Second blast rocks Japanese nuclear plant

More than 180,000 people have evacuated the area in recent days, and up to 160 may have been exposed to radiation.

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A screen grab taken from news footage by Japanese public broadcaster NHK on March 14, 2011 shows the moment of a hydrogen explosion at the Fukushima No. 1 nuclear power station number three reactor on March 14, 2011.  An explosion shook a quake-damaged Japanese nuclear power plant on March 14 and plumes of smoke rose from the building, live television showed.  Japan's nuclear safety agency said the blast, at the number 3 reactor at the Fukushima No. 1 plant, was believed to be caused by hydrogen.  JAPAN OUT AFP PHOTO / HO / NHK

SOMA, JAPAN // The second hydrogen explosion in three days rocked Japan’s struck Fukushima Dai-ichi nuclear plant today, sending a massive column of smoke into the air and wounding 11 workers. Hours later, the US said it had shifted its offshore forces away from the plant after detecting low-level radioactive contamination.

The aircraft carrier USS Ronald Reagan was about 160 kilometres offshore when it detected the radiation, which US officials said was about the same as one month’s normal exposure to natural background radiation in the environment.

It was not clear if the radiation had leaked during Monday’s explosion. That blast was felt 40 kilometres away, but the plant’s operator said radiation levels at the reactor were still within legal limits.

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The explosion at the plant's Unit 3, which authorities have been frantically trying to cool after a system failure after Friday's massive earthquake and tsunami, triggered an order for hundreds of people to stay indoors, said the chief cabinet secretary, Yukio Edano. The two disasters left at least 10,000 people dead.

Operators knew an explosion was a possibility as they struggled to reduce pressure inside the reactor containment vessel, but apparently felt they had no choice if they wanted to avoid a complete meltdown. In the end, the hydrogen in the released steam mixed with oxygen in the atmosphere and set off the blast.

Tokyo Electric Power, which operates the plant, said radiation levels at Unit 3 were well under the levels where a nuclear operator must file a report to the government.

A similar explosion occurred on Saturday at the plant’s Unit 1, injuring four workers and causing mass evacuations. Shortly after today's explosion, Tokyo Electric warned it had lost the ability to cool Unit 2. Takako Kitajima, a company official, said plant workers were preparing to inject seawater into the unit to cool the reactor, a move that could lead to an explosion there as well.

The Unit 3 reactor’s inner containment vessel holding nuclear rods was intact, Mr Edano said, allaying some fears of the risk to the environment and public.

More than 180,000 people have evacuated the area in recent days, and up to 160 may have been exposed to radiation – pouring misery onto those already devastated by the twin disasters.

Officials have declared states of emergency at six Fukushima reactors, where Friday’s twin disasters knocked out the main cooling systems and backup generators. Three are at Dai-ichi and three at the nearby Fukushima Daini complex.

Most attention, though, has been focused on Dai-ichi units 1 and 3, where operators have been funnelling in seawater in a last-ditch measure to cool the reactors. A complete meltdown – the melting of the radioactive core – could release radioactive contaminants into the environment and pose major, widespread health risks.

Mr Edano said no Fukushima reactor was near that point, and he was confident of escaping the worst scenarios.

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