, Units 1 and 2 - Request for Additional Information from Reactor Systems Branch Related to License Amendment Request No. 247 Spent Fuel Pool Storage Criticality Control - Round 3 (TAC Nos. MD9321 and MD9322), October 22, 2009, ADAMS Accession number ML092930502
- 5. Safety Evaluation by the Office of Nuclear Reactor Regulation related to Amendment No. 125 to Facility Operating License No. NPF-41. Amendment No. 125 to Facility Operating License No. NPF-51, and Amendment No. 125 to Facility Operating License No. NPF-74, Arizona Public Service Company, et al. Palo Verde Nuclear Generating Station, Units 1, 2, and 3 Docket Nos. STN 50-528, STN 50-529, and STN 50-530, March 2000, ADAMS Accession Number ML003689038
- 6. Safety Evaluation by the Office of Nuclear Reactor Regulation related to Amendment No. 227 to Facility Operating License No. NPF-4 and Amendment No. 208 to Facility Operating License No. NPF-7, Virginia Electric and Power Company, North Anna Power Station, Unit Nos. 1 And 2 Docket Nos. 50-338 and 50-339, June 2001, ADAMS Accession Number ML011700557
- 7. Evaluation of the French Haut Taux de Combustion (HTC) Critical Experiment Data, NUREG/CR-6979, Oak Ridge National Laboratory, September 2008, ADAMS Accession Number ML082880452
- 8. NRC presentation during NRC public meeting, February 26, 2009, ADAMS Accession Number ML090830604
- 9. Affidavit of Laurence I. Kopp in support of the NRC staff brief and summary of relevant facts, data and arguments upon which the staff proposes to rely at oral argument on technical contention 2, ASLBP No. 99-762-02-LA, January 2000, ADAMS Accession Number ML003673795
- 10. Safety Evaluation by the Office of Nuclear Reactor Regulation related to Amendment No. 150 to Facility Operating License No. DPR-43, Nuclear Management Company, LLC, Kewaunee Nuclear Power Plant, Docket Nos. 50-305, January 2001, ADAMS Accession Number ML010240051
- 11. Safety Evaluation by the Office of Nuclear Reactor Regulation related to Amendment
No. 215 to Facility Operating License No. DPR-16, Amergen Energy Company, LLC, Oyster Creek Nuclear Generating Station, Docket No. 50-219, September 2000, ADAMS Accession Number ML003730135
- 12. Safety Evaluation by the Office of Nuclear Reactor Regulation related to Amendment No. 87 to Facility Operating License No. NPF-87 and Amendment No. 87 to Facility Operating License No. NPF-89, TXU Electric, Comanche Peak Steam Electric Station, Docket Nos. 50-445 and 50-446, October 2001, ADAMS Accession Number ML012560143
- 13. Nuclear Criticality Safety in Operations with Fissionable Material Outside Reactors, American Nuclear Society, September 9, 1998.
- 14. Validation of Neutron Transport Methods for Nuclear Criticality Safety Calculations, American Nuclear Society, March 16, 2007.
- 15. Guide for Validation of Nuclear Criticality Safety Calculational Methodology, Science Applications International Corporation, January 2001, ADAMS Accession Number ML050250061
- 16. Letter from Exelon Nuclear to NRC dated June 4, 2009, ADAMS Accession Number ML091550832
- 17. Palo Verde Nuclear Generating Station (PVNGS) Units 1, 2 and 3 Docket Nos. STN 50528/
529/530 Request for Amendment to Technical Specification 3.7.15, Fuel Storage Pool Boron Concentration; 3.7.17, Spent Fuel Assembly Storage; and 4.3.1, Criticality, June 1999, ADAMS Accession Number ML9906150275
- 18. Palo Verde Nuclear Generating Station, Units 1, 2 and 3 - Issuance of Amendments on Fuel Storage Pool Capacity (TAC Nos. MA5685, MA5686, and MA5687), March 2000, ADAMS Accession Number ML003689038