Emergency workers seemed to try everything they could think of Thursday to douse one of Japan's 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, but an even greater danger has emerged. Japanese and U.S. concerns were increasingly focusing on the pools used to store spent nuclear fuel: Some of the pools are dry or nearly empty and 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 in a third they were also not completely submerged.
If the fuel is not fully covered, rising temperatures and pressure 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.
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, and said he was asking for a comprehensive review of U.S. nuclear plant safety.
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.
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 Unit 3's reactor and cooling pool.
Two Japanese military CH-47 Chinook helicopters began dumping seawater on Unit 3 on Thursday morning, defense ministry spokeswoman Kazumi Toyama said. The choppers doused the reactor with at least four loads of water in just the first 10 minutes, though television footage showed much of it appearing to disperse in the wind.
Chopper crews flew missions of about 40 minutes each to limit their radiation exposure, passing over the reactor with loads of about 2,000 gallons (7,500 liters) of water. Another 9,000 gallons (35,000 liters) of water were blasted from military trucks with high-pressure sprayers used to extinguish fires at plane crashes, though the vehicles had to stay safely back from areas deemed to have too much radiation.
Special police units with water cannons were also tried, but they could not reach the targets from safe distances and had to pull back, said Yasuhiro Hashimoto, a spokesman for Japan's nuclear safety agency.
Unit 3's reactor uses a fuel that combines plutonium, better known as an ingredient in nuclear weapons, and reprocessed uranium. The presence of this mixed oxide fuel, or MOX, means potentially that two very harmful radioactive products could be released into the environment.
Tokyo Electric Power said it believed workers were making headway in staving off a catastrophe both with the spraying and, especially, with efforts to complete an emergency power line to restart the plant's own electric cooling systems.
"This is a first step toward recovery," said Teruaki Kobayashi, a facilities management official at the power company. He said radiation levels "have somewhat stabilized at their lows" and that some of the spraying had reached its target, with one reactor emitting steam.
"We are doing all we can as we pray for the situation to improve," Kobayashi said. Authorities planned to spray again Friday, and Kobayashi said: "Choices are limited. We just have to stick to what we can do most quickly and efficiently."
Work on connecting the new power line to the plant was expected to begin Friday and take 10 to 15 hours, said Nuclear Safety Agency spokesman Minoru Ohgoda. But the utility is not sure the cooling systems will still function. If they don't, electricity won't help.
Four of the plant's six reactors have seen fires, explosions, damage to the structures housing reactor cores, partial meltdowns or rising temperatures. Officials also recently said temperatures are rising even in the spent fuel pools of the other two reactors.
The troubles at the nuclear complex were set in motion by last Friday's 9.0-magnitude earthquake and tsunami, which knocked out power and destroyed backup generators needed for the reactors' cooling systems. That added a nuclear crisis on top of twin natural disasters that likely killed well more than 10,000 people and left hundreds of thousands homeless.
Mario V. Bonaca, a physicist sits on an advisory committee to the U.S. Nuclear Regulatory Commission, said he believes the focus of the effort has shifted to the spent fuel pools. "I understand that they've controlled the cooling of the cores," said Bonaca, who said he was basing his understanding on NRC and industry sources.
The storage pools need a constant source of cooling water. Even when removed from reactors, uranium rods are still extremely hot and must be cooled for months, possibly longer, to prevent them from heating up again and emitting radioactivity.
While a core team of 180 emergency workers has been rotating in and out of the complex to avoid exposure, experts said that anyone working close to the reactors was almost certainly being exposed to radiation levels that could, at least, give them much higher cancer risks.
Experts note, though, that radiation levels drop quickly with distance from the complex. While elevated radiation has been detected well outside the evacuation zone, experts say those levels are not dangerous.
U.S. officials were taking no chances. In Washington, the State Department warned U.S. citizens to consider leaving the country and offered voluntary evacuation to family members and dependents of U.S. personnel in the cities of Tokyo, Yokohama and Nagoya.
The first flight left Thursday, with fewer than 100 people onboard, Undersecretary of State Patrick Kennedy said. Plans also call for airlifting several thousand family members of U.S. armed forces personnel as well as nonessential staff stationed in Japan in the coming days.
The U.S. evacuation zone is far bigger than that established by Japan, which has called for a 12-mile zone and has told those within 20 miles to stay indoors. Daniel B. Poneman, U.S. deputy secretary of energy, said at the briefing that his agency agreed with the 50-mile zone -- but said Japan's measures were also prudent.
Nearly a week after the earthquake and tsunami, police said more than 452,000 people were staying in schools and other shelters, as supplies of fuel, medicine and other necessities ran short. Both victims and aid workers appealed for more help, as the chances of finding more survivors dwindled.
Noriko Sawaki lives in a battered neighborhood in Sendai that is still without running water and food or gasoline supplies and that, she said, makes life exhausting. "It's frustrating, because we don't have a goal, something to strive for. This just keeps on going," said the 48-year-old.
In the town of Kesennuma, people lined up to get into a supermarket after a delivery of key supplies, such as instant rice packets and diapers.
Each person was only allowed to buy 10 items, NHK television reported.
With diapers hard to find in many areas, an NHK program broadcast a how-to session on fashioning a diaper from a plastic shopping bag and a towel.
Yamaguchi reported from Tokyo. Associated Press writers George Jahn in Vienna, Elaine Kurtenbach, Shino Yuasa, Jeff Donn and Tim Sullivan in Tokyo contributed to this report.