Development of Approach to Establishment of Risk Informed Accident Management Class in Severe Accident(#21)

Transcription

1 PSAM14 Sep. 17, 2018 Development of Approach to Establishment of Risk Informed Accident Management Class in Severe Accident(#21) Shinya Kamata 1 Nobuyuki Ueda Central Research Institute of Electric Power Industry 1

2 Severe Accident Management Subcommittee! The Great East Japan Earthquake of a magnitude 9.0 occurred on March 11, 2011.! The SAM Subcommittee was set up under the System Safety Technical Committee (STC) for the Standard Committee (SC)of Atomic Energy Society of Japan (AESJ) in order to establish the Severe Accident Management(SAM)standard.! The SAM standard provides technical requirements and satisfactory methods concerning maintenance and update of severe accidents management strategies for existing nuclear plants.! The SAM standard was established on March 2014.! The SAM Subcommittee resumed to revise the standard on Sep ! The first revision of the standard was established on June 2018 after about three years of discussion and reviews in AESJ. 2

3 Target of Revision for SAM standard(rev.1) " Accident management and management classification should be established based on plant vulnerability and latent risk including external events unique to each NPP. " Optimized accident management and management classification can be determined based on integrated risk informed decision making process using key element such as risk reduction, safety margin and Defense-in-Depth (DiD). " Most recent findings including revised IAEA NS-G-2.15, regulatory progressions in the U.S. NRC and other overseas regulators should be taken into the main text and appendices. 3

4 Events as Object of Accident Management (Concept of Constant Risk Approach) (Beyond Design Basis Accident) Frequency Design Basis Accident Internal, External events and combined events including DEC (LT- SBO, ATWS etc.,) Significant loss of safety function (Aircraft Impact, Extensive damage, etc.,) frequency, high impact events (Meteorite fall etc.,) Consequence Category of Events and Extent of Accident Management 4

5 AM Optimization and Establishment Block-Chart (1/2) Assume internal, external or combined events Assume significant loss of safety function Preliminary qualitative screening ) Utilize PRA if internal events Identify significant sequences by deterministic evaluation or engineering judgments, etc. Bounding or conservative analysis No Significant events to be assumed PRA? Yes Identify significant sequence by PRA ) Identify events leading to the significant damage of safety facilities and equipment such as aircraft impact Significant sequence group Identify plant response capability Extract individual plant-specific vulnerabilities 5

6 Impact Propagation Extent by Initiating Events Plant A Internal events (including multiplied events) Significant loss of safety function External events (including multiplied events) Plant B (Emergency operation facilities, adjacent plant, storage warehouse) Supply and support equipment or fuel from off-site Supply and accommodate equipments Supply and support equipment or fuel from off-site 6

7 AM Optimization and Establishment Block-Chart (2/2) Identify plant response capability Extract individual plant-specific vulnerabilities Assume AM candidate for significant sequence Verify balance*of AM in Defense-in-Depth Assess risk reduction* using AM candidates balance* Assignment of AM in SA progression phases risk reduction* PRA, NEI Verify feasibility and safety margin of AM Accident management, Management classification 7

8 Classification of Risk Significance using CDF, CFF Percentile of total CDF(%) Perform risk assessment using IPE or an alternative method for relevant plant and identify vulnerability in the plant. Classify the sequence groups into three graded risk significance based on the criteria of NEI 91-04(Rev.1) Middle Core Damage Frequency(/ry) CDF 10-6 /ry or CDF 10-7 /ry and CDF Total CDF 20% Significant accident sequence group Percentile of total CFF(%) Middle Containment Failure Frequency(/ry) 8

9 Establishment of AM Candidates in SA Progression Phases SA Progression Phases in 4 th layer AM Candidates Alternative high pressure injection Prevent Core Damage Middle Terminate SA progress. RCIC black start Middle Effective Alternative high voltage AC power sources Enhancement of battery capacity voltage AC power from neighboring plant voltage AC power from neighboring plant (Example using PRA results of Internal and Seismic event for BWR) Middle Effective Maintain CV Integrity Minimize FP Release Achieve long term stable st. Mobile power sources Alternative low pressure injection Effective Middle Alternative mobile injection Effective Middle PCV venting Middle Effective 9

10 Example of the Management Classification flow(1/2) AM Candidate Middle AM Candidate Defense-in- Depth Middle Significance of the Accident Sequence Result of Risk analysis AM Candidate AM Candidate Defense-in- Depth Assume the AM candidates as measure according to each accident sequence group. With regard to the AM candidates, verify from the point of view of securing defense-in-depth whether they are appropriately assigned to each level, or not. 10

11 Example of the Management Classification flow(2/2) Manage. Class 1 Manage. Class 2 Manage. Class 3 Engineering Judgement Engineering Judgement Engineering Judgement Safety Margin Safety Margin Middle Risk Reduction Risk Reduction AM Candidate Risk Reduction Middle AM Candidate AM Candidate Assess the risk reduction in the case of adopting the AM candidate using existing PRA results and NEI reliability method (NEI 16-06). Analyze feasibility and safety margin in the AM candidate using SA analysis code. Determine feasible AM and management classification using integrated risk informed decision making process. 11

12 Applicability of Semi-Quantitative Approach(NEI 16-06) Q FLEX = Q HE *TM*CC*EF+Q EA, ΔCDF=Σ i {CDF base i -CDF base i *Q FLEX i } Q FLEX : Unreliability for AM (FLEX) Q HE : Failure probability for AM TM : Time margin for deployment (transport, connect, initiate) CC : Command and Control (Direction, Staffing for strategy) EF : Environmental factors (temperature, radiation) accessibility, operability Q EA : Unavailability of equipment 12

13 Example of Safety Classification for AM(BWR) Risk reduction effect using NEI reliability method(%) Contribution to the total CDF( SBO + Seismic event ) 10%~100% Middle 0.1%~10% ~0.1 % ΔCDF(/reactor year) Total Short term Middle term 13

14 Results of Trial Assessment for BWR " The largest ΔCDF values are provided by mobile power sources, alternative high voltage AC power sources and alternative mobile injection which are effective against long term SBO due to the earthquake. (Management Class 1) " The second largest ΔCDF values are provided by alternative high pressure injection, RCIC black start and alternative DC power sources which are effective against short term SBO. (Management Class 2) " Since power sources from neighboring plant using tie line is effective against only SBO caused by internal event, CDF reduction is limited. (Management Class 3) 14

15 Conclusion " The trial assessment using the SAM standard shows that accident management and management classification can be established based on plant vulnerability and latent risk including external events unique to each NPP, and that the optimized program in accident management can be developed. " It is significant to assign resources to optimized management based on classification for system modification, addition, maintenance, procedures and training as accident management. " In near future, it is recommended that the pilot studies will be performed in the existing plant, the feasibility and optimization of the accident management will be verified with checking the applicability. 15

16 Thank you for your kind attention! 16