A build up in pressure in one of the reactors at a Japanese nuclear power plant damaged in the huge earthquake yesterday has caused an explosion, according to the government.

They are assuring people that there is no immediate danger, that the metal container housing the nuclear material was not damaged, but do we really believe them? This is an extraordinarily dangerous situation with Japanese nuclear experts saying that not one, but two power generating sites are affected and at least 5 reactors may be at risk of melting down.

Washington Post:

Later Saturday an official with Japan's nuclear safety commission said a meltdown was possible because of the overheating, the Associated Press reported. Ryohei Shiomi added that even if there were a meltdown, it wouldn't affect residents outside the evacuated six-mile radius.

Meanwhile, Tokyo Electric said it had decided to vent slightly radioactive steam and gas to relieve pressure that had increased sharply in the containment building at unit No. 1. The company said on its Web site that the increase was "assumed to be due to leakage of reactor coolant." It remained unclear where the leak was. The company said it did not think there was leakage of reactor coolant in the containment vessel "at this moment."

[...]

There were also reports of elevated radiation levels inside the control room of that reactor unit. NISA said levels were 1,000 times the norm. The AP later quoted an official from NISA as saying that a measurement of radiation levels outside the plant was eight times as high as normal. Even that level of radiation still posed little danger to residents, nuclear experts said. They also said the release of steam and gas from containment buildings posed little danger.

But in an update on its Web site, Tokyo Electric said that one of its employees working in unit No. 1 was treated for radiation exposure by a special physician. Tokyo Electric also said, shortly before the explosion, that even though it was injecting water into the reactor vessel, levels were still dropping. It said that water levels in unit No. 1 were lower than normal, but stable.

The status of Tokyo Electric's Daina plants remained unclear. Earlier, they had been said to have completed automatic shutdowns. But Saturday, Tokyo Electric suggested that they were having problems similar to the ones at the other nuclear complex because of disruptions in the power supply needed to run cooling facilities.

The time to discuss the politics of this nuclear accident will be later, after the danger has passed. For the moment, we will note that Japan depends heavily on nuclear power and that it is unclear if these plants, not constructed to withstand such a massive quake and Tsunami, will be able to survive.