Japanese engineers conceded on Friday that burying a crippled nuclear plant in sand and concrete may be a last resort to prevent a catastrophic radiation release, the method used to seal huge leakages from Chernobyl in 1986.
But they still hoped to solve the crisis by fixing a power cable to two reactors by Saturday to restart water pumps needed to cool overheating nuclear fuel rods. Workers also sprayed water on the No.3 reactor, the most critical of the plant's six.
It was the first time the facility operator had acknowledged burying the sprawling complex was possible, a sign that piecemeal actions such as dumping water from military helicopters or scrambling to restart cooling pumps may not work.
"It is not impossible to encase the reactors in concrete. But our priority right now is to try and cool them down first," an official from the plant operator, Tokyo Electric Power Co, told a news conference.
As Japan entered its second week after a 9.0-magnitude earthquake and 10-metre (33-foot) tsunami flattened coastal cities and killed thousands of people, the world's worst nuclear crisis since Chernobyl looked far from over.
The nuclear disaster has triggered global alarm and reviews of safety at atomic power plants around the world.
"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," Gregory Jaczko, chairman of the U.S. Nuclear Regulatory Commission, told a news conference at the White House.
Millions of people in Tokyo continued to work from home, some fearing a blast of radioactive material from the complex, 240 km (150 miles) to the north, although the International Atomic Energy Agency said radiation levels in the capital were not harmful.
That is little solace for about 300 nuclear plant workers toiling in the radioactive wreckage. They are wearing masks, goggles and protective suits whose seams are sealed off with duct tape to prevent radioactive particles from creeping in.
"My eyes well with tears at the thought of the work they are doing," Kazuya Aoki, a safety official at Japan's Nuclear and Industrial Safety Agency, told Reuters.
Even if engineers restore power at the plant, the pumps may be too damaged from the earthquake, tsunami or subsequent explosions to work. The first step is to restore electricity to pumps for reactors No. 1 and 2 by Saturday.
By Sunday, the government expects cooling pumps for badly damaged reactors No.3 and No.4 to have power, said Hidehiko Nishiyama, Japan's nuclear agency spokesman.
Asked about burying the reactors in sand and concrete, he said: "That solution is in the back of our minds, but we are focused on cooling the reactors down."
Some experts said dumping water from helicopters to try to cool spent-fuel pools would have little impact.
"One can put out forest fires like this -- by pouring water from far above," said Gennady Pshakin, a Russian nuclear expert. "It is not clear where this water is falling. There is no control."
Japan raised the incident level at the crippled plant to 5 on a scale called INES to rank nuclear accidents, up from 4 on a 1-7 scale.
That puts it on a par with the Three Mile Island accident in the United States in 1979, although some experts say it is more serious. Chernobyl was a 7 on the INES scale.
DOLLAR GAINS AS FINANCIAL LEADERS INTERVENE
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 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 ended up 2.7 percent, recouping some of the week's stinging losses. It has lost 10.2 percent this week.
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.
MANY STILL WITHOUT ELECTRICITY, WATER, POWER
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, heating oil and fuel are low at evacuation centres, where many survivors wait bundled in blankets. Many elderly lack proper medical supplies. Food is often rationed.
The government said on Friday it was considering moving some of the hundreds of thousands of evacuees to parts of the country unscathed by the devastation.
Nearly 320,000 households in the north were still without electricity in near-freezing weather as of Friday afternoon, 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 6,539 deaths from the quake and tsunami disaster, exceeding 6,434 who died after the Kobe earthquake in 1995. But 10,354 people are still missing.
The 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.
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. (Additional reporting by Linda Sieg, Nathan Layne, Elaine Lies, Leika Kihara and Chris Gallagher; Writing by Jason Szep; Editing by Dean Yates and John Chalmers)