Unclear whether copters, fire trucks aiding Japan's reactor battle; US says it may take weeks

Japan (AP) - Emergency workers seemed to try everything they could think of Thursday to douse Japan's most dangerously overheated nuclear reactors: helicopters, heavy-duty fire trucks, even water cannons normally used to quell rioters. But they couldn't be sure any of it was easing the peril at the tsunami-ravaged facility.

Three reactors have had at least partial meltdowns at the Fukushima Dai-ichi nuclear plant, where wisps of white steam rose from the stricken units Friday morning. But Japanese and U.S. officials believe a greater danger exists in the pools used to store spent nuclear fuel: Fuel rods in one pool were believed to be at least partially exposed, if not dry, and others were in danger. Without water, the rods could heat up and spew radiation.

It could take days and "possibly weeks" to get the complex under control, U.S. Nuclear Regulatory Commission Chairman Gregory Jazcko said. He defended the U.S. decision to recommend a 50-mile evacuation zone for its citizens, a much stronger measure than Japan has taken.

A senior official with the U.N.'s nuclear safety agency said there had been "no significant worsening" at the Fukushima Dai-ichi nuclear plant but that the situation remained "very serious." Graham Andrew told reporters in Vienna that nuclear fuel rods in two reactors were only about half covered with water, and they were also not completely submerged in a third.

If the fuel is not fully covered, rising temperatures will increase the chances of complete meltdowns that would release much larger amounts of radioactive material than the failing plant has emitted so far.

Low levels of radiation have been detected well beyond Tokyo, which is 140 miles (220 kilometers) south of the plant, but hazardous levels have been limited to the plant itself. Still, the crisis triggered by last week's earthquake and tsunami has forced thousands to evacuate and drained Tokyo's normally vibrant streets of life, its residents either leaving town or holing up in their homes.

The official death toll from the disasters stood at 5,692 as of Friday morning, with 9,522 missing, the national police agency said.

President Barack Obama appeared on television to assure Americans that officials do not expect harmful amounts of radiation to reach the U.S. or its territories. He also said the U.S. was offering Japan any help it could provide. He reaffirmed America's commitment to nuclear power and said he was asking for a comprehensive safety review.

Japanese and American assessments of the crisis have differed, with the plant's owner denying Jazcko's report Wednesday that Unit 4's spent fuel pool was dry and that anyone who gets close to the plant could face potentially lethal doses of radiation. But a Tokyo Electric Power Co. executive moved closer to the U.S. position Thursday.

"Considering the amount of radiation released in the area, the fuel rods are more likely to be exposed than to be covered," Yuichi Sato said.

Another utility official said Wednesday that the company has been unable to get information such as water levels and temperatures from any of the spent fuel pools in the four most troubled reactors.

Workers have been dumping seawater when possible to control temperatures at the plant since the quake and tsunami knocked out power to its cooling systems, but they tried even more desperate measures on Units 3 and 4