I know a little about nuclear reactors. This isn’t good.

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From NYT:

Japanese officials issued broad evacuation orders on Saturday for people living near two nuclear power plants whose cooling systems broke down as a result of the earthquake. The officials warned that small amounts of radioactive material were likely to leak from the plants.

The Fukushima nuclear power plant before and after a reported explosion that affected a building at the bottom, left on Saturday. More Photos »

Reuters quoted Jiji news agency as saying there had been an explosion at one of the plants — the 40-year-old Daiichi number one reactor — and television footage showed vapor rising from the plant about 150 miles north of Tokyo. The country’s nuclear safety agency did not confirm the reported incident.

The power plants, known as Daiichi and Daini and operated by Tokyo Electric Power, experienced critical failures of the cooling systems after the plants were shut down, as they were during the quake.

Ryohei Shiomi, an official with Japan’s nuclear safety commission, said that a meltdown was possible at one of the two Daiichi reactors, The Associated Press reported. Japanese television reported that the country’s Nuclear and Industry Safety Agency said it had detected cesium near one of those reactors.

An agency official said that a valve had been successfully opened to reduce pressure inside that reactor.

Naoto Sekimura, a professor at Tokyo University, told NHK, Japan’s public broadcaster, that “only a small portion of the fuel has been melted. But the plant is shut down already, and being cooled down. Most of the fuel is contained in the plant case, so I would like to ask people to be calm.” (more…)