The American Nuclear Society in cooperation with the American Society of Mechanical Engineers through a Joint Committee on Nuclear Risk management (JCNRM) is developing a standard titled “Low Power and Shutdown PRA Methodology, ANSI/ANS-58.22”. This LPSD Standard sets forth requirements for low power and shutdown probabilistic risk assessments (PRAs) and also requirements for shutdown qualitative risk assessment (QLRA) that can be used to support risk-informed decisions for commercial nuclear power plants. This Standard also prescribes a method for applying these requirements for specific applications.

The PRA requirements in this Standard are intended to be used together with other PRA standards that cover different aspects of PRA scope. Specifically, they are intended to be used directly with the PRA Standard developed by the American Society of Mechanical Engineers (ASME) and the American Nuclear Society (ANS) (“Standard for Level 1 / Large Early Release Frequency Probabilistic Risk Assessment for Nuclear Power Plant Applications,” ASME-RA-Sa-2009[1]). The ASME/ANS Combined Standard covers PRAs for both internal hazard events and external hazard events for a commercial nuclear power plant operating at full power.

The PRA scope covered by the Low Power and Shutdown (LPSD) PRA Methodology Standard is limited to analyzing accident sequences initiated by “internal hazard events” or “external hazard events”, excluding only internal fire hazards. Therefore, this Standard covers all potential accident initiators arising at low power and shutdown conditions, except for internal fires. The only other initiators explicitly excluded are accidents resulting from purposeful human-induced security threats (e.g., sabotage).

This Standard’s PRA technical requirements are presented in support of a quantitative PRA for annual average risk. For applications involving a specific outage, modifications to the technical requirements are noted.

The quantitative PRA requirements in this Standard are further restricted to requirements for: (a) a full Level 1 analysis of the core damage frequency (CDF), and (b) a limited Level 2 analysis sufficient to evaluate the large early release frequency (LERF). The scope is also limited to analyzing accident sequences involving fuel while it is in the reactor vessel. Events involving fuel while it is in the spent fuel pool are not covered.

All comments received on earlier drafts of the LPSD standard have been responded to and the standard adjusted accordingly. At present, these responses and revisions are being reviewed by those who offered comments. In April, 2012 the LPSD PRA Methodology Standard is being submitted to the JCNRM to determine its readiness for a formal ballot. A subcommittee of the JCNRM, consisting of persons not involved in the LPSD writing group, will perform the readiness review. The schedule for the next re-ballot will be determined in part by the outcome of this readiness review and is expected to take place later 2012.

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