ML26006A151

9. NEI - Realism and Risk Informed Applications - V.Anderson

ML26006A151
Person / Time
Site:	Nuclear Energy Institute
Issue date:	10/01/2025
From:	Anderson V Nuclear Energy Institute
To:	NRC/RES/DRA
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Download: ML26006A151 (12)

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©2026 Nuclear Energy Institute NRC Public Workshop on PRA Realism Victoria Anderson, NEI Realism and Risk Informed Regulatory Applications October 1, 2025

©2026 Nuclear Energy Institute 2 What are the basic premises of risk informed regulation?

What is the context of realism for various forms of risk informed regulation?

How can lack of realism impact risk informed regulatory decisions?

What role do SDPs and supporting documents play in the level of realism available for risk informed decisions?

How do the principles of risk informed decision making interact with realism?

Overview

©2026 Nuclear Energy Institute 3 Deterministic approaches Risk insights Better regulatory and operational decisions Basic Premises of Risk Informed Regulation

©2026 Nuclear Energy Institute 4 Basic Premises of Risk Informed Regulation Improves safety by highlighting vulnerabilities and hidden dependencies Improves efficiency by allowing flexibility (e.g., in maintenance, testing, inspection) where risk is very low

©2026 Nuclear Energy Institute 5 Context of Realism in Various Risk Informed Applications Context-specific realism depends on how PRA is used in decision-making Generally, best-estimate modeling that preserves the correct relative importance of risk contributors Realism may refer to a PRA model, or scenario specific risk

• Average risk

• Facility modifications

• Risk Informed Tech Spec Surveillance Frequencies

• Scenario specific risk

• Risk Informed Tech Spec Completion Times

• Significance Determination Process

©2026 Nuclear Energy Institute 6 Impact of Lack of Realism on Risk-Informed Decision Making: EDG Example Conservative Assumption

• Instead of using the best-estimate failure probability of the EDGs (e.g., based on actual plant data), the PRA uses a conservative bounding value

• This inflates the calculated contribution of EDG failures to plant risk Resulting PRA Output

• EDGs appear to dominate risk

• Risk-informed evaluation conclude that EDG allowed outage times cannot be extended to support maintenance flexibility Incorrect Decision

• Resources are over-concentrated on EDG surveillance and maintenance.

• Other contributors to actual plant risk are under-addressed because they appear less important in the PRA model

©2026 Nuclear Energy Institute 7 Impact of Lack of Realism on Risk-Informed Decision Making: Fire Growth Assumptions Conservative Assumption

• Any electrical cabinet fire grows to full-room involvement with no credit for suppression

• Realistic data show many fires are detected and suppressed within a few minutes.

Effect on PRA

• The calculated conditional core damage probability from cabinet fires is exaggerated

• Fire risk dominates the total plant risk Incorrect Decision

• Plant management invests heavily in cabinet fire barriers and redundant suppression systems.

• More meaningful risk reductions (like early detection upgrades) are overlooked.

©2026 Nuclear Energy Institute 8 PRA conservatism shifts the apparent risk profile, which misleads prioritization in risk-informed applications PRA conservatism shifts the apparent risk profile, which misleads prioritization in risk-informed applications Best-estimate values with uncertainty characterization give much more useful insights for decision-making Best-estimate values with uncertainty characterization give much more useful insights for decision-making Importance of Realism in Risk-Informed Decision Making

©2026 Nuclear Energy Institute 9 SDPs and guidance supporting their conduct (e.g. RASP Handbook) can lack realism in key areas Reliance on generic CCF values leads to over-prediction of CCF risk, especially in cases where actual evidence or design features might reduce CCF likelihood Limited opportunities for refining conservatisms when better data are available SPAR-H has known limitations Potentially overstates the risk of findings compared to what would be found with refined modeling Creates disconnect when utilities use realistic models for licensing applications and daily operational decisions Critical to move forward on improving realism in SDPs to support consistency in risk informed decision making Role of SDPs and supporting documents

©2026 Nuclear Energy Institute 10 Realism and the Principles of Risk Informed Decision Making PRA realism is central to all five principles Realism ensures that risk-informed regulation is credible, balanced, and effective

©2026 Nuclear Energy Institute 11 Realism and the Principles of Risk Informed Decision Making

• A realistic PRA demonstrates that proposed changes are consistent with the regulations

• If overly conservative: Could wrongly suggest non-compliance, blocking justified changes Compliance with Current Regulations

• PRA realism helps identify where layers truly provide protection

• If conservative: Might suggest defense-in-depth is eroded when it isnt.

Defense-in-Depth is Maintained

• Realistic modeling of uncertainties and plant performance ensures safety margins are neither understated nor overstated

• If conservative: May imply safety margins are too thin and block changes with net benefit to safety Safety Margins are Maintained

©2026 Nuclear Energy Institute 12 Realism and the Principles of Risk Informed Decision Making

• Realistic PRA is critical to correctly quantify CDF and LERF.

• If conservative: Inflates CDF/LERF, preventing risk-informed improvements.

Changes in Risk are Small and Consistent with Safety Goals

• A realistic PRA baseline makes performance monitoring meaningful deviations stand out

• If conservative: Creates false alarms performance looks worse than it really is Implementation is Monitored Using Performance Measurement Strategies