A standard for Procedure of Seismic Probabilistic Risk Assessment (PRA) for Nuclear Power Plants: 2015 has been established and issued by the Atomic Energy Society of Japan (AESJ) through the discussions at the Seismic PRA Subcommittee under the Risk Assessment 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 earthquakes at nuclear power plants during power operation, and the concrete methods of meeting it.
This 2015 version standard is regular revision. In revising the 2007 version standard, we updated various requirements to reflect advancements in Seismic 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.
The Standards Committee and the Risk Technical Committee reopened the Seismic PRA Subcommittee and three Working Groups (WG); Seismic Hazard Evaluation WG, Building and Component Fragility Evaluation WG, Accident Sequence Evaluation WG, and resumed the discussion of the Seismic PRA Standard. In the first place the Subcommittee found essential topics for revision of the Seismic PRA Standard and classified them to short term ones, which were able to be provided as implementation requirements at an early date, and medium- and long-term ones, which were considered to require medium to long-term research initiatives. The amendment reflected solutions to short-term topics as requirements. This standard showed information about research and technical development for medium- and long-term topics as referential attachments, too. These attachments are useful for information sharing with users. The revised standard has several remarkable points. First of all, this standard covers all the areas on seismic PRA and includes not only seismic events but also seismic induced events, e.g. seismic induced fire, seismic induced internal flooding, and seismic event combined with tsunami. To be specific, seismic hazard evaluation method comes from this standard, accident sequence evaluation method is based on each PRA standard, and fragility evaluation method of SSCs (structures, systems and components) for seismic induced event PRA is based on the fragility chapter of this standard. For example, fragility evaluation method of equipment for tsunami ( a seawall, a watertight door and others) is stipulated in this standard in case of seismic event PRA combined with tsunami.
Seismic PRA or seismic induced event PRA can provide risk information to improve the safety level of nuclear power plants by using this revised Seismic PRA Standard, which includes the important and useful points stated above.