An explosion blew the roof and outer walls off of the number 3 unit Monday at the Fukushima nuclear power plant in earthquake-stricken northern Japan.
A similar explosion struck the number 1 unit on Saturday at the plant, where cooling systems were knocked out by Friday's devastating earthquake and tsunami.
Officials said the inner containment vessel remained intact after Monday's explosion and they did not believe there had been a major release of radiation.
However engineers were still struggling to gain control of a new threat at the number 2 unit, where officials reported a total failure of the cooling system. The operators of the plant, Tokyo Electric Power Company, said the coolant had evaporated exposing the fuel rods, raising the risk of them melting down.
If the heat from a meltdown ruptures the containment vessel, that could permit a major release of radiation with serious health consequences. Engineers were desperately pumping sea water into all three units at Fukushima in an effort to cool them down.
Engineers have also vented steam from the plants to release a build-up of pressure. Hydrogen in the steam is believed to have mixed with oxygen, causing the two explosions.
Evacuations
Almost 200,000 people have already been evacuated from areas around Fukushima and other troubled nuclear power stations. About 600 people still in homes near the Fukushima plant were advised to remain indoors.
The commander of the aircraft carrier USS Ronald Reagan, which has been assisting in relief efforts from about 160 kilometers offshore, said the American fleet had moved away from the Fukushima plant after low levels of radiation were detected on 17 crewmen. He said the radiation was easily washed off.
The nuclear crisis has diverted attention away from a massive rescue and relief effort. The total death toll from the quake and tsunami is expected to reach more than 10,000.
The search for survivors
About 100,000 Japanese troops, backed by relief teams from more than a dozen countries, are searching for survivors in the debris of Friday's earthquake and tsunami, which reduced whole towns and villages to rubble. Power shortages and massive damage to infrastructure are complicating efforts to reach the hardest-hit areas.
More than 1,000 bodies were found Monday along the shores of one northeastern province, and the Kyodo news agency said another 1,000 bodies were found at a second location in hard-hit Miyagi province.
Also Monday, officials sounded tsunami warnings and ordered residents to head for higher ground along the devastated northeastern coast near the city of Sendai following a major aftershock. The alert turned out to be a false alarm but further jangled the nerves of a frightened population.
Markets tumble
In Tokyo, residents returned to shops and offices at the start of a new work week, coping with rolling blackouts and reduced transit service as authorities deal with limited power-generating capacity. Fears that industrial production will be disrupted helped push the main stock index to its lowest levels since November.
There is no electricity at all in vast stretches of the northeast region, where tens of thousands of homeless residents spent a third night in near-freezing temperatures without heat or running water. Relief crews are rushing to provide food and water, but are hard-pressed to reach many of those in greatest need.
Two U.S. search-and-rescue teams with 144 staff and 12 dogs were among the teams that began clawing through the ruins at first light Monday in search of survivors. A 15-member Chinese team was also at work and Japan's Kyodo news agency said the Defense Ministry will activate reserves to assist in relief operations, the first time it has ever done so.
Friday's earthquake had a magnitude of 8.9, making it the most powerful ever recorded in Japan and the fifth strongest since 1900.