By Alister Doyle, Environment Correspondent
OSLO (Reuters) - Winds are set to blow low-level radiation from Japan's quake-crippled nuclear power plant out over the Pacific Ocean in coming hours, easing health worries after drifting toward Tokyo early on Tuesday, experts said.
After the more serious 1986 Chernobyl disaster in the Soviet Union, radiation blew around the northern hemisphere in about three weeks. One U.N. study said Chernobyl may eventually cause up to 9,000 deaths, mainly from extra cancers near the plant.
"The cloud is going in the direction of Tokyo for the next 15 to 20 hours or so," said Gerhard Wotawa, of the Austrian weather service ZAMG who is advising the International Atomic Energy Agency (IAEA).
"Then it will go out toward the Pacific," he told Reuters. "At some point this will also spread around the world." With prevailing winds, Canada and the United States might be first to detect a much diluted cloud.
New explosions on Tuesday at the Fukushima plant, 240 km (180 miles) north of the capital, released low levels of radiation, worsening the crisis caused by Japan's worst quake on record on Friday which triggered a devastating tsunami. More than 10,000 are feared dead.
Japan told the U.N. nuclear watchdog a spent fuel storage "pond" was on fire and radioactivity was being released directly into the atmosphere. Prime Minister Naoto Kan urged people within 30 kms of the plant to stay indoors.
In Geneva, the World Meteorological Organization (WMO) set up extra monitoring of satellite and other data.
"At this point, all the meteorological conditions are offshore so there are no implications, for Japan or other countries near Japan," Maryam Golnaraghi, chief of WMO's disaster risk reduction division, told a briefing.
The 1986 Chernobyl explosion blew large amounts of radioactive material high into the atmosphere, where it was more easily carried in the jet stream, experts say. Japan's radiation releases have been smaller and lower in the atmosphere.
Experts disagree sharply about the risks facing people near the plant or downwind of it.
"This is only the beginning, the worst is yet to come," said Sebastian Pflugbeil, head of the private German-based Society for Radiation Protection. "It is hard to say if this will be worse than Chernobyl -- much depends on unknowns and what happens in the coming days."
Professor Malcolm Sperrin, Director Of Medical Physics And Clinical Engineering, Royal Berkshire Hospital, told reporters in London: "This is not a Chernobyl, this is absolutely not a Chernobyl event."
In the worst case, in which an explosion breaches the container housing a reactor, radioactive material could be blown up to about 20 kms, the limit of the exclusion zone, he said.
"It will fall down to earth inside that exclusion zone. So anyone walking and working inside that exclusion zone will have to be very carefully monitored, it will be a significant issue, and outside that zone even if it's extended that contour of risk will be very well controlled," he said.
Assuming that leaks began on Saturday when the first explosion occurred, Wotawa said the low level radiation would have been carried over Japan and toward the Pacific, rather than toward countries such as China and Russia.
The World Health Organization estimated in 2006 that up to 9,000 people could eventually die, in Ukraine, Belarus and Russia, from the Chernobyl disaster. The environmental group Greenpeace disputes that figure, projecting 93,000 cancer deaths.
That huge discrepancy shows the difficulty of estimating the health risks. Kyodo news agency reported that radiation levels in Maebashi, 100 km north of Tokyo, were 10 times normal.
"That's a high level but it's not that dangerous," said Ingar Amundsen, of the Norwegian Radiation Protection Agency.
He said that rules for nuclear transport, for instance, allowed radiation of 50 to 100 times background levels around containers.
(Additional reporting by Stephanie Nebehay in Geneva, Janet Lawrence, Gerard Wynn, Ben Hirschler in London, Elaine Lies in Tokyo; editing by Tim Pearce)