ML21049A310

CFR 50.69 Alternative Categorization LAR

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 1 Presentation Title

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 2 Presentation Title

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 3 Presentation Title

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 4 Presentation Title

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 5 Presentation Title

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 6 Presentation Title

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 7 Presentation Title

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 8 Presentation Title

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).

9 Presentation Title

Summary and Conclusions

• LGS PRA model adequacy has been previously demonstrated

• 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

• Exelon will be submitting pilot LGS LAR in early March 2021 10 Presentation Title

Proposed Alternate 10 CFR 50.69 Categorization Open Discussion 11 Presentation Title