SOMA, Japan — The second hydrogen explosion in three days rocked Japan's stricken Fukushima Dai-ichi nuclear plant Monday, sending a massive column of smoke into the air and wounding 11 workers. Hours later, the U.S. 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 100 miles (160 kilometers) offshore when it detected the radiation, which U.S. 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 25 miles (40 kilometers) away, but the plant's operator said radiation levels at the reactor were still within legal limits.
The explosion at the plant's Unit 3, which authorities have been frantically trying to cool after a system failure in the wake of Friday's massive earthquake and tsunami, triggered an order for hundreds of people to stay indoors, said 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 Co., 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 Saturday at the plant's Unit 1, injuring four workers and causing mass evacuations.
Shortly after Monday'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, Edano said, allaying some fears of the risk to the environment and public. TV footage of the building housing the reactor appeared to show damage similar to Saturday's blast, with outer walls shorn off, leaving only a skeletal frame.
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.