OSLO In the worst case, any radioactive cloud from Japan's damaged nuclear plant is likely to be limited to the densely populated nation -- unlike the wider fallout from the Chernobyl disaster, experts say.
The 1986 blast in then-Soviet Ukraine, when the reactor exploded, contaminated large parts of Europe in the world's worst nuclear disaster. At the Fukushima plant, the explosive potential within the six reactors is easing with time.
"In the worst case, a radioactive cloud would not go that far up in the atmosphere," said Jan Beranek, head of environmental group Greenpeace's International Nuclear Campaign.
"That is good news for the world, but bad news for Japan."
Despite assurances by Japanese authorities about low health risks, the crisis at the Fukushima plant has worsened since Friday's quake-caused tsunami, with desperate, unsuccessful attempts on Wednesday to water-bomb the facility.
"We are at the beginning of the catastrophic phase," Sebastian Pflugbeil, president of the private, German-based Society for Radiation Protection, said of Japan's efforts to pull the Fukushima plant back from the brink.
"Maybe we have to pray," he said, adding that a wind blowing any nuclear fallout east into the Pacific would limit any damage for Japan's 127 million population in case of a meltdown or other releases, for instance from spent fuel storage "ponds."
Japan placed top priority on Wednesday on efforts to cool down a plutonium-fuelled nuclear reactor at Fukushima -- the only one of six not fuelled by less hazardous uranium. Some countries advised their nationals to leave the country.
Many experts expect the outcome to be worse than the partial reactor meltdown at Three Mile Island in the United States in 1979, which caused no widespread health damage, but less severe than Chernobyl.
A U.N. study estimated that there could be 4,000 to 9,000 extra cancer deaths from Chernobyl, but Greenpeace has said that the disaster could cause more than 250,000 cancer cases, including 100,000 fatalities.
"In Chernobyl the whole plant core exploded," said Malcolm Crick, Secretary of the U.N. Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). "But there was also a lot of heat and that lifted it high into the atmosphere."
He said Fukushima was "a serious situation but it's too early to say" what the worst outcome could be.
Malcolm Grimston, a nuclear expert at the Chatham House think-tank in Britain, said Fukushima was not like Chernobyl.
"We're nearly five days after the fission process was stopped, the levels of radioactive iodine will only be about two-thirds of where they were at the start, some of the other very short-lived, very radioactive material will be gone altogether by now," he said.
"The situation may recede or deteriorate and lead to a massive radiation leak to the atmosphere," said Professor Javier Dies, head of Nuclear Engineering at the Polytechnic University of Barcelona. "As things stand, this cannot be ruled out."
Greenpeace's Beranek said that heavy pollution from caesium could make some areas of Japan near the plant uninhabitable, at least for decades, as happened around Chernobyl. Pflugbeil also said some areas might be off-limits.
Laurence G. Williams, Professor of Nuclear Safety at the John Tyndall Institute for Nuclear Research in Britain, said he did not see a Chernobyl-type blast as likely.
"I can't think of anything at the moment that would drive that explosive force," he said. "It will just be a melting, or a degrading, heating up of the fuel which will just crumble into a heap like what happened at Three Mile Island."
Richard Wakeford, of the Dalton Nuclear Institute at the University of Manchester, said in a statement that words like "apocalypse" and "catastrophe" were "utterly inappropriate" about the situation at Fukushima and could cause unnecessary panic.
Leaks from Fukushima have already spread some radiation from the plant, briefly raising levels in Tokyo to 10 times normal levels, but are far below the level of a catastrophic release that would pose a wide threat to human health, experts say.
Crick at UNSCEAR said that long-term exposure after Chernobyl for people living in Ukraine, Belarus and Russia was about 10 millisieverts, the equivalent of the radiation in one CT scan, a special type of x-ray. People pick up about 2.4 millisieverts a year in background radiation.
(Additional reporting by Gerard Wynn in London, Vera Eckert in Frankfurt, Sylvia Westall in Vienna and Martin Roberts in Madrid; editing by Mark Trevelyan)
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