Next-generation reactors using passive cooling systems, application to current BWRs

Lessons learned from the analysis of TMI, Chernobyl and Fukushima nuclear accidents were summarized as follows: assurance of valve operation at station blackout, preparation of boric acid solution and cooling method with final heat sink, various and multiple cooling methods based on defense in depth principle, and enforcement of containment cooling and filter venting to mitigate dissipation of radioactive materials at severe accidents. Based on such lessons, next-generation reactors such as ESBWR (Economic Simplified Boiling Water Reactor) and AP1000 had been developed using passive cooling systems such as isolation condenser system, passive containment cooling and passive residual heat removal system. Passive cooling systems could improve reliability by simplified plants and reduced equipment. Possible application to current BWRs was also discussed. As reflecting Fukushima accident, defense in depth layer 5 should include 'restoration' as well as 'prevention of disaster'. (T. Tanaka)