Radiation levels spiked on Tuesday after a blast at a reactor in a quake-damaged nuclear power plant in north-eastern Japan. The plant's operator said it feared the reactor containment vessel had been damaged.

The explosion happened in reactor number 2 at a plant in Fukushima that is home to six nuclear reactors.

Earlier blasts had occurred in reactors 1 and 3 there, and a fire was also reported Tuesday in reactor number 4. It was put out, the Kyodo News agency reported, citing the operator, the Tokyo Electric Power Co (TEPCO).

Radioactivity around the damaged nuclear reactors have reached dangerous levels, top government spokesman Yukio Edano said Tuesday.

"We are talking now about radiation levels that can endanger human health," the chief cabinet secretary said.

TEPCO said that after the 6:10 am (2110 GMT Monday) blast at reactor number 2, radiation levels rose to 965.5 microsievert per hour, well above the legal limit of 500 microsievert per hour.

At 8:31 am, radiation had exceeded 8,217 microsievert, more than eight times the allowed annual exposure level, Kyodo reported, citing TEPCO.

TEPCO also admitted that a meltdown, a critical nuclear accident in which fuel rods melt and are destroyed, was possible.

Winds at the time of the explosion at the plant 240 km north of Tokyo were blowing to the south, which could raise the risk of radioactive particles being blown toward the capital.

TEPCO said it had evacuated workers at the power plant, leaving only about 50 necessary to continue work to try to cool the reactors and prevent meltdowns.

Residents living within a 30-km radius of the reactors should close their doors and windows and not wear clothing that has hung outside, Mr. Edano said.

People living within 20 km of the facility had been ordered to evacuate at the weekend, and Prime Minister Naoto Kan said Tuesday that most had done so.

The danger decreases the farther one is away from the reactors, Mr. Edano said. "We can continue to lead our normal lives," he said.

Friday's magnitude-9 earthquake and tsunami caused the cooling systems of reactors at Fukushima to fail, leading to the explosions and fears of reactor meltdowns.

Water was continuing to be pumped into reactor 1, 2 and 3 to try to cool them down and it was believed temperatures could be kept down, Mr. Edano said.

Kan appeals for calm

Mr. Kan urged the population to remain calm in the face of the escalating crisis at the damaged plant while also warning that further radiation leaks were possible.

Buildups of hydrogen were blamed for the explosions in reactors 1 and 3.

Prior to the latest blast, workers at the plant had been struggling overnight to keep reactor 2?s nuclear fuel rods cool, repeatedly trying to pump seawater into the reactor.

Seawater has also been used to cool the cores of reactors 1 and 2, a process that will make the reactors unusable in the future.

Earlier Tuesday, Mr. Kan said he would personally lead an integrated headquarters for handling the nuclear crisis, combining both government officials and TEPCO.

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