SOMA, Japan -- The second hydrogen explosion in three days rocked a Japanese nuclear plant Monday, devastating the structure housing one reactor and injuring 11 workers. Water levels dropped precipitously at another reactor, completely exposing the fuel rods and raising the threat of a meltdown.

The morning explosion in Unit 3 of the Fukushima Dai-ichi plant was felt 25 miles, but the plant's operator said the radiation levels at the affected reactor were still within legal limits. Hours later, officials reported that the fuel rods at another reactor, Unit 2, were fully exposed, at least temporarily.

Authorities began pouring sea water into that unit to re-cover the rods -- as they are at the plant's two other troubled reactors after cooling system failures in the wake of Friday's massive earthquake and tsunami, which killed at least 10,000 people. The latest explosion triggered an order for hundreds of people to stay indoors, said Chief Cabinet Secretary Yukio Edano.

Operators knew the sea water flooding at Unit 3 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.

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