Nature: Reactors, residents and risk

http://www.nature.com/news/2011/110421/full/472400a.html
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Conventionally, nuclear plant operators have considered some accident sequences so unlikely that they have not built in complete safeguards — such accidents are called 'beyond design basis' events. . . . The Kashiwazaki-Kariwa plant has seven reactors, making it the world's biggest in terms of electrical output at 7,965 megawatts.