Low concentrations of radioactive particles from Japan's disaster-hit nuclear power plant have been heading eastwards and are expected to reach North America in days, a Swedish official said today.
Lars-Erik De Geer, research director at the Swedish Defence Research Institute, a government agency, was quoting data from a network of international monitoring stations set up to detect signs of any nuclear weapons tests.
Stressing that the levels were not dangerous for people, he predicted the particles would eventually also continue across the Atlantic and reach Europe.
"It is not high from any danger point of view."
The New York Times earlier said a forecast of the possible movement of the radioactive plume showed it churning across the Pacific, and touching the Aleutian Islands today before hitting southern California late on Friday.
It said the projection was made by the Comprehensive Test Ban Treaty Organisation, a Vienna-based independent body for monitoring possible breaches of the test ban.
The US Nuclear Regulatory Commission said yesterday: "All the available information continues to indicate Hawaii, Alaska, the US Territories and the US west Ccast are not expected to experience any harmful levels of radioactivity."
The New York Times said health and nuclear experts emphasised radiation would be diluted as it travelled and at worst would have extremely minor health consequences in the United States.
Meanwhile Japan tried high-pressure water cannons, fire trucks and even helicopters that dropped batches of seawater in increasingly frantic attempts today to cool the overheated nuclear complex as US officials warned the situation was deteriorating.
The top US nuclear regulatory official gave a far bleaker assessment of the crisis than the Japanese, and the US ambassador warned US citizens within 80 kilometers of the Fukushima Dai-ichi plant on the northeast coast to leave the area or at least remain indoors.
The Japanese government said it had no plans to expand its mandatory 20-kilometer exclusion zone around the plant along the northeastern coast, while also urging people within 30 kilometers to stay inside.
The troubles at the nuclear complex were set in motion by last week's 9.0-magnitude earthquake and tsunami 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 probably killed more than 10,000 people and left hundreds of thousands homeless.
Four of the plant's six reactors have faced serious crises involving fires, explosions, damage to the structures housing reactor cores, partial meltdowns or rising temperatures in the pools used to store spent nuclear fuel. Officials also recently announced that temperatures are rising in the spent fuel pools of the last two reactors.
Two Japanese military CH-47 Chinook helicopters began dumping seawater on the complex's damaged Unit 3 at 9.48am (0048 GMT), defence ministry spokeswoman Kazumi Toyama said. The choppers dumped at least four loads on the reactor 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 7,500 litres of water.
The dousing is aimed at cooling the Unit 3 reactor, as well as replenishing water in that unit's cooling pool, where used fuel rods are stored, Ms Toyama said. The plant's owner, Tokyo Electric Powe, said earlier that pool was nearly empty, which would cause the rods to overheat and emit even more radiation.
Defence Minister Toshifumi Kitazawa told reporters that emergency workers had no choice but to try the water dumps before it was too late.
Along with the helicopter water drops, military vehicles designed to extinguish fires at plane crashes were also being used to spray the crippled Unit 3, said Mitsuru Yamazaki, a military spokesman. The high-pressure sprayers were to allow emergency workers to get water into the damaged unit while staying safely back from areas deemed to have too much radiation.
But special police units trying to use water cannons normally used to quell rioters failed in their attempt to cool the unit when the water failed to reach its target from safe distances, said Yasuhiro Hashimoto, a spokesman for the Nuclear And Industrial Safety Agency.
US officials, meanwhile, said Unit 4 also was seriously at risk.
the chairman of the US Nuclear Regulatory Commission, Gregory Jaczko said at a congressional hearing in Washington that all the water was gone from that unit's spent fuel pool. Mr Jaczko said anyone who gets close to the plant could face potentially lethal doses of radiation.
"We believe radiation levels are extremely high," he said.
Tokyo Electric executives said today that they believed the rods in that pool were covered with water, but an official with Japan's nuclear safety agency later expressed scepticism about that and moved closer to the US position.
"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.
Emergency workers were forced to temporarily retreat from the plant yesterday when radiation levels soared, losing precious time. While the levels later dropped, they were still too high to let workers get close.
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.
A core team of 180 emergency workers has been at the forefront of the struggle at the plant, rotating in and out of the complex to try to reduce their radiation exposure.
But 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.
"I don't know any other way to say it, but this is like suicide fighters in a war," said Keiichi Nakagawa, associate professor of the Department of Radiology at University of Tokyo Hospital.
Experts said, 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.
Japanese officials raised hopes of easing the crisis early today, saying they may be close to bringing power back to the plant. The new power line would revive electric-powered pumps, making it easier for workers to control the high temperatures.
Tokyo Electric officials said they hoped to have the new power line working later Thursday, and had electricians standing by to connect the power plant.
Nearly a week after the disaster, 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.
"There is enough food, but no fuel or gasoline," said Yuko Niuma, 46, as she stood looking out over Ofunato harbor, where trawlers were flipped on their sides.
Along the tsunami-savaged coast, people must stand in line for food, gasoline and kerosene to heat their homes. In the town of Kesennuma, they 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.
More than 5,300 people are officially listed as dead, but officials believe the toll will climb to more than 10,000.
Other countries have complained that Japan has been too slow and vague in releasing details about its rapidly evolving crisis at the complex of six reactors along Japan's northeastern coast.
Reuters and Associated Press