ML24173A193
| ML24173A193 | |
| Person / Time | |
|---|---|
| Issue date: | 06/27/2024 |
| From: | Weerakkod S NRC/NRR/DRA |
| To: | |
| Rezai, A., NRR/DNRL, 301-415-1328 | |
| Shared Package | |
| ML24159A553 | List: |
| References | |
| Download: ML24173A193 (23) | |
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Risk-Informed Decision Making (RIDM) Relating to Risk-Informing Material Assessment (RIMA) Issues:
Perspectives from a PRA Practitioner Sunil D. Weerakkody Senior Level Advisor, PRA Division of Risk Assessment Office of Nuclear Reactor Regulation June 27, 2024 1
Views expressed constitute presenter's professional views as opposed to NRCs formal staff positions to enable a constructive open dialogue.
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Outline A. Identify key PRA related policies that provide guidance to NRC staff about how to use RIDM in its regulatory functions.
B. Discuss strengths and limitations of PRA models relating to incorporating risks associated with passive components.
C. Discuss licensees responsibility to maintain PRA models to reflect operating experience.
D. Identify challenges in risk-informing RIMA issues and how they may be addressed.
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A. PRA Policies There are two PRA related Commission policies that provide high-level guidance to NRC staff on how to use risk-informed decision making in risk-informed material assessments.
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PRA Policies, Contd.
Excerpts from the PRA Policy Statement (60 FR 42622) (1995)
- The use of PRA technology should be increased in all regulatory matters to the extent supported by the state-of-the-art in PRA methods and data and in a manner that complements the NRCs deterministic approach and supports the NRCs traditional defense-in-depth philosophy.
- PRA and associated analyses (e.g., sensitivity studies, uncertainty analyses, and importance measures) should be used in regulatory matters, where practical within the bounds of the state-of-the-art, to reduce unnecessary conservatism associated with current regulatory requirements, regulatory guides, license commitments, and staff practices. It is, of course, understood that the intent of this policy is that existing rules and regulations shall be complied with unless these rules and regulations are revised.
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PRA Policies, Contd.
- Risk-Informed Decision-Making - SRM SECY 98-0144: White Paper on Risk-Informed and Performance-Based Regulation
- A risk-informed approach represents a philosophy whereby risk insights are considered together with other factors to establish requirements that better focus attention on issues commensurate with their importance to public health and safety.
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PRA Policies, Contd.
- Risk-Informed Approach (Based on SRM-SECY-98-0144)
- Risk-informed decision-making includes key principles/factors addressed in RG 1.174:
oQuantitative or qualitative risk inputs/insights oSafety margins oDefense-in-depth oFeedback mechanisms 7
PRA Policies, Contd.
- Risk-Informed Approach (Based on SRM-SECY-98-0144)
- Risk-informed approach could include factors, such as:
- Engineering judgement
- Deterministic insights
- Exposure to radiation workers 8
B. Strengths and Limitations on PRA Models
- Several decades of experience have demonstrated that strengths of PRA models far outweigh their weaknesses.
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Strengths and Limitations of PRA Models, Contd.
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Strengths of PRA Models
- PRA models have the ability to capture risk associated with material-related issues impacting passive components implicitly or explicitly.
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Strengths of PRA Models PRA models have the ability to capture risk associated with material-related issues impacting passive components implicitly or explicitly.
Integrate this information in a logical manner to obtain risk insights Use of plant-
- specific, industry-wide, and world-wide experience to develop probabilities Structured process to identify and model a spectrum of initiating events Structured process to identify failure modes, system interactions and use realistic calculations Structured process to identify and model human errors 12
Weaknesses of PRA Models Assumptions Approximations Model Uncertainty Completeness Uncertainty Parametric Uncertainty 13
C: Licensees Responsibility to Maintain PRA Models to Reflect Operating Experience.
When licensees are given additional operational flexibilities using insights or inputs from PRA models, they are also expected to maintain PRA models of acceptable quality.
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Licensees Responsibility to Maintaining PRA Models of Acceptable Quality All operating reactor licensees have adopted at least one risk-informed initiative that requires them to maintain PRA models of acceptable quality.
10 CFR 50.69 (e) Feedback and process adjustment(1) RISC-1, RISC-2, RISC-3 and RISC-4 SSCs. The licensee shall review changes to the plant, operational practices, applicable plant and industry operational experience, and, as appropriate, update the PRA and SSC categorization and treatment processes. The licensee shall perform this review in a timely manner but no longer than once every two refueling outages.
TSTF-505/RICT Example License Condition: The risk assessment approaches and methods shall be acceptable to the NRC. The plant PRA shall be based on the as-built, as-operated, and maintained plant; and reflect the operating experience at the plant, as specified in Regulatory Guide 1.200, Revision 2. Methods to assess the risk from extending the completion times must be PRA methods used to support this license amendment, or other methods approved by the NRC for generic use; and any change in the PRA methods to assess risk that are outside these approval boundaries require prior NRC approval.
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D. Stakeholders must collaborate to identify and overcome challenges.
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Challenges in Risk-Informing Materials Issues Alignment on the adequacy of passive component modeling in PRA models.
Alignment on the adequacy of performance monitoring necessary to ensure that evaluations, assumptions and approximations used in the PRA model remain valid.
Alignment on how to apply risk-informed factors, defense-in-depth and safety margins in RIMA.
Alignment on deterministic and any other factors considered in risk-informed decision making.
Recognizing the different interpretations of key terms among different technical communities (e.g.,
interpretation of the term risk-significance could be different among subject matter experts).
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Adequacy of PRA Models (Completeness of Uncertainty)
Concern: PRA models do not explicitly model many passive components due to their assumed low failure probability.
- Use surrogate to capture the impact (e.g., impact of degraded cable that is not explicitly modeled in the PRA can be captured by pumps or valves that are modeled in the PRA).
- Use bounding assessments (e.g., assume that the pipe has ruptured to capture direct and indirect impacts of that pipe rupture on using components modeled in the PRA).
- Modify the level of detail in the model to meet the need of the specific application.
- Others?
Means to mitigate concern:
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Adequacy of Performance Monitoring Concern: Adequacy of performance monitoring to ensure that evaluation, assumptions, and approximation used in the PRA model remain valid.
- Use ALL means available (NRC inspections, NRCs access to licensees inspections, operating experience collected by industry entities ) to monitor performance.
- Explore the need/means to update PRA models to reflect operating experience during the required periodic PRA updates for passive components.
Means to mitigate concern:
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Alignment on Interpretation of DID and SM in RIDM for RIMA Concern: Align on how to apply risk-informed factors, defense-in-depth and safety margins in RIMA.
- Align on risk-informed and deterministic factors applicable to RIMA.
- Align on detailed documents that enable implementation of the RIDM key principles (e.g., Code compliance for Safety Margins, EPRI TR 3002014783, LIC-504).
- Collaborate to develop detailed guidance within the overarching principles.
Means to mitigate challenge:
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Alignment on Other Factors that Must be Considered in Risk-Informed Decisionmaking Should factors such as exposure to radiation workers be considered in decision making?
Compliance with regulatory requirements Defense-in-depth Safety Margins Performance measurement strategies/performance monitoring plans Exposure to radiation workers What are the factors that should be considered in decision making?
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Recognizing the Different Interpretation of Key Terms Within NRC, material engineers and risk analysts are collaborating to address the emerging challenges (e.g., LIC-206).
Increase collaboration among risk analysts and material engineers in the regulated community.
Increase collaboration among technical staff in the regulating and regulated communities.
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Concluding Remarks PRA models have unique capabilities to provide risk insights to support risk-informed decision making.
PRA model configuration control to reflect the as-built as-operated plant and establishment of effective feedback mechanisms to incorporate operating experience is critical to maintain the fidelity of PRA models.
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