Japan raced to avert a meltdown of two reactors at a quake-hit nuclear plant Monday as the death toll from the disaster on the ravaged northeast coast was forecast to exceed 10,000.
An explosion at the ageing Fukushima No. 1 atomic plant blew apart the building housing one of its reactors Saturday, a day after the biggest quake ever recorded in Japan unleashed a monster tsunami.
The atomic emergency escalated as crews struggled to prevent overheating at a second reactor where the cooling system has also failed, and the government warned that it too could suffer a blast.
Nuclear contamination: The options
Prime Minister Naoto Kan said the situation at the stricken power plant remained grave, and that Japan was facing its worst crisis since the end of World War II -- which left the defeated country in ruins.
"The current situation of the earthquake, tsunami and the nuclear plants is in a way the most severe crisis in the 65 years since World War II," Kan said in a televised national address Sunday.
"Whether we Japanese can overcome this crisis depends on each of us," said the premier, who was dressed in an emergency services suit.
Rolling power outages were due to start later Monday as the quake and tsunami crippled nuclear power plants in the northeast. Millions of people were left without electricity after the disaster hit Friday. Japan's nuclear industry provides about a third of its power needs.
Top government spokesman Yukio Edano said it was highly likely that a partial meltdown had occurred at the plant's number one reactor, and a second was possible at the plant 250 kilometres (160 miles) northeast of Tokyo.
"There is the possibility of an explosion in the number three reactor," he said, while voicing confidence that it would withstand the blast as the first reactor had.
A meltdown occurs when a reactor core overheats and causes damage to the facility, potentially unleashing radiation into the environment.
Facts: Fukushima nuclear plant
Edano said that some radiation had escaped in the accident, but that the levels released into the air were so far not high enough to affect human health.
Plant operator Tokyo Electric Power said that despite continuing efforts, it had not managed to ensure that the tops of the fuel rods in the two troubled reactors remained submerged. Exposed rods increase the risk of a meltdown.
Focus: Japan quake threatens setback for nuclear energy
France's Institute of Radiation Protection and Nuclear Safety (IRSN) said "very large" amounts of radioactivity were "produced simultaneously with the explosion" at Fukushima.
"During the explosion the rate of release at the edge of the site would have attained one millisievert (mSv) per hour," compared with naturally present radioactivity of 0.0001 mSv per hour, it said Sunday.
The government has said the radiation released into the air so far had not reached levels high enough to affect human health.
A cooling pump at another plant 120 kilometres from Tokyo, the Tokai No. 2, had failed, but a back-up was working and cooling the reactor, a plant spokesman said early Monday.
The United Nations said a total of 590,000 people had been evacuated in the quake and tsunami disaster, including 210,000 living near the Fukushima nuclear plants.
The colossal 8.9 magnitude tremor sent waves of churning mud and debris racing over towns and farmland in Japan's northeast, destroying everything in its path and reducing swathes of countryside to a swampy wasteland.
Scene: Japanese towns become wastelands
The immense force of the quake had moved Honshu -- the main Japanese island -- 2.4 metres (eight feet), the US Geological Survey said.
In the small port town of Minamisanriku alone some 10,000 people were unaccounted for -- more than half the population of the town, which was practically erased, public broadcaster NHK reported.
The police chief in Miyagi prefecture -- where Minamisanriku is situated -- said the death toll was certain to exceed 10,000 in his region.
The national police agency said the confirmed death toll now stood at 1,597.
But in a rare piece of good news, a man who was swept 15 kilometres out to sea along with his house by the tsunami was plucked to safety Sunday after being spotted clinging to a piece of the roof.
Hiromitsu Shinkawa, 60, was discovered by a Japanese destroyer and transported by helicopter to hospital, where he was found to be in surprisingly good health.
With ports, airports, highways and manufacturing plants shut down, the government predicted "considerable impact on a wide range of our country's economic activities".
The Bank of Japan plans to pump "massive" funds into markets Monday in a bid to help them stabilise following the linked disasters, Dow Jones Newswires said.
Leading risk analysis firm AIR Worldwide said the quake alone would exact an economic toll estimated at between $14.5 billion and $34.6 billion (10 billion to 25 billion euros), without taking into account the effects of the tsunami.
The yen gained ground against major currencies in early Asian trade Monday, briefly touching 80.60 against the dollar, its highest since November 9.
Otomo Miki was with her husband, three children and their 82-year-old grandfather when the quake hit their home in Sendai. They managed to get to their car and speed to safety before the tsunami roared through.
"I had to keep zig-zagging around people and water to get to safety," she said. "We've lost our house and we have no idea what's going to happen next."
Her older sister was in a bus when the wave some 10 metres high crashed through.
"The bus driver told everybody to get out of the bus and run," Miki said. "My sister got out but some people just couldn't run fast enough," she said, adding that they were swept away in the waves.
While the world's third-largest economy struggled to assess the full extent of the disaster, groups of hundreds of bodies were being found along the shattered coastline.
Many survivors were left without water, electricity, fuel or enough food, as authorities appeared overwhelmed by the monumental scale of the disaster.
Scene: Huge petrol queues stretch Japan survivors' nerves
Japan committed 100,000 troops -- about 40 percent of the armed forces -- to spearhead a mammoth rescue and recovery effort with hundreds of ships, aircraft and vehicles headed to the Pacific coast area.
"There are so many people who are still isolated and waiting for assistance. This reality is very stark," said Defence Minister Toshimi Kitazawa.
The world rallied behind the disaster-stricken nation, with offers of help even from Japan's traditional rival China.
The US aircraft carrier Ronald Reagan reached waters off the northeast coast Sunday, part of a flotilla sent by Japan's close ally which has nearly 50,000 military personnel in the country. US Navy helicopters were transporting relief supplies for quake and tsunami survivors.
Pope Benedict XVI hailed the "courageous" Japanese people and called for prayers for the victims.
Japan sits on the "Pacific Ring of Fire", and Tokyo is in one of its most dangerous areas, where three continental plates are slowly grinding against each other, building up enormous seismic pressure.
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