From bad to worse:
An explosion early Tuesday morning damaged the No. 2 reactor at Japan’s Fukushima Daiichi Nuclear Power Station, the third in a series of blasts that have now hit each of the three crippled reactors at the plant, plant officials said. [...]
This explosion, reported to have occurred at 6:14 a.m., happened in the “pressure suppression room” in the cooling area of the reactor and inflicted some degree of damage on the pool of water used to cool the reactor, officials of Tokyo Electric Power said. But they did not say whether or not the incident had impacted the integrity of the steel containment structure that shields the nuclear fuel.
Radiation levels around plant spiked after the explosion to 8,217 microsieverts an hour from 1,941 about 40 minutes earlier, the company said. Some emergency workers there were evacuated, though the levels would have to rise far higher to pose an immediate threat to health, officials said.
Any damage to the steel containment vessel of a nuclear reactor is considered critical because it raises the prospect of an uncontrolled release of radioactive material and full meltdown of the nuclear fuel inside.
Updates as they become available.
For more discussion, see Joieau's diary.
Update: From The New York Times:
Japan faced the likelihood of a catastrophic nuclear accident Tuesday morning, as an explosion at the most crippled of three reactors at the Fukushima Daiichi Nuclear Power Station damaged its crucial steel containment structure, emergency workers were withdrawn from the plant, and much larger emissions of radioactive materials appeared immiment, according to official statements and industry executives informed about the developments. [...]
“It’s way past Three Mile Island already,” said Frank von Hippel, a physicist and professor at Princeton. “The biggest risk now is that the core really melts down and you have a steam explosion.”