Japan Nuclear Plant Explodes After the Earthquake

TOKYO — An explosion at a crippled nuclear power plant in northern Japan on Saturday blew the roof off one building and caused a radiation leak of unspecified proportions, escalating the emergency confronting Japan’s government a day after an earthquake and tsunami devastated parts of the country’s northeastern coast.

Japanese television showed a cloud of white-gray smoke from the explosion billowing up from a stricken reactor at the Fukushima Daiichi Nuclear Power Station Saturday afternoon, and officials said leaks of radiation from the plant prompted them to expand the evacuation area around the facility to a 12-mile radius.

Government officials said that the explosion, caused by a build-up of pressure in the reactor after the cooling system failed, destroyed the concrete structure surrounding the reactor but did not collapse the critical steel container inside. They said that raised the chances that they could prevent the release of large amounts of radioactive material and could avoid a core meltdown at the plant.

"We’ve confirmed that the reactor container was not damaged. The explosion didn’t occur inside the reactor container. As such there was no large amount of radiation leakage outside," Japan’s Chief Cabinet Secretary Yukio Edano said in a news conference Saturday evening. "At this point, there has been no major change to the level of radiation leakage outside, so we’d like everyone to respond calmly."

Tokyo Electric Power, which operates the plant, which is located 160 miles north of Tokyo, now plans to fill the reactor with sea water to cool it down and reduce pressure. The process would take five to 10 hours, Mr. Edano said, expressing confidence that the operation could “prevent criticality.”

But the crisis at the aging plant confronted Japan with its worst nuclear accident — and perhaps the biggest mishap at a nuclear plant since the Chernobyl nuclear disaster in 1986.

Japanese nuclear safety officials and international experts said that because of crucial design differences the release of radiation at the Fukushima plant would likely be much smaller than at Chernobyl even if the Fukushima plant has a complete core meltdown, which they said it had not. But the problems at the plant are certain to worsen concerns about the safety record and reliability of Japan’s extensive nuclear power facilities, which have been criticized for major safety violations in the past.

The vulnerability of nuclear plants to earthquakes was also underscored by ongoing problems at the cooling system of reactors at a second nearby plant, known as Daini, which prompted a smaller evacuation from surrounding communities.

Tokyo Electric Power said the explosion happened “near” the No. 1 reactor at Daiichi at around 3:40 p.m. Japan time on Saturday. It said four of its workers were injured in the blast.

Officials said even before the explosion that they had detected cesium, an indication that some of the nuclear fuel was already damaged.

In the form found in reactors, radioactive cesium is a fragment of a uranium atom that has been split. In normal operations, some radioactivity in the cooling water is inevitable, because neutrons, the sub-atomic particles that carry on the chain reaction, hit hydrogen and oxygen atoms in the water and make those radioactive. But cesium, which persists far longer in the environment, comes from the fuel itself.

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.”

Both the Daiichi and Daini plants were shut down during Friday’s earthquake. But the loss of power in the area and damage to the plant’s generators from the subsequent tsunami crippled the cooling systems, which need to function after a shut down to cool down nuclear fuel rods.
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