Can someone provide background information on the type of reactor at Fukusima, it's age, and it's failsafe mechanism. I was under the impression that no modern reactor would sustain criticality in the event of a failure.
http://en.wikipedia.org/wiki/Boiling_Water_Reactor
It lacks the safety mechanisms of an abwr. Namely even after the control rods are deployed, heat can continue to build up. Hence the plant requires forced cooling, in this case provided by diesel and electric pumps. After the earthquake the diesel pumps failed and cooling was effected by battery power. While the government rushed in new diesel generators, they were not installed in time to prevent the build up of pressure resulting in this explosion.
Agency officials said a team of National Institute of Radiological Sciences detected cesium, a radioactive substance contained in nuclear fuel rods, near the No. 1 reactor of the No. 1 nuclear plant, leading them to suspect nuclear fuel rods in the reactor began melting amid the high temperatures.
The nuclear safety agency said in the afternoon that the level of cooling water in the No. 1 reactor likely had dropped to 1.7 meters below the top of nuclear fuel rods, leading officials to suspect that about half of the rods' length had been exposed.
Reactor meltdown feared / Quake disabled Fukushima N-plants' cooling systems
"A portion of the reactor's fuel rods, which create heat through a nuclear reaction, had become exposed due to the cooling-system failure. The spokesman for Tepco said 1.5 meters of the 4.5 meter long fuel rods were exposed. It was unclear Saturday afternoon whether the water added by workers had re-covered the rods."
http://online.wsj.com/article/SB1000142405274870355540457619...
http://news.ycombinator.org/item?id=2316851
It's going to take a couple days for any nuclear reactor to burn off decay heat.