Overview of Revised Level 2 PRA Standard in Japan

In this paper, we introduce the overview of the standard for Procedure of Level 2 Probabilistic Risk Assessment (PRA) for nuclear power plants established and issued by the Atomic Energy Society of Japan (AESJ). The first edition of the standard was published in 2008 through the discussions at the Level 2 Subcommittee under the Risk Technical Committee of the Standards Committee. As an enforcement standard based on the PRA procedure, the standard specifies the requirements which should have the PRA dealing with incidents resulting from internal events at nuclear power plants during power operation, and the concrete methods of meeting it. This new version standard is regular revision. In revising the 2008 version standard, we updated various requirements to reflect advancements in Level 2 PRA techniques based on new technological findings after the publication of the previous standard and to improve the quality and transparency of this standard. In particular, the lessons learned and new findings from the severe accidents of Fukushima Dai-ichi nuclear power plants, which occurred on March 11 of 2011, were significant. The reason was that three cores were melted down and large amounts of FP were released in the accidents. We investigated the latest documents relevant to severe accident research, and the measure against a severe accident established after the severe accidents of Fukushima Dai-ichi nuclear power plants. Furthermore, we extracted the matter, which should be reflected by comparison with international standard for PRA, ASME/ANS standard and IAEA SSG-4. Here, we introduce the outline and the feature of the AESJ standard for level 2 PRA. We also introduce the future renewal plan of the standard including the extension of the scope for external event, such as an earthquake and tsunami.