The deep tank at the reactor unit contains used fuel rods which are extremely radioactive and normally are kept immersed in cooling water.

Unlike the fuel rods that are used in the reactor vessel, the spent rods are not surrounded by a steel-and-concrete containment vessel, which is designed to confine leaks of radioactive gas and particles.

Instead, they are housed in the overall building covering the No. 4 unit, which has been "badly damaged," the Nuclear Safety Authority (ASN) said in a statement.

Fires broke out on Tuesday and again early Wednesday in the area of the fuel-rod pool before being extinguished, the agency said.

"The state of the fuel rods stored in this pool, which have been potentially affected by the fire, is not known," it said.

"The evaporation of water in the pool is continuing," the ASN said.

If the tanks run dry, the rods can overheat and the metal sheaths that surround the fuel can be ruptured, the ASN said.

"The Japanese authorities have consequently indicated there is the possibility of releases of radioactivity directly into the atmosphere."

earlier related report
Japan's nuclear crisis deepens, emperor addresses nation
Sendai, Japan (AFP) March 16, 2011 - Japan's Emperor Akihito delivered a rare address to a jittery nation in dread of nuclear catastrophe Wednesday as millions struggled in desperate conditions after quake and tsunami disasters.

The television appearance by the emperor emphasised the severity of the crisis gripping Japan in the wake of the devastating quake and tsunami that hit on Friday, killing thousands and crippling a nuclear power plant.

Akihito said he was "deeply concerned" about the "unpredictable" situation at the stricken Fukushima No.1 nuclear power plant, which has been hit by a series of explosions after the quake knocked out reactor cooling systems.

"I sincerely hope that we can keep the situation from getting worse," Akihito said.

Japanese crews grappling with the world's worst nuclear incident since Chernobyl briefly suspended work over fears of an increase in radiation at the plant 250 kilometres (155 miles) northeast of Tokyo.

And already jangled nerves were frayed further by a strong 6.0 magnitude earthquake that swayed buildings in Tokyo.

The official toll of the dead and missing after the quake and tsunami flattened Japan's northeast coast rose to more than 11,000, with 3,676 confirmed killed, police said.

However, after the Tokyo stock exchange's biggest two-day sell-off in 24 years sparked a global market rout, the headline Nikkei share index closed up 5.68 percent on bargain hunting.

The Bank of Japan pumped another 3.5 trillion yen ($43.3 billion) into the financial system, adding to trillions spent this week since the 9.0 magnitude earthquake and towering tsunami crippled a large swathe of the economy.

The evacuation order at the Fukushima nuclear power plant came as a tall white cloud was seen billowing into the sky over the stricken complex.

Earlier, crews at Fukushima -- who have been hailed as heroes -- contended with a new fire and feared damage to the vessel containing one of the plant's six reactor cores.

Japanese military helicopters were due to dump water on the stricken nuclear plant, which has been hit by four explosions and two fires, to help contain the overheating, but were forced back due to radiation, reports said.

The containment vessel around the core of reactor number three may have suffered damage, and the "likeliest possibility" for the white cloud was steam escaping from the vessel, chief government spokesman Yukio Edano said.

Engineers have been desperately battling a feared meltdown at the 40-year-old plant since the earthquake and tsunami knocked out cooling systems and fuel rods began overheating.

But Edano said radiation levels from the plant posed no immediate health threat outside a 20-kilometres exclusion zone that has already been evacuated.

France's Nuclear Safety Authority said the disaster now equated to a six on the seven-point international scale for nuclear accidents, ranking the crisis second only in gravity to the level-seven Chernobyl disaster in 1986.

But Yukiya Amano, the Japanese chief of the International Atomic Energy Agency, insisted Tuesday there was no comparison to the Chernobyl crisis, when radiation spewed across Europe.

The head of the UN's atomic watchdog said that unlike Chernobyl, the Fukushima reactors have primary containment vessels and had also shut down automatically when the earthquake hit, so there was no chain reaction going on.

The full scale of the quake and tsunami was meanwhile becoming clear as more details emerged of the staggering death and devastation in the worst-hit northeast.

"The number of people killed is increasing day by day and we do not know how many people have fallen victim," said the emperor, who is held in deep respect by many Japanese. "I pray for the safety of as many people as possible."

"People are being forced to evacuate in such severe conditions of bitter cold, with shortages of water and fuel... I cannot help praying that rescue work is done swiftly and people's lives get better, even a little."

A spokesman for the Imperial Household Agency said it was the first time the emperor had addressed the nation on television in the wake of a natural disaster.

Millions have been left without water, electricity, fuel or enough food and hundreds of thousands more are homeless, stoically coping with freezing cold and wet conditions in the northeast.

Aomori governor Shingo Mimura said he desperately needed central government assistance to get hold of oil and relief supplies.

"We cannot possibly get out to rescue survivors nor reconstruct the devastated areas without oil," he said.

"There are a variety of problems, such as shortages of water, food and blankets as well as difficulties in delivering supplies," added Ryu Matsumoto, state minister in charge of disaster management.

Tokyo Electric Power Co said three-hour power outages Wednesday would affect 10.89 million households.

With nerves on edge across the world's third-biggest economy and beyond, people across Asia have been stripping shelves of essentials for fear of a major emission of radiation from the power plant on the east coast.

The Japanese government has warned that panic buying in towns and cities that have not been directly affected by the twin disasters could hurt its ability to provide aid to the devastated areas.

The normally heaving streets and subways of Tokyo were quieter than usual on Wednesday morning. The number of people sporting paper face masks has shot up, although the masks offer no real protection against radiation.

Radiation levels in the capital's vast urban sprawl of 30 million people have see-sawed without ever reaching harmful levels, according to the government.

But it has warned people living up to 10 kilometres (six miles) beyond a 20-kilometre exclusion zone around the Fukushima plant to stay indoors. More than 200,000 people have already been evacuated from the zone.

President Barack Obama, who has dispatched a naval flotilla led by a US aircraft carrier to aid in the quake-tsunami rescue operation, said he was "deeply worried" about the potential human cost of the disaster in Japan.

The government in Tokyo said it was ready to draft in more help from the 49,000 US military personnel stationed in Japan.

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