http://www.reuters.com/article/2011/03/11/us-japan-quake-nuclear-us-idUSTRE72A8DD20110311Tepco said at about 2:46 p.m. local time Units 1, 2 and 3 at its Fukushima Daiichi plant -- three of six at the plant -- shut following the earthquake. All are boiling water reactors.
Tepco said the reactors shut due to the loss of offsite power due to the malfunction of one of two off-site power systems. That triggered the emergency diesel generators to startup and provide backup power for plant systems.
About an hour after the plant shut down, however, the emergency diesel generators stopped, leaving Units 1, 2 and 3 with no power for important cooling functions.
Nuclear plants need power to operate motors, valves and instruments that control the systems that provide cooling water to the radioactive core.
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