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KORIYAMA, Japan - Japanese officials warned of a possible second explosion at a nuclear plant crippled by the earthquake and tsunami as they raced to stave off multiple reactor meltdowns, but they provided few details about whether they were making progress.
More than 180,000 people have evacuated the area, and up to 160 may have been exposed to radiation.
More than 3200 people have died or remain unaccounted for in the wake of Friday's tsunami, according to figures confirmed by police.
However, 10,000 of the 17,000 residents remain missing in the destroyed coastal town of Minami-Sanriku.
Japanese Prime Minister Naoto Kan said: "This is the toughest crisis in Japan's 65 years of postwar history", but added, "I'm convinced that we can overcome the crisis."
Head of the Miyagi prefectural police, said in a local disaster task force meeting: "We have no choice but to deal with the situation on the premise that it (the death toll) will undoubtedly be numbered in the ten thousands."
Miyagi is a province in the north-east of Japan. Its capital is the earthquake-devastated city Sendai.
Fukushima Daiichi nuclear complex worst hit
Meanwhile four nuclear plants in northeastern Japan have reported damage, but the danger appeared to be greatest at the Fukushima Daiichi nuclear complex, where one explosion occurred Saturday and a second was feared. Operators have lost the ability to cool three reactors at Dai-ichi and three more at another nearby complex using usual procedures, after the quake knocked out power and the tsunami swamped backup generators.
Chief Cabinet Secretary Yukio Edano said overnight that a hydrogen explosion could occur at Daiichi's Unit 3, the latest reactor to face a possible meltdown. That would follow a hydrogen blast Saturday in the plant's Unit 1.
"At the risk of raising further public concern, we cannot rule out the possibility of an explosion," Edano said. "If there is an explosion, however, there would be no significant impact on human health."
Operators have been dumping seawater into units 1 and 3 in a last-ditch measure to cool the reactors. They were getting water into the other four reactors with cooling problems without resorting to corrosive sea water, which likely makes the reactors unusable.
Edano said residents within about 20 kilometres of the Daiichi plant were ordered to evacuate as a precaution, and the radioactivity released into the environment so far was so small it didn't pose any health threats.
Emergency centres set up
Such statements, though, did little to ease public worries.
"First I was worried about the quake," said Kenji Koshiba, a construction worker who lives near the plant. "Now I'm worried about radiation." He spoke at an emergency centre in Koriyama, about 60 kilometres from the most troubled reactors an 190 kilometres north of Tokyo.
At the makeshift centre set up in a gym, a steady flow of people - mostly the elderly, schoolchildren and families with babies - were met by officials wearing helmets, surgical masks and goggles.
About 1,500 people had been scanned for radiation exposure, officials said.
People exposed to radiation
Up to 160 people, including 60 elderly patients and medical staff who had been waiting for evacuation in the nearby town of Futabe, and 100 others evacuating by bus, might have been exposed to radiation, said Ryo Miyake, a spokesman from Japan's nuclear agency. It was unclear whether any cases of exposure had reached dangerous levels.
A foreign ministry official briefing reporters said radiation levels outside the Daiichi plant briefly rose above legal limits, but had since declined significantly.
Edano said none of the Fukushima Daiichi reactors was near the point of complete meltdown, and he was confident of escaping the worst scenarios.
State of emergency called at reactor sites
Officials, though, have declared states of emergency at the six reactors where cooling systems were down - three at Daiichi and three at the nearby Fukushima Daini complex.
The International Atomic Energy Agency said a state of emergency was also declared Sunday at another complex, the Onagawa power plant, after higher-than-permitted levels of radiation were measured there. However investigations at the site indicate that no emissions of radioactivity have occurred from any of the three units at Onagawa.
Japanese officials believe the increased levels may have been due to the release of radioactive material from the Fukushima Daiichi nuclear power plant.
A pump for the cooling system at yet another nuclear complex, the Tokai Dai-Ni plant, also failed after Friday's quake but a second pump operated normally as did the reactor, said the utility, the Japan Atomic Power Co. It did not explain why it did not announce the incident until Sunday.
Edano denied there had been a meltdown in the Fukushima Daiichi complex, but other officials said the situation was not so clear.
Hidehiko Nishiyama, a senior official of the Economy, Trade and Industry Ministry, indicated the reactor core in Unit 3 had melted partially, telling a news conference, "I don't think the fuel rods themselves have been spared damage," according to the Kyodo News agency.
Worst case scenario
A complete meltdown - the melting of the radioactive core - could release uranium and dangerous contaminants into the environment and pose major, widespread health risks.
The steel reactor vessel could melt or break from the heat and pressure. A concrete platform underneath the reactor is supposed to catch the molten metal and nuclear fuel, but the intensely hot material could set off a massive explosion if water has collected on the platform. Radioactive material also could be released into the ground if the platform fails.
The explosion that destroyed the walls and ceiling of Dai-chi Unit 1`s containment building was much less serious that a meltdown would be - in fact, it was operators' efforts to avoid a meltdown that caused it.
Officials vented steam from the reactor to reduce pressure, and were aware that there was an explosion risk because the steam contained hydrogen, said Shinji Kinjo, spokesman for the government's Nuclear and Industrial Safety Agency. The explosion occurred when hydrogen reacted with oxygen outside the reactor.
It is unclear how far the impact of a meltdown might reach. In the United States, local communities plan for evacuation typically within 16km of a nuclear plant. However, states must be ready to cope with contamination of food and water as far as 80km away. Radioactivity can also be carried to faraway places by the winds, as it was in the 1986 Chernobyl disaster, though it will become increasingly diffuse. Acute radiation deaths would normally be expected only much closer to the plant.
The reactor that exploded at Chernobyl, sending a cloud of radiation over much of Europe, was not housed in a sealed container as those at Dai-ichi are. The Japanese reactors also do not use graphite, which burned for several days at Chernobyl.
9.0 earthquake, up to 10,000 missing
Japan's nuclear crisis was triggered by twin disasters on Friday, when an 9-magnitude earthquake, the most powerful in the country's recorded history, was followed by a tsunami that savaged its northeastern coast with breathtaking speed and power.
More than 1,500 people have now been confirmed dead and hundreds more were missing, according to officials, but police in one of the worst-hit areas estimated the toll there alone was more than 10,000.
All of the reactors in the region shut down automatically when the earthquake hit. But with backup power supplies also failing, shutting down the reactors is just the beginning of the problem, scientists said.
"You need to get rid of the heat," said Friedrich Steinhaeusler, a professor of physics and biophysics at Salzburg University and an adviser to the Austrian government on nuclear issues. "You are basically putting the lid down on a pot that is boiling."
"They have a window of opportunity where they can do a lot," he said, such as using sea water as an emergency coolant. But if the heat is not brought down, the cascading problems can eventually be impossible to control. "This isn't something that will happen in a few hours. It's days."
Japan's nuclear safety agency said 1,450 workers were at the Daiichi plant on Sunday, its usual staffing. The workers were in protective gear and were taking shorter turns than usual in units 1 and 3 to limit their exposure, agency spokesman Yoshihiro Sugiyama said.
The emergencies at the nuclear plants have led to an electricity shortage in Japan, where nearly 2 million households were without power Sunday. Starting Monday, power will be rationed with rolling blackouts in several cities, including Tokyo.
Japan has a total of 55 reactors spread across 17 complexes nationwide.
- AP
