Overview of Implementation of Probabilistic-Risk-Assessment-Based Seismic Margin Analyses in Design Certification and Combined License Applications

The licensing process for new reactors in the United States was established in accordance with Title 10 of the Code of Federal Regulations (10 CFR) Part 52, “Licenses, Certifications, and Approvals for Nuclear Power Plants,” which provides requirements for early site permit (ESP), standard design certification (DC), and combined license (COL) applications. In this process, an application for a COL may incorporate by reference a DC, an ESP, both, or neither. This approach allows for the early resolution of safety and environmental issues. The safety issues resolved by the DC and ESP processes are not reconsidered during a COL review. However, a COL application that incorporates a DC by reference must demonstrate that pertinent site-specific characteristics are confined within the envelopes established by the DC’s site parameters. This paper provides an overview of the implementation of probabilistic risk assessment (PRA) based seismic margin analyses in DC and COL applications. In addressing the severe accident preventions and mitigations for new reactors, 10 CFR 52.47(a)(27) requires that the final safety analysis report for a DC application describe the design-specific PRA and its results. Regulatory Guide 1.206, “Combined License Applications for Nuclear Power Plants (LWR Edition),” issued June 2007, further states that the scope of this assessment should be a Level 1 and Level 2 PRA that includes internal and external hazards and addresses all plant operating modes. However, the staff recognized that it is not practical for a DC applicant to perform a seismic PRA because a DC application would not contain site-specific seismic hazard information. As an alternative approach to a seismic PRA, the staff proposed a PRA-based seismic margin analysis in SECY-93-087, “Policy, Technical, and Licensing Issues Pertaining to Evolutionary and Advanced Light-Water Reactor (ALWR) Designs,” dated April 2, 1993, and the Commission approved it in the corresponding staff requirements memorandum, dated July 21, 1993. This analysis preserves key elements of a seismic PRA to the maximum extent possible and estimates the design-specific plant seismic capacity in terms of sequence-level high confidence of low probability of failure capacities and fragility for all sequences leading to core damage or containment failures up to approximately 1.67 times the ground motion acceleration of the design-basis safe-shutdown earthquake. Using this approach, the analysis can demonstrate acceptably low seismic risk for a DC. This paper discusses the implementation aspects of PRA-based seismic margin analyses in support of a DC application and post-DC updating activities, including COL updates to incorporate site- and plant-specific features and post-COL verifications.