SOMA, Japan - (AP) - Water levels dropped precipitously Mondayinside a stricken Japanese nuclear reactor, twice leaving theuranium fuel rods completely exposed and raising the threat of ameltdown, hours after a hydrogen explosion tore through thebuilding housing a different reactor.
Water levels were restored after the first decrease but the rodsremained exposed late Monday night after the second episode,increasing the risk of the spread of radiation and the potentialfor an eventual meltdown.
The cascading troubles in the Fukushima Dai-ichi plantcompounded the immense challenges faced by the Tokyo government,already struggling to send relief to hundreds of thousands ofpeople along the country's quake- and tsunami-ravaged coast whereat least 10,000 people are believed to have died.
Later, a top Japanese official said the fuel rods in all threeof the most troubled nuclear reactors appeared to be melting.
Of all these troubles, the drop in water levels at Unit 2 hadofficials the most worried.
"Units 1 and 3 are at least somewhat stabilized for the timebeing," 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 itmight seem.
The blast actually lessened pressure building inside thetroubled reactor, and officials said the all-important containmentshell - thick concrete armor around the reactor - had not beendamaged. In addition, officials said radiation levels remainedwithin legal limits, though anyone left within 12 miles (20kilometers) of the scene was ordered to remain indoors.
"We have no evidence of harmful radiation exposure," deputyCabinet secretary Noriyuki Shikata told reporters.
On Saturday, a similar hydrogen blast destroyed the housingaround the complex's Unit 1 reactor, leaving the shell intact butresulting in the mass evacuation of more than 185,000 people fromthe area.
So the worst case scenario still hung over the complex, andofficials were clearly struggling to keep ahead of the crisis.
Late Monday, the chief government spokesman said there weresigns that the fuel rods were melting in all three reactors, all ofwhich had lost their cooling systems in the wake of Friday'smassive earthquake and tsunami
"Although we cannot directly check it, it's highly likelyhappening," 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 througha reactor's innermost chamber but not through the outer containmentshell.
By contrast, a complete reactor meltdown, where the uranium coremelts through the containment shell, would release a wave ofradiation and result in major, widespread health problems.
The Monday morning explosion at the Fukushima Dai-ichi plant'sUnit 3 injured 11 workers and came as authorities were trying touse sea water to cool the complex's three reactors.
While four Japanese nuclear complexes were damaged in the wakeof Friday's twin disasters, the Dai-ichi complex, which sits justoff the Pacific coast and was badly hammered by the tsunami, hasbeen the focus of most of the worries over Japan's deepeningnuclear crisis. All three of the operational reactors at thecomplex now have faced severe troubles.
Operators knew the sea water flooding would cause a pressurebuildup in the reactor containment vessels - and potentially leadto an explosion - but felt they had no choice if they wanted toavoid complete meltdowns. Eventually, hydrogen in the releasedsteam mixed with oxygen in the atmosphere and set off the twoblasts.
Japan's meteorological agency did report one good sign. It saidthe prevailing wind in the area of the stricken plant was headingeast into the Pacific, which experts said would help carry away anyradiation.
Across the region, though, many residents expressed fear overthe situation.
People in the port town of Soma had rushed to higher groundafter a tsunami warning Monday - a warning that turned out to befalse alarm - and then felt the earth shake from the explosion atthe Fukushima reactor 25 miles (40 kilometers) away. Authoritiesthere ordered everyone to go indoors to guard against possibleradiation contamination.
"It's like a horror movie," said 49-year-old Kyoko Nambu asshe stood on a hillside overlooking her ruined hometown. "Ourhouse is gone and now they are telling us to stay indoors.
"We can see the damage to our houses, but radiation? ... Wehave no idea what is happening. I am so scared."
Meanwhile, 17 U.S. military personnel involved in helicopterrelief missions were found to have been exposed to low levels ofradiation after the flew back from the devastated coast to the USSRonald Reagan, an aircraft carrier about 100 miles (160 kilometers)offshore.
U.S. officials said the exposure level was roughly equal to onemonth's normal exposure to natural background radiation, and the 17were declared contamination-free after scrubbing with soap andwater.
As a precaution, the U.S. said the carrier and other 7th Fleetships involved in relief efforts had shifted to another area.
While Japan has aggressively prepared for years for majorearthquakes, reinforcing buildings and running drills, the impactof the tsunami - which came so quickly that not many people managedto flee to higher ground - was immense.
By Monday, officials were overwhelmed by the scale of thecrisis, with millions of people facing a fourth night withoutelectricity, water, food or heat in near-freezing temperatures.
International scientists say there are serious dangers butlittle risk of a catastrophe like the 1986 blast in Chernobyl,where there was no containment shells.
And, some analysts noted, the length of time since the nuclearcrisis began indicates that the chemical reactions inside thereactor were not moving quickly toward a complete meltdown.
"We're now into the fourth day. Whatever is happening in thatcore is taking a long time to unfold," said Mark Hibbs, a seniorassociate at the nuclear policy program for the Carnegie Endowmentfor International Peace. "They've succeeded in prolonging thetimeline of the accident sequence."
He noted, though, that Japanese officials appeared unable tofigure out what was going on deep inside the reactor. In part, thatwas probably because of the damage done to the facility by thetsunami.
"The real question mark is what's going on inside the core,"he said.
Overall, more than 1,500 people had been scanned for radiationexposure in the area, officials said.