SOMA, Japan (AP) -- 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 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.
So the worst case scenario still hung over the complex, and officials were clearly struggling to keep ahead of the crisis.
Late Monday, the chief government spokesman said there were signs that the fuel rods were melting in all three reactors, all of which had lost their cooling systems in the wake of Friday's massive earthquake and tsunami
"Although we cannot directly check it, it's highly likely happening," Chief Cabinet Secretary Yukio Edano told reporters.
Some experts would consider that a partial meltdown. Others, though, reserve that term for times when nuclear fuel melts through a reactor's innermost chamber but not through the outer containment shell.
Officials held out the possibility that, too, may be happening.
"It's impossible to say whether there has or has not been damage" to the vessels, nuclear agency official Naoki Kumagai said.
If a complete reactor meltdown -- where the uranium core melts through the outer containment shell -- were to occur, a wave of radiation would be released, resulting in major, widespread health problems.
The Monday morning explosion at the Fukushima Dai-ichi plant's Unit 3 injured 11 workers and came as authorities were trying to use sea water to cool the complex's three reactors.
While four Japanese nuclear complexes were damaged in the wake of Friday's twin disasters, the Dai-ichi complex, which sits just off the Pacific coast and was badly hammered by the tsunami, has been the focus of most of the worries over Japan's deepening nuclear crisis. All three of the operational reactors at the complex now have faced severe troubles.
Operators knew the sea water flooding would cause a pressure buildup in the reactor containment vessels -- and potentially lead to an explosion -- but felt they had no choice if they wanted to avoid complete meltdowns. Eventually, hydrogen in the released steam mixed with oxygen in the atmosphere and set off the two blasts.
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