A Technical Overview of The Japan’s Standards for Risk-Informed Decision Making

In Japan, a lot of efforts have been made on severe accident study, and development and application of the probabilistic safety assessment (PSA) technique. The PSA was applied to the examination of the accident management (AM) plan in the beginning of 1990s and was performed for all the nuclear power plants (NPPs) to evaluate the effects of the AM. Furthermore, the PSA has been performed as part of periodic safety review (PSR) to review the safety of individual plant. In recent years, discussions have started to apply risk information to the safety regulation or safety related activities to improve rationality, accountability and transparency. As the technical foundations, the Nuclear Safety Commission (NSC) showed the safety goals and policy toward risk informed decision making (RIDM), and the Nuclear and Industrial Safety Agency (NISA) developed guidelines for risk informed regulation (RIR). Consensus standards have been developed in the Atomic Energy Society of Japan (AESJ), the Japan Society of Mechanical Engineers, and the Japan Electric Association. Especially, the AESJ established the Standards Committee in 1999 and has made several PSA standards. Now, there are seven PSA standards. The most important one is the seismic PSA standard. It was developed ahead of the world as a concrete manual. Lots of illustrations and useful example are included for ease of use and to make decision adequately. There were needs to develop a standard, which provides the basic requirements and specific procedures commonly applicable to respective fields of utilization of RIDM regarding changes in safety related activities. Responding to such needs, an implementation standard has been developed on use of risk information in changing the safety related activities. It stands over individual standards that will be developed in future, and shows the common and basic rules. It requires being consistent with the defense-in-depth philosophy, to maintain sufficient safety margins, and to clarify the influence to safety by comparing with some criteria. And it also requires as a final step that a comprehensive decision be made by considering various items, e.g. the defense-in-depth, safety margins, risk indices, and implementation and monitoring program. We will continue to make an effort toward RIDM and develop the standard to assess dominant risk hazards, e.g. fire risk and internal flooding risk. Moreover, it is necessary to develop the standard for individual applications in future.