The Meaning of “Beyond the Scope of the Accident Scenario” in the Fukushima Nuclear Power Plant Accident

A strong earthquake with a magnitude of 9.0 occurred offshore of the Tohoku area in March 2011. This earthquake was accompanied by a big tsunami with a height of 15 meters on the shore of Fukushima Prefecture. The earthquake and tsunami damaged the site of nuclear power plants, called “Fukushima dai-itchi hatsudensho” in Japanese, and damaged the safety systems of four plants. The most serious damage was the loss of all electric power supply systems, that is, the external power supply system (caused by the earthquake) and the emergency power supply system (caused by the tsunami). Due to this electric power loss, the core cooling ability was also lost, which led to a severe accident with a core meltdown, a hydrogen explosion and wide radioactive contamination in the environment by a large amount of released radioactive material. This paper discusses the causes of the accident from the standpoint of safety theory and risk evaluation through the meaning of “Beyond the scope of the accident scenario,” or “souteigai” in Japanese.departmental bulletin pape