By Kiyoshi Takenaka and Terril Jones
TOKYO (Reuters) - Japanese engineers toiled frantically to avert a catastrophic release of radiation from a crippled nuclear power plant north of Tokyo on Friday, but the United States said it could take weeks to cool the facility's overheating fuel rods.
Officials said they hoped to fix a power cable to at least two of the six reactors in the hope of restarting water pumps and were preparing to douse them in the afternoon with water from fire trucks.
However, no one was holding out hope that the crisis -- about to enter its second week after last Friday's 9.0-magnitude earthquake and tsunami -- could be overcome anytime soon.
Japan's nuclear agency spokesman conceded that a "Chernobyl solution" of burying the reactors in sand and concrete was in the back of the authorities' minds.
Millions in Tokyo remained indoors on Friday, fearing a blast of radioactive material from the complex 240 km (150 miles) to the north, though prevailing winds would likely carry contaminated smoke or steam away from the densely populated city to dissipate over the Pacific Ocean.
Japan's nuclear disaster, the world's worst since Chernobyl in Ukraine 25 years ago, has triggered alarm and reviews of safety at atomic power plants around the globe.
President Barack Obama, who stressed the United States did not expect harmful radiation to reach its shores, announced that he had ordered a comprehensive review of domestic nuclear plants and pledged Washington's support for Japan.
"In the coming days, we will continue to do everything we can to ensure the safety of American citizens and the security of our sources of energy," he said. "And we will stand with the people of Japan as they contain this crisis, recover from this hardship, and rebuild their great nation."
The Group of Seven rich nations, stepping in together to calm global financial markets after a tumultuous week, agreed to join in rare concerted intervention to restrain a soaring yen.
The United States' top nuclear regulator said it could take weeks to reverse the overheating of fuel rods at the Fukushima Daiichi plant.
"This is something that will take some time to work through, possibly weeks, as you eventually remove the majority of the heat from the reactors and then the spent-fuel pools," Nuclear Regulatory Commission Chairman Gregory Jaczko told a news conference at the White House.
Yukiya Amano, head of the Vienna-based International Atomic Energy Agency (IAEA), was due back in his homeland later on Friday with an international team of experts after earlier complaining about a lack of information from Japan.
Graham Andrew, his senior aide, called the situation at the plant "reasonably stable" but the government said white smoke or steam was still rising from three reactors and helicopters used to dump water on the plant had shown exposure to small amounts of radiation.
"The situation remains very serious, but there has been no significant worsening since yesterday," Andrew said.
The nuclear agency said the radiation level at the plant was as high as 20 millisieverts per hour. The limit for the workers was 100 per hour.
COOLING PUMPS MAY NOT WORK
Even if the engineers manage to connect the power at the Fukushima Daiichi plant, it is not clear the pumps will work as they may have been damaged in the earthquake or subsequent explosions and there are real fears of the electricity shorting and causing another explosion.
Nuclear agency spokesman Hidehiko Nishiyama said it was unclear how effective spraying water on the reactors from helicopters had been on Thursday, but the priority now was to get water into the spent-fuel pools.
"We have to reduce the heat somehow and may use seawater," he told a news conference. "We need to get the reactors back online as soon as possible and that's why we're trying to restore power to them."
Jaczko said the cooling pool for spent-fuel rods at the complex's reactor No.4 may have run dry and another was leaking.
DOLLAR GAINS AS FINANCIAL LEADERS INTERVENE
The U.S. dollar surged more than two yen to 81.80 after the G7's pledge to intervene, leaving behind a record low of 76.25 hit on Thursday.
Japan's Nikkei share index climbed 3.0 percent, recouping some of the week's stinging losses.
U.S. markets, which had tanked earlier in the week on the back of the crisis, rebounded on Thursday but investors were not convinced the advance would last.
The yen has seen steady buying since the earthquake, as Japanese and international investors closed long positions in higher-yielding, riskier assets such as the Australian dollar, funded by cheap borrowing in the Japanese currency.
Expectations that Japanese insurers and companies would repatriate billions of dollars in overseas funds to pay for a reconstruction bill that is expected to be much costlier than the one that followed the Kobe earthquake in 1995 also have helped boost the yen.
RADIATION LEVELS IN TOKYO BARELY ABOVE AVERAGE
The government had warned Tokyo's 13 million residents on Thursday to prepare for a possible large-scale blackout but later said there was no need for one. Still, many firms voluntarily reduced power, submerging parts of the usually neon-lit city in darkness.
The U.S. embassy in Tokyo has urged citizens living within 80 km (50 miles) of the Daiichi plant to evacuate or remain indoors "as a precaution," while Britain's foreign office urged citizens "to consider leaving the area." Other nations have urged nationals in Japan to leave the country or head south.
Japan's government has told everyone living within 20 km (12 miles) of the plant to evacuate, and advised people within 30 km (18 miles) to stay indoors.
At its worst, radiation in Tokyo has reached 0.809 microsieverts per hour this week, 10 times below what a person would receive if exposed to a dental x-ray. On Thursday, radiation levels were barely above average.
The plight of hundreds of thousands left homeless by the earthquake and tsunami worsened following a cold snap that brought heavy snow to worst-affected areas.
Supplies of water and heating oil are low at evacuation centers, where many survivors wait bundled in blankets.
About 30,000 households in the north were still without electricity in near-freezing weather, Tohuku Electric Power Co. said, and the government said at least 1.6 million households lacked running water.
The National Police Agency said on Friday it had confirmed 5,692 deaths from the quake and tsunami disaster, while 9,522 people were unaccounted for in six prefectures.
(Additional reporting by Linda Sieg, Nathan Layne, Elaine Lies, Leika Kihara and Mayumi Negishi; Writing by John Chalmers; Editing by Dean Yates)