SOMA, Japan -- The second hydrogen explosion in three days rocked a Japanese nuclear plant Monday, sending a massive cloud of smoke into the air and injuring 11 workers. The blast was felt 25 miles away, but the plant's operator said the radiation levels at the affected unit were still within legal limits.
Later Monday, fuel rods at a separate reactor in the plant were fully exposed after it lost its ability to cool down, officials said. The exposure raises the risk of the unit overheating and adds to fears of a potential third explosion at the plant.
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The morning blast occurred in Unit 3, which authorities have been trying to cool with sea water 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 the sea water flooding would cause a pressure buildup in the reactor containment vessel -- and potentially lead to an explosion -- but 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.
The inner containment shell surrounding the Unit 3 reactor was intact, Edano said, allaying some fears of the risk to the environment and public. But the outer building around the reactor appeared to have been devastated, with only a skeletal frame remaining.