The operating reactors at Fukushima Daiichi power station automatically shut down during the earthquake.

But after subsequent cooling failures, two of them went into partial meltdown.

The Crippled Japanese Nuclear Reactors

Japanese officials were scrambling to avoid meltdowns at two nuclear reactors damaged after Friday’s earthquakes.

http://www.nytimes.com/interactive/2011/03/12/world/asia/the-explosion-at-the-japanese-reactor.html