SOMA, Japan – Water levels dropped precipitously Monday inside a stricken Japanese nuclear reactor, twice leaving the uranium fuel rods completely exposed and raising the threat of a meltdown, hours after a hydrogen explosion tore through the building housing a different reactor.

Water levels were restored after the first decrease but the rods remained exposed late Monday night after the second episode, increasing the risk of the spread of radiation and the potential for an eventual meltdown.

The cascading troubles in the Fukushima Dai-ichi plant compounded the immense challenges faced by the Tokyo government, already struggling to send relief to hundreds of thousands of people along the country's quake- and tsunami-ravaged coast where at least 10,000 people are believed to have died.

Later, a top Japanese official said the fuel rods in all three of the most troubled nuclear reactors appeared to be melting.

Of all these troubles, the drop in water levels at Unit 2 had officials the most worried.

"Units 1 and 3 are at least somewhat stabilized for the time being," said Nuclear and Industrial Agency official Ryohei Shiomi "Unit 2 now requires all our effort and attention."

In some ways, the explosion at Unit 3 was not as dire as it might seem.

The blast actually lessened pressure building inside the troubled reactor, and officials said the all-important containment shell — thick concrete armor around the reactor — had not been damaged. In addition, officials said radiation levels remained within legal limits, though anyone left within 12 miles (20 kilometers) of the scene was ordered to remain indoors.

"We have no evidence of harmful radiation exposure," deputy Cabinet secretary Noriyuki Shikata told reporters.

On Saturday, a similar hydrogen blast destroyed the housing around the complex's Unit 1 reactor, leaving the shell intact but resulting in the mass evacuation of more than 185,000 people from the area.