The reactors at Fukushima were typical boiling water reactors, BWR-3 (Unit 1) and BWR-4 (Units 2 – 5) designs. Prior to the accident Units 1, 2 and 3 were operating, Unit 4 was defueled, but not operating (planned maintenance), and Units 5 and 6 fueled, but not operating (planned outage).
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