By Taiga Uranaka and Ki Joon Kwon
FUKUSHIMA, Japan (Reuters) - Japan struggled on Monday to avert a nuclear disaster and care for millions of people without power or water, three days after an earthquake and tsunami killed an estimated 10,000 people or more in the nation's darkest hour since World War Two.
The world's third-largest economy opens for business later on Monday, a badly wounded nation that has seen whole villages and towns wiped off the map by a wall of water, leaving in its wake an international humanitarian effort of epic proportion.
A grim-faced Prime Minister Naoto Kan described the crisis at Japan's worst since 1945, as officials confirmed that three nuclear reactors were at risk of overheating, raising fears of an uncontrolled radiation leak.
"The earthquake, tsunami and the nuclear incident have been the biggest crisis Japan has encountered in the 65 years since the end of World War II," Kan told a news conference.
"We're under scrutiny on whether we, the Japanese people, can overcome this crisis."
As he spoke, officials worked desperately to stop fuel rods in the damaged reactors from overheating. If they fail, the containers that house the core could melt, or even explode, releasing radioactive material into the atmosphere.
The most urgent crisis centers on the Fukushima Daiichi complex, where all three reactors are threatening to overheat, and where authorities say they have been forced to release radioactive steam into the air to relieve reactor pressure.
The complex, 240 km (150 miles) north of Tokyo, was rocked by an explosion on Saturday, which blew the roof off a reactor building. The government did not rule out further blasts there but said this would not necessarily damage the reactor vessels.
Authorities have poured sea water in all three of the complex's reactor to cool them down.
FEARS OVER OTHER REACTORS
The complex, run by Tokyo Electric Power Co, is the biggest nuclear concern but not the only one: on Monday, the U.N. nuclear watchdog said Japanese authorities had notified it of an emergency at another plant further north, at Onagawa.
But Japan's nuclear safety agency denied problems at the Onagawa plant, run by Tohoku Electric Power Co, noting that radioactive releases from the Fukushima Daiichi complex had been detected at Onagawa, but that these were within safe levels at a tiny fraction of the radiation received in an x-ray.
Shortly later, a cooling-system problem was reported at another nuclear plant closer to Tokyo, in Ibaraki prefecture.
Fukushima's No. 1 reactor, where the roof was ripped off, is 40 years old and was originally set to go out of commission in February but had its operating license extended by 10 years.
Prime Minister Kan said the crisis was not another Chernobyl, referring to the nuclear disaster of 1986 in Soviet Ukraine.
"Radiation has been released in the air, but there are no reports that a large amount was released," Jiji news agency quoted him as saying. "This is fundamentally different from the Chernobyl accident."
Nevertheless, France recommended its citizens leave the Tokyo region, citing the risk of further earthquakes and uncertainty about the nuclear plants.
Broadcaster NHK, quoting a police official, said more than 10,000 people may have been killed as the wall of water triggered by Friday's 8.9-magnitude quake surged across the coastline, reducing whole towns to rubble.
Almost 2 million households were without power in the freezing north, the government said. There were about 1.4 million without running water. Kyodo news agency said about 300,000 people were evacuated nationwide.
Authorities have set up a 20-km (12-mile) exclusion zone around the Fukushima Daiichi plant and a 10 km (6 miles) zone around another nuclear facility close by.
The nuclear accident, the worst since Chernobyl, sparked criticism that authorities were ill-prepared for such a massive quake and the threat that could pose to the country's nuclear power industry.
Chief Cabinet Secretary Yukio Edano said there might have been a partial meltdown of the fuel rods at the No. 1 reactor at Fukushima. Engineers were pumping in seawater, trying to prevent the same happening at the No. 3 reactor, he said in apparent acknowledgement they had moved too slowly on Saturday.
"Unlike the No.1 reactor, we ventilated and injected water at an early stage," Edano told a news briefing.
The No. 3 reactor uses a mixed-oxide fuel which contains plutonium, but plant operator Tokyo Electric Power Co (TEPCO) said it did not present unusual problems.
TEPCO said radiation levels around the Fukushima Daiichi plant had risen above the safety limit but that it did not mean an "immediate threat" to human health.
The wind over the plant would continue blowing from the south, which could affect residents north of the facility, an official at Japan's Meteorological Agency said.
SEARCH FOR THE MISSING
Kan said food, water and other necessities such as blankets were being delivered by vehicles but because of damage to roads, authorities were considering air and sea transport. He also said the government was preparing to double the number of troops mobilized to 100,000.
Thousands spent another freezing night huddled in blankets over heaters in emergency shelters along the northeastern coast, a scene of devastation after the quake sent a 10-meter (33-foot) wave surging through towns and cities in the Miyagi region, including its main coastal city of Sendai.
A Japanese official said 22 people have been confirmed to have suffered radiation contamination and up to 190 may have been exposed. Workers in protective clothing used handheld scanners to check people arriving at evacuation centers.
The government, in power less than two years and which had already been struggling to push policy through a deeply divided parliament, came under criticism for its handling of the disaster.
"Crisis management is incoherent," blared a headline in the Asahi newspaper, saying information and instructions to expand the evacuation area around the troubled plant were too slow.
There has been a proposal of an extra budget to help pay for the huge cost of recovery.
The Bank of Japan is expected to pledge on Monday to supply as much money as needed to prevent the disaster from destabilizing markets and its banking system. It is also expected to signal its readiness to ease monetary policy further if the damage from the worst quake since records began in Japan 140 years ago threatens a fragile economic recovery.
The earthquake was the fifth most powerful to hit the world in the past century. It surpassed the Great Kanto quake of September 1, 1923, which had a magnitude of 7.9 and killed more than 140,000 people in the Tokyo area.
The 1995 Kobe quake killed 6,000 and caused $100 billion in damage, the most expensive natural disaster in history. Economic damage from the 2004 Indian Ocean tsunami was estimated at about $10 billion.
(Additional reporting by Risa Maeda and Leika Kihara in Tokyo and Chris Meyers and Kim Kyung-hoon in Sendai; Writing by Nick Macfie; Editing by John Chalmers)