NUCLEAR ENERGY AGENCY COMMITTEE ON THE SAFETY OF NUCLEAR INSTALLATIONS Specialist Meeting on the Seismic Probabilistic Safety Assessment of Nuclear Facilities

In 2003, the American Nuclear Society (ANS) issued a standard entitled, “External Events PRA Methodology” (ANSI/ANS-58-21-2003). This ANS standard is one in a series of standards relevant to the probabilistic risk analysis methods which either have been published or are under preparation. The primary objective of these standards is to facilitate the use of risk-informed approaches. The NRC supports this facilitation process by publication of regulatory guides which outline the criteria for PRA technical acceptability. While the ANS standard covers both natural external events, such as, earthquake, high winds, and external floods, and humanity-generated external events, such as, airplane crashes, explosions from nearby industrial sources, and impact of nearby transportation activities, the focus in this paper is on seismic events. Generally, the requirements for PRA analysis of accident sequences initiated by a seismic event that might occur while a nuclear power plant is at full power are described. The requirements are for both a Level 1 and for a limited Level 2 PRA sufficient to evaluate the large early release frequency. Requirements are also provided for a seismic margins assessment (SMA). The requirements for both the EPRI and the NRC SMAs are described along with screening analysis criteria and criteria for demonstrably conservative or bounding analysis. In February, 2004, the NRC published Regulatory Guide1.200, for trial use, entitled, “An Approach for Determining the Technical Adequacy of Probabilistic Assessment Results for Risk-Informed Activities” (NRC, 2004a). Regulatory Guide 1.200 describes an acceptable approach for determining that the quality of a PRA is sufficient to provide confidence in the results such that the PRA can be used in regulatory decision making. The guidance is intended to be consistent with NRC’s PRA Policy Statement and additional detailed guidance in Regulatory Guide 1.174, “An Approach for Using Probabilistic RISK Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis”. It is also intended to reflect and endorse guidance provided by standard-setting and nuclear industry organizations. “Draft Guide (DG) 1138 (Appendix C to RG 1.200) (NRC, 2004b) was issued in August 2004; this DG provides the staff position on ANSI/ANS-58-21-2003.” Background This paper is intended to provide an update of the development and application of seismic probabilistic risk analysis in the United States since the topic was last addressed in Tokyo in August, 1999, OECD, (1999). At that time one of my co-authors addressed the initial lessons learned from the seismic portion of the Individual Plant Examination for External Events program (IPEEE). Since then the final conclusions from that program were published as NUREG-1742, Perspectives Gained from the Individual Plant examination of External Events (IPEEE) Program, USNRC (2002a). The overall perspectives gained included: 1. Corresponding Author, Senior Technical Advisor, U.S. Nuclear Regulatory Commission, Mail Stop T-10-D-20, Washington, DC USA, 301-415-6011, [email protected] 2. The NRC staff is in the early stages of developing guidance of the treatment of uncertainties and alternate methods in risk-informed decisionmaking.