ML21049A310
| ML21049A310 | |
| Person / Time | |
|---|---|
| Site: | Limerick  |
| Issue date: | 02/19/2021 |
| From: | Exelon Generation Co, Exelon Nuclear |
| To: | Dukehart C Plant Licensing Branch 1 |
| Cory Dukehart-NRR/DORL 301-415-3545 | |
| References | |
| Download: ML21049A310 (12) | |
Text
Limerick 10 CFR 50.69 Alternative Categorization LAR Pre-submittal Meeting February 23, 2021
Agenda and Objective
- Introductions
- Objective: Discuss Limerick license amendment request (LAR) as a pilot for the 10 CFR 50.69 alternative categorization process
- Background - Model Adequacy
- Proposed Alternate 10 CFR 50.69 Categorization
- Limerick Pilot LAR - References and License Condition
- Summary and Conclusions
- Open Discussion Presentation Title 1
Background - Model Adequacy
- Limerick PRA models used for 10 CFR 50.69 meet ASME/ANS RA-Sa-2009 PRA standard high-level requirements for initiating events, accident sequences, system analysis, and success criteria
- Limerick PRA model adequacy has been previously demonstrated for 50.69 and TSTF-505 (RICT) applications
- Previous NRC approval of 10 CFR 50.69 for Limerick via Amendment Nos. 230 and 193, dated July 31, 2018
- Common Cause modeling sufficiency is demonstrated by addressing Peer Review findings related to Accident Sequence Analysis and Success Criteria Initiating Event Frequencies and Truncation Common Cause Groupings Presentation Title 2
Defense-in-Depth: Current Guidance in NEI 00-04
- Core Damage Defense-in-Depth
-Utilizes PRA success criteria and initiating event frequencies
-Multiple (identical) redundant trains cannot be credited
-Does not acknowledge defense-in-depth within same system
- Containment Defense-in-Depth
-Uses assumptions and documentation from PRA model without using the quantitative results
-Most considerations could be evaluated using LERF insights Presentation Title 3
Proposed Approach for Core Damage Defense-in-Depth
- Replaces NEI 00-04, Section 6.1
- Uses FPIE PRA Model to identify only those cutsets with an initiating event and a single basic event
- Based on success criteria in the PRA model
- Single point vulnerabilities captured
- Cutsets with initiating events < 1E-04/yr excluded
- Associated SSCs retained as high safety significant
- Pressure boundary failure events that lead to internal flooding screened out (addressed in the pressure boundary analysis)
- Functions/SSCs outside scope of the PRA are not evaluated
- Does not replace IDP defense in depth evaluation
- Meets Rule and RG 1.174 Risk-Informed Principles Presentation Title 4
Proposed Approach for Containment Defense-in-Depth
- Replaces NEI 00-04, Section 6.2
-Process analogous to that used for core damage
-Based on success criteria in the Level 2 FPIE PRA model
-Captures single point vulnerabilities
-Associated SSCs retained as high safety significant
- Retains NEI 00-04, Section 6.2, Long-Term Containment Integrity consideration (not evaluated in the PRA)
Does SSC support a system function not considered in CDF and LERF, but would be only means for preserving long-term containment integrity post-core damage (e.g., containment heat removal)?
- Does not replace IDP defense in depth evaluation
- Meets Rule and RG 1.174 Risk-Informed Principles Presentation Title 5
Pressure Boundary Categorization
- Current pressure boundary (passive) categorization requires a significant amount of effort to complete the analysis
- Propose using new approach in EPRI Report 3002015999, Enhanced Risk-Informed Categorization Methodology for Pressure Boundary Components
- Discussed with the NRC in December 2019 Public Meeting
- Improved guidance addresses gaps and inconsistencies
- LAR will demonstrate three prerequisites for Limerick:
Robust internal events model, including internal flooding Robust programs for localized corrosion, FAC and erosion Protective measures for internal flooding events Presentation Title 6
Seismic Tier 1 Approach
- Limerick currently uses an IPEEE Seismic Margins Assessment (SMA) equipment list where all SSCs on list are HSS
- Propose use of EPRI 3002017583, Alternative Approaches for Addressing Seismic Risk in 10 CFR 50.69 Risk-Informed Categorization, February 2020.
-Employs a systematic process to evaluate the seismic hazard which is integrated into the categorization process
-Considers likelihood and magnitude of the seismic hazard
-Considers margin to the site-specific seismic design basis
- Reference NRC approval of Seismic Tier 1 alternative for Calvert Cliffs (Amendment Nos. 332 and 310), February 28, 2020 Presentation Title 7
Limerick Pilot LAR - References
- Limerick 50.69 Alternative Categorization LAR will reference following Topical Reports (available for NRC audit)
- PWROG-20015-NP, Alternate 10 CFR 50.69 Categorization Process
- EPRI 3002015999, Enhanced Risk-Informed Categorization Methodology for Pressure Boundary Components
- EPRI 3002017583, Alternative Approaches for Addressing Seismic Risk in 10 CFR 50.69 Risk-Informed Categorization Presentation Title 8
Limerick Pilot LAR - Proposed License Condition Exelon is approved to implement 10 CFR 50.69 using the processes for categorization of Risk-Informed Safety Class (RISC)-1, RISC-2, RISC-3, and RISC-4 structures, systems, and components (SSCs) using: Probabilistic Risk Assessment (PRA) models to evaluate risk associated with internal events, including internal flooding, and internal fire; the shutdown safety assessment process to assess shutdown risk; the Arkansas Nuclear One, Unit 2 (ANO-2) passive categorization method to assess passive component risk for Class 2 and Class 3 SSCs and their associated supports; and the results of non-PRA evaluations that are based on the IPEEE Screening Assessment for External Hazards, i.e., seismic margin analysis (SMA) to evaluate seismic risk, and a screening of other external hazards updated using the external hazard screening significance process identified in ASME/ANS PRA Standard RA-Sa-2009; as specified in Unit [1] License Amendment No. [230]
dated July 31, 2018.
In addition, Exelon is approved to implement 10 CFR 50.69 using any of the following alternative processes for categorization of RISC-1, RISC-2, RISC-3, and RISC-4 SSCs: the defense-in-depth approach contained in Section 2.1 of PWROG-20015-NP; the passive pressure boundary categorization approach described in EPRI Topical Report 3002015999; and the seismic approach as described in Exelon's submittal letter dated March XX, 2021, as specified in Unit [1]
License Amendment No. [XXX] dated [DATE].
Prior NRC approval, under 10 CFR 50.90, is required for a change to the categorization process specified above (e.g., change from a seismic margins approach to a seismic probabilistic risk assessment approach).
Presentation Title 9
Summary and Conclusions
- Considerable experience with implementation of 50.69 at LGS
- The alternate defense-in-depth approach takes advantage of the improved PRA modeling in place and use of model logic:
Improves consistency and removes subjectivity Assures that key safety functions are still maintained by redundant SSCs
- Pilot results using the proposed alternative approaches
- Demonstrate that existing methods are conservative
- Represent process improvements
- SSCs previously categorized wont necessarily be reevaluated
Open Discussion Presentation Title 11 Proposed Alternate 10 CFR 50.69 Categorization