Cf. http://www.nytimes.com/interactive/2011/03/16/science/plume-graphic.html?ref=science
Status of the Nuclear Reactors at the Fukushima Daiichi Power Plant
None of the six reactors at the plant have operated since the earthquake. But explosions have damaged four of the buildings, and fuel in the reactors and spent fuel stored in the buildings is in danger of melting and releasing radioactive materials.
Cf. http://www.nytimes.com/interactive/2011/03/16/world/asia/reactors-status.html?ref=science
How a Reactor Shuts Down and What Happens in a Meltdown
Cf. http://www.nytimes.com/interactive/2011/03/12/world/asia/the-explosion-at-the-japanese-reactor.html?ref=science
U.N. Scientists Project Path of Radiation Plume