Spent Fuel Safety Studies and Analyses

Transcription

1 Spent Fuel Safety Studies and Analyses Steven Jones Division of Safety Systems Office of Nuclear Reactor Regulation National Academy of Sciences Meeting January 29, 2015

2 Agenda History of SFP Analyses Recent Spent Fuel Pool Study (NUREG- 2161) Recent Expedited Transfer Analysis (COMSECY ) Risks associated with expedited transfer Conclusion 2

3 History of Regulatory Activities Transition to High- Density SFP Racking (starting in late 70s) Action Plan Activities to Increase SFP Cooling Reliability (mid-90s) NUREG-2161 Spent Fuel Pool Study (2013) National Academy of Sciences Study (2004) Comprehensive Site Level 3 PRA Study (~2018) Early SFP Consequence Studies (e.g., NUREG/CR- 0649) and High-Density Racking Review Criteria Development (late 70s) NUREG-1353 Resolution of Generic Issue 82, Beyond Design Basis Accidents in Spent Fuel Pools (late-80s) NUREG-1738 Study for Decommissioning ( ) Post-9/11 Security Activities ( ) Post-Fukushima Activities ( ) COMSECY Expedited Transfer Analysis (2013) 3

4 Past SFP Studies Annual frequency of SFP fuel uncovery as reported in previous SFP risk studies NUREG-1353* Seismic Cask / Heavy Load Drop NUREG-1738** Seismic Cask / Heavy Load Drop Other Other * Total frequency of fuel uncovery 2.0 E-06/yr, based on PWR best estimate results. Results for BWRs higher. ** Total frequency of fuel uncovery 2.3 E-06/yr based on Livermore seismic hazard curves Very strong seismic events found to be the dominant contributor to fuel uncovery; low frequency did not support regulatory action Conservative evaluation of accident progression and consequences Information used in expedited fuel transfer analysis to supplement results described in NUREG

5 Overview of SFPS (NUREG-2161) Uses state-of-the-art tools to evaluate accident progression after a large earthquake BWR4 Mark I was used as reference plant Two conditions considered: Representative of the current situation for the reference plant (i.e., high-density loading and a relatively full SFP) Representative of expedited movement of older fuel to a dry cask storage facility (i.e., low-density loading) Study updates publicly available consequence estimates for spent fuel pool accidents 5

6 NUREG-2161 Results 6

7 Overview (COMSECY ) (NUREG-2161) (NUREG-2161) 7

8 Regulatory Analysis Process Safety Goal Screening Evaluation Based on the Commission Safety Goal Policy Statement Used the Quantitative Health Objectives (QHOs) to evaluate achievement of the safety goals Cost/Benefit Analysis Intended to identify maximum potential benefit Analyzes costs and benefits for representative pool design groups Sensitivity Studies Evaluates key factors to illustrate their effect on the final result 8

9 Expedited Transfer Dry Cask Loading Estimates Source: Technical Report , Impacts Associated with Transfer of Spent Nuclear Fuel from Spent Fuel Storage Pools to Dry Storage After Five Years of Cooling, EPRI, November

10 Assumptions to Maximize Benefit Release fraction and mitigation effectiveness assumptions provide conservative estimate of potential benefit Regulatory Baseline Maintain the Existing Spent Fuel Storage Requirements High cesium release fractions (SFPS value of ~40% for elevated pools and past study value of 75% for other groups in base case) Ineffective mitigation (most accidents result in release) Expedited Transfer Alternative - Low-density Spent Fuel Pool Storage Low cesium release fractions (SFPS value of 3% for all groups in base case) Effective mitigation (most accidents result in no release) Sensitivity to evaluate effect of successful mitigation 10

11 Risks Associated with Expedited Transfer Risk of spent fuel storage: Dominated by high decay heat assemblies (SFP) Cask loading affects frequency of cask drop events Density of fuel storage affects event consequences Actions to improve safety: Mitigation strategies/equipment Dispersed fuel storage Risks depend on site-specific features: Seismicity of site SFP design and operation Reliability of mitigating features 11

12 Safety Perspectives Pools provide adequate protection and defense-in-depth Overall estimated frequency of damage to stored fuel is low Base case release frequencies for existing pools are on the order of a few times in a million years These frequencies exclude effective deployment of mitigation capability and generally exclude consideration of air cooling No early fatalities and 1.5x10-8 latent cancer fatality risk Spent Fuel Pool Maintains Defense-in-Depth Defense-in-depth consists of layers of protection with reliability of each layer commensurate with the frequency of challenges SFP designed to prevent coolant inventory loss under accident conditions, which results in a low frequency of coolant inventory loss Safety improvements have been made such as dispersal of spent fuel, coolant makeup, and spray capabilities 12

13 COMSECY (2013) Staff Evaluation and Recommendation for Japan Lessons-Learned Tier 3 Issue on Expedited Transfer of Spent Fuel Concluded that expedited transfer would provide only a minor or limited safety benefit, and that its expected implementation costs would not be warranted. 13

14 SECY (2014) Response to Commission Direction on Spent Fuel Pool Limited Term Operational Vulnerabilities Refers to the allowable period of time for licensee s to achieve a dispersed (e.g. 1 x 4) spent fuel configuration in the SFP following discharge from the reactor core. Concludes that current requirements for SFP mitigation measures are sufficient to ensure adequate protection of public health and safety, as well as common defense and security. 14

15 Conclusion Health risks from SFP accidents are less than 1% of the Quantitative Health Objectives Regulatory analysis of expedited transfer of spent fuel to dry cask storage showed that the costs outweigh the expected benefits Commission has concluded that no further regulatory action is needed on this issue 15

16 Backup Slides: Expedited Transfer of Spent Fuel

17 Generic Regulatory Analysis Spent Fuel Pool Study (Appendix D) and Tier 3 Regulatory Analysis consider initiating events beyond the event in SFPS: more severe earthquake cask drop loss of power/loss of coolant inventory events Tier 3 Regulatory Analysis covers all SFP designs used with operating reactors in the U.S. PWRs and BWRs with Mark III containments (spent fuel stored in at-grade pool separate from reactor building) new reactors (AP-1000) Assessment of security events handled separately regulatory changes implemented (e.g., 10 CFR 50.54(hh)) effect of security changes reflected in regulatory baseline 1

18 Safety Goal Screening Results Limited safety benefit based on comparison with QHOs No risk of fatalities due to nature of release Potential benefit is a very small fraction (0.76%) of latent cancer goal Cancer risk relatively insensitive to magnitude of release due to slow accident progression and effective protective actions (SFPS) Sufficient margin to QHOs that regulatory analysis guidelines call for no cost-benefit analysis 2

19 Attributes Considered in a Cost-Benefit Analysis Principal Attributes Public Health (Routine) Public Health (Accident) Occupational Health (Routine) Industry Operation Occupational Health (Accident) Offsite Property Industry Implementation Onsite Property NRC Implementation 3

20 Cost-Benefit Analysis Evaluated one alternative - Expedited Transfer Transfer fuel with more than 5 years decay to dry casks Store remaining fuel in low-density configuration in existing racks Established SFP Groups Four groups evaluated representing operating and new plants Groups representing shutdown/decommissioning reactors not evaluated due to low risk Major Assumptions (Regulatory Analysis Table 2) Initiating SFP Event Frequencies and Accident Progression Economic modeling (e.g., definition of representative plants, future spent fuel discharge projections, etc.) Timing (e.g., dry cask storage loading, occupational dose, etc.) Established a base case Performed sensitivity studies 4

21 Base Case Analysis Staff considers base case appropriate for decision whether to pursue additional research to refine assumptions Base case includes appropriately conservative assumptions, but not bounding values, for the following: Initiating Events (USGS 2008 information for Peach Bottom seismic hazard, and NUREG-1738 and NUREG-1353 for other initiators) Seismic liner fragilities (based on results of SFPS and past studies) Cesium inventories for each group (based on SFP capacity, reactor power, and fuel burnup for reactors in group) Plume dispersion (uses MAACS2 and Peach Bottom Meteorology) Population density and economic activity (used data for Surry) Industry implementation costs (EPRI information modified for representative site) 5

22 Base Case Analysis (Con t) Uncertainty regarding spent fuel pool conditions (i.e., pool water level, fuel distribution, and location of liner tears) Conservative assumption to bound effects Generally make bounding assumption of inadequate heat removal if fuel is uncovered for base case Results in dominant initiating events progressing to release without mitigation Conservative because SFPS and other studies indicate substantial potential for air cooling when pool is drained or decay heat is low Exception for Mark I and II BWRs SFPS reduces uncertainty for specific scenario evaluated Used SFPS information of 8% inadequate cooling for SFPS quake scenario 6

23 Cost-Benefit Analysis Results Base case costs outweigh benefits Benefits based on $2000/person-rem within 50 miles Changes in discount rate do not change result Sensitivity Analyses ($4000/person-rem and analysis beyond 50 miles) produce marginal benefits Base case costs outweigh benefits for Groups 1 & 2 Base case benefits marginally outweigh costs for Groups 3 & 4 The staff considers the base case an appropriately conservative analysis for use as the primary basis for the staff s recommendation. 7

24 Effect of Burnup on Decay Heat Source: Technical Report , Impacts Associated with Transfer of Spent Nuclear Fuel from Spent Fuel Storage Pools to Dry Storage After Five Years of Cooling, Rev. 1 EPRI,