TOKYO (Reuters) - 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 40-year-old 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-meter (33-foot) tsunami flattened coastal cities and killed thousands, the world's worst nuclear crisis since Chernobyl and Japan's worst humanitarian crisis since World War Two looked far from over.
Around 6,500 people have been confirmed dead from the earthquake and tsunami while 10,300 are missing, many feared dead.
Some 390,000 people including many elderly are homeless and battling near-freezing temperatures in makeshift shelters in northeast coastal areas. Food, water, medicine and heating fuel is in short supply.
The government signaled it could have moved faster in dealing with the multiple disasters.
"An unprecedented huge earthquake and huge tsunami hit Japan. As a result, things that had not been anticipated in terms of the general disaster response took place," Chief Cabinet Secretary Yukio Edano told a news conference.
Japan also raised the severity rating of the nuclear crisis from Level 4 to Level 5 on the seven-level INES international scale, putting it on a par with America's Three Mile Island accident in 1979, although some experts say it is more serious.
Chernobyl was a 7 on the INES scale.
Tourists, expatriates and many Japanese continue to leave Tokyo, fearing a blast of radioactive material from the nuclear complex 240 km (150 miles) to the north, even though health officials and the U.N. atomic watchdog have said radiation levels in the capital were not harmful.
That is little solace for about 300 nuclear plant workers toiling in the radioactive wreckage, wearing masks, goggles and protective suits with seams sealed off by duct tape to keep out radioactive particles.
"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 Fukushima Daiichi plant, the pumps may be too damaged from the earthquake, tsunami or subsequent explosions to work.
The first step is to restore power to pumps for reactors No. 1 and 2, and possibly 4, by Saturday, said Hidehiko Nishiyama, Japan's nuclear safety agency spokesman.
By Sunday, the government expects to connect electricity to pumps for its badly damaged reactor No.3 -- a focal point in the crisis because of its use of mixed oxides, or mox, containing both uranium and highly toxic plutonium.
Asked about burying the reactors in sand and concrete, Nishiyama said: "That solution is in the back of our minds, but we are focused on cooling the reactors down."
Burying the reactors would leave part of Japan off-limits for decades. "It's just not that easy," Murray Jennex, a San Diego State University in California professor said when asked about the so-called Chernobyl option to bury the reactors.
"They are kind of like a coffee maker. If you leave it on the heat, they boil dry and then they crack," he said. "Putting concrete on that wouldn't help keep your coffee maker safe. But eventually, yes, you could build a concrete shield and be done with it."
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 .N225 ended up 2.7 percent, recouping some of the week's stinging losses. It has lost 10.2 percent this week, wiping $350 billion off market capitalization.
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.
The plight of those 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 centers, where many survivors wait bundled in blankets. Many elderly lack proper medical supplies. Food is often rationed. Rescue workers report acute fuel shortages.
The government said on Friday it was considering moving some 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, officials said, and the government said at least 1.6 million households lacked running water.
The government has told everyone living within 20 km (12 miles) of the crippled 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 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