IMPROVEMENT OF EXTERNAL EVENT ( TSUNAMI SEISMIC) PSA APROACH FOR SEVERE ACCIDENTS OF NUCLEAR POWER PLANTS

摘要

Fukushima Daiichi nuclear power plant was subjected to and experienced significant damages from a large tsunami induced by the East Great Japan Earthquake. The cores at three units of the plant were severely damaged. The main challenges at the plant were preventing tsunami inundation to the site, preventing flooding of the vital safety systems and components and implementing effective accident management to mitigate the station blackout (SBO) and loss of ultimate heat sink. The lessons learned from the accident revealed the importance of provisions of effective tsunami protection as well as the provisions of effective severe accident strategies in all nuclear power plants. To respond to these lessons learned there is an urgent need to development tsunami-seismic PSA methods and to investigate and develop suitable and effective severe accident management measures for nuclear power plants. This paper describes the improvement of tsunami-seismic PSA methods and considers accident management measures for the two important functions; electric power supply and cooling water injection. This evaluation uses sample tsunami-seismic PSA analyses for a typical nuclear power plant. In this study, the strategic use of PSA approaches are discussed, including long-term earthquake occurrence probability estimation methods. To facilitate a timely implementation of improved strategies, a probabilistic safety margin analysis method (PSMA) is proposed for developing specific safety enhancement systems and accident management measures to cope with severe accidents.