Japan scrambled on Saturday to avert a disastrous meltdown at a nuclear plant damaged when a massive earthquake and tsunami struck the northeast coast, killing at least 1,300 people.
Jiji news agency said there had been an explosion at the tricken 40-year-old Daichi 1 reactor and TV footage showed vapour rising from the plant, which lies 240 km north of Tokyo.
The country's nuclear safety agency could not confirm the reported incident, which came as plant operator Tokyo Electric Power Co (Tepco) worked desperately to reduce pressures in the core of the reactor that - if not contained - could lead to a release of radiation into the atmosphere.
"An unchecked rise in temperature could cause the core to essentially turn into a molten mass that could burn through the reactor vessel," political risk information service Stratfor said in a report. "This may lead to a release of an unchecked amount of radiation into the containment building that surrounds the reactor."
NHK television said the outer structure of the building that houses the reactor appeared to have blown off, which could suggest the containment building had already been breached.
Earlier the operator released what it said was a tiny amount of radioactive steam to reduce the pressure and the danger was minimal because tens of thousands of people had already been evacuated from the vicinity.
Media reports estimate at least 1,300 people may have been killed by the 8.9 quake, the biggest since records began in Japan 140 years ago, and the 10-metre tsunami that swept ferociously inland after it struck.
Italy's National Institute of Geophysics and Volcanology said the earth's axis had shifted 25 cm as a result of the quake and the US Geological Survey said the main island of Japan had actually shifted 2.4 metres.
Japanese officials and experts have been at pains to say that while there would be radiation leaks, they would be very small and have dismissed suggestions of a repeat of a Chernobyl-type disaster.
10,000 people unaccounted for in Japan port town: NHK
Around 10,000 people are unaccounted for in the Japanese port town of Minamisanriku in quake-hit Miyagi prefecture, public broadcaster NHK reported Saturday.
The figure is more than half of the population of roughly 17,000 in the town on the Pacific coast, it said.
Local authorities are trying to find their whereabouts with the help of Self-Defence Forces, NHK said.
Authorities have so far confirmed that around 7,500 people were evacuated to 25 shelters after Friday's quake, but they have been unable to contact the other 10,000, NHK said.
Tomohiko Kato, an official of the disaster bureau of Miyagi, told AFP that it had contacted at least 7,500 local residents at shelters and houses.
"But our monitoring operations have been hampered with debris and mad," Kato said. "Even helicopters can't approach some of the shelters. I'm afraid that it will take more time to finish our confirmation procedures."
Minamisanriku is one of the areas hardest-hit by the 8.9-magnitude earthquake which triggered a massive tsunami in a disaster that has left at least 1,000 people dead.
Friday's tremor was so huge that thousands fled their homes from coastlines around the Pacific Rim, as far away as North and South America, fearful of a tsunami.
Most appeared to have been spared anything more serious than some high waves, unlike Japan's northeast coastline which was hammered by the huge tsunami that turned houses and ships into floating debris as it surged into cities and villages, sweeping aside everything in its path.
"I thought I was going to die," said Watauga Fuji, a 38-year-old sales representative in Koriyama, Fukushima, north of Tokyo and close to the area worst hit by the quake.
"Our furniture and shelves had all fallen over and there were cracks in the apartment building, so we spent the whole night in the car... Now we're back home trying to clean."
The unfolding natural disaster, which has been followed by dozens of aftershocks, prompted offers of search and rescue help from 50 countries.
The central bank said it would cut short a two-day policy review scheduled for next week to one day on Monday and promised to do its utmost to ensure financial market stability.
The disaster struck as the world's third-largest economy had been showing signs of reviving from an economic contraction in the final quarter of last year. It raised the prospect of major disruptions for many key businesses and a massive repair bill running into tens of billions of dollars.
The earthquake was the fifth most powerful to hit the world n the past century. It surpassed the Great Kant 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 caused $100 billion in damage and was the most expensive natural disaster in history.
Earlier, Tokyo Electric Power Co also said fuel may have been damaged by falling water levels at the Daiichi facility, one of its two nuclear power plants in Fukushima, some 240km north of Tokyo.
Officials said so far the level of radiation leakage was small. And Naoto Sekimura, a professor at the University of Tokyo, said a major radioactive disaster was unlikely.
"No Chernobyl is possible at a light water reactor. Loss of coolant means a temperature rise, but it also will stop the reaction," he said.
"Even in the worst-case scenario, that would mean some radioactive leakage and equipment damage, but not an explosion. If venting is done carefully, there will be little leakage. Certainly not beyond the 3km radius."
Kyodo news agency reported that authorities were evacuating about 20,000 people from the vicinity of the other Fukushima nuclear facility, the Daini plant.
Meanwhile, More than 215,000 people were in emergency shelters in eastern and northern Japan on Saturday, the National Police Agency said.
The number included more than 100,000 people who took refuge in the northern prefecture of Fukushima, including residents ordered to evacuate areas around two nuclear power plants.
The full scale of those left homeless was believed to be much higher, with police saying they had not received a tally from Miyagi prefecture, a hard-hit northern Japan province where hundreds of deaths have been reported.
Prime Minister Naoto Kan, who flew by helicopter to view Fukushima by air, had earlier ordered that residents within a 10 km radius be evacuated from the Daiichi plant.
UAE citizens safe
Meanwhile, Director of Media Affairs and Government Communications at the Ministry of Foreign Affairs Sultan Mohammed Al Ali on Saturday called UAE Ambassador to Japan Saeed Al Nowais to inquire about the safety of 70 UAE students studying in Japan, as well as the UAE citizens there, following the devastating earthquake which hit northern and eastern coasts of Japan.
Al Nowais affirmed that all UAE citizens in Japan, including the embassy staff, are safe, reported Wam.
The Operations Department at the Ministry is coordinating with the Japanese embassy in Abu Dhabi to follow the situation there and to ensure safety of the UAE citizens.
Prime Minister Naoto Kan's right-hand man and top spokesman, Chief Cabinet Secretary Yukio Edano, said "it is believed that more than 1,000 people have lost their lives".
Amid a mass rescue effort there were grim updates indicating appalling loss of life from along the hard-hit east coast of northern Honshu island, where the monster waves destroyed more than 3,000 homes.
On Saturday, the day after the devastating catastrophe, the National Police Agency said 413 people had been confirmed dead and 784 missing, with 1,128 injured in the massive quake and tsunami disaster. International search and rescue teams rushed to Japan Saturday
Tepco said it had lost ability to control pressure in some of the reactors at its Daini plant as it had with the Daiichi plant. Pressure was stable inside the reactors of the Daini plant but rising in the containment vessels, a spokesman said.
Pressure at one Daiichi reactor may have risen to 2.1 times the designed capacity, the trade ministry said.
The quake and tsunami cut the supply of off-site power to the plant and diesel generators intended to provide back-up electricity to the cooling system.
The cooling problems at the Japanese plant raised fears of a repeat of 1979's Three Mile Island accident, the most serious in the history of the US nuclear power industry.
Equipment malfunctions, design problems and human error led to a partial meltdown of the reactor core at the Three Mile Island plant, but only minute amounts of dangerous radioactive gases were released.
Radiation levels detected at the control unit of the reactors at the Daiichi plant were 1,000 times the normal level, but not yet at a level requiring workers to evacuate, a trade ministry official said.
The Union of Concerned Scientists, a US-based nonprofit organisation, said the power failure resulted in one of the most serious conditions that can affect a nuclear plant - a station blackout - during which off-site power and on-site emergency alternating current (AC) power is lost.
Nuclear plants generally need AC power to operate the motors, valves and instruments that control the systems that provide cooling water to the radioactive core. If all AC power is lost, the options to cool the core are limited.
If the core overheats, then the fuel would become damaged and a molten mass could melt through the reactor vessel, releasing a large amount of radioactivity into the containment building surrounding the vessel, the UCS said.
It added that it was not clear if the quake had undermined the containment building to contain pressure from any meltdown and allow radioactivity to leak out. The reactors shut down due to the earthquake account for 18 percent of Japan's nuclear power generating capacity.
Nuclear power produces about 30 percent of the country's electricity. Many reactors are located in earthquake-prone zones such as Fukushima and Fukui on the coast.
The IAEA estimates that around 20 percent of nuclear reactors around the world are currently operating in areas of significant seismic activity.
It said the sector began putting more emphasis on external hazards after an earthquake hit TEPCO's Kashiwazaki-Kariwa plant in July 2007, until then the largest to ever affect a nuclear facility.
When the earthquake hit the Kashiwazaki-Kariwa nuclear power plant, four reactors shut down automatically. Water containing radioactive material was released into the sea, but without an adverse effect on human health or the environment, it said.
Tepco had been operating three out of six reactors at the Fukushima Daiichi nuclear plant at the time of the quake, all of which shut down.