PROPOSED METHODOLOGY, BASIS AND STANDARD FOR SHUTDOWN QUALITATIVE RISK ASSESSMENTS

摘要

Qualitative assessments have been applied to shutdown risk management of U.S. nuclear plants since 1991. For the purposes of managing nuclear safety risk during shutdown, guidance provided in NUMARC 91-06 applies the concept of defense-in-depth (DID) of key safety functions. Although the application of Qualitative Risk Assessments (QLRA) for shutdown conditions has been well established in the industry, reviews of both the modeling methods and decision criteria have indicated moderate to low consistency. Comparison of quantitative risk assessment and QLRA results have shown inconsistent correlation between reported risk levels. These observations suggest a standardized methodology for QLRA could be a valuable tool in reducing the disparity among DID assessments, such that they reflect risk more consistently. A methodology is proposed to provide guidance for development of standardized QLRA models based on key safety function defense-in-depth. The proposed methodology provides a structured approach to consider the various attributes of a shutdown QLRA that will consistently reflect risk and be effective in assessing and managing configuration risk during shutdown operations. Bases are provided to show that the methodology is capable of providing a consistent measure of risk. The methodology provides guidance on the important attributes of a standard QLRA, as applied to shutdown risk assessments. Additionally, an implementing Standard has been proposed to provide a means to evaluate shutdown QLRAs against objective criteria, in order to ensure the technical capability level of the QLRA is sufficient for the intended application.