The Sendai Earthquake cut the power supply to the pumps at Fukushima Daiichi nuclear plant. With no cooling water being pumped through the reactors, the nuclear fuel rods heated themselves until reactor 1 melted. But it should have been OK. The containment building should have kept the radiation from escaping. Then an explosion blew the containment building apart.
Radioactive cesium, a product of nuclear reactions, has been detected in the atmosphere around the plant. The Japanese Government says that there is no danger to the population because of the 6-mile exclusion zone that was put in place when the emergency started.
The experts say it was a hydrogen/oxygen explosion. This can happen when molten nuclear fuel rods fall into the water that usually carries their excess heat away. The water molecules quickly disassociate into hydrogen and oxygen due to the heat, with ignition once the hydrogen gets to combustible level. But the experts are still hopeful the explosion didn’t breach the inner containment around the core and is not a sign of a widespread radiation release. It’s possible the explosion occurred between the inner and out containment walls, leaving the inner one intact.
It’s hard to share their optimism.
Five other reactors, two of them at the same plant, are experiencing the same problem of fuel rod heat accumulating, threatening meltdown.
If the core has been breached and radioactive dust is carried into the sky to fall over a wide area, we’re looking at a second Chernobyl.
Update: the Japanese are saying the hydrogen gas resulting from the partial meltdown of nuclear fuel rods, which burnt through their casings into their cooling water. The gas had been pumped out of the steel inner containment of the reactor into the concrete outer containment building and it was between the two containment layers that the explosion occurred. They say the inner containment layer is intact. Their plan now is to flood the reactor with seawater to cool it down, which should take 10 days. If that succeeds, they’re going to be left with an awful lot of radioactive mess to dispose of safely but at least there should be no large radiation release.
Good article on how a meltdown occurs (written before the partial meltdown).