Present state of nuclear power facilities in Japan and severe accident

This article reviewed present state of nuclear power development and related facilities in Japan and safety research conducted during the development stage. Before the Fukushima nuclear disaster, 54 units of LWR had been installed in Japan with 40% of PWR and 60% of BWR. Principle of nuclear power and fuel cycle was described to show necessity of after-heat removal. Nuclear facilities consisted of research reactors and fuel facilities for nuclear power with no serious problem in case of all power losses (SBO). Nuclear power station was designed for large size rupture as the design basis accidents. However at TMI-2 small size rupture LOCA was experienced leading to partial meltdown or severe accident. Safety research in Japan had been conducted as Safety Assessment and Facilities Establishment (SAFE) project, Rig of Safety Assessment (ROSA) project and others. After TMI-2 accident several research including steam explosion had been performed focusing basic research and relying on overseas large-scale experiments. Event sequence of the Fukushima Daiichi nuclear accident was reviewed in detail and key issues of accident management were commented. Severe accident research results and overseas good practices should be followed and total organizational problems covering government, regulators and operators should be solved. (T. Tanaka)