By Alister Doyle and Fredrik Dahl
OSLO/VIENNA (Reuters) - Any Israeli attacks on Iran's nuclear facilities are unlikely to cause a Fukushima-scale disaster unless a Russian-built reactor is destroyed, experts say.
They could, however, release toxic chemicals - rather than high levels of radiation - causing local contamination affecting health and the environment. That was also the case from U.S.-led strikes on nuclear facilities in Iraq during the Gulf Wars.
"I doubt that the radiation effects would be great," said Hans Blix, a former head of U.N. nuclear watchdog the International Atomic Energy Agency (IAEA).
Iran says all its facilities are for peaceful purposes. Israel, which in 1981 bombed Iraq's Osirak nuclear reactor before it came online, has led international accusations that Tehran is secretly developing atomic bombs.
"There could be some chemical hazard (from an Israeli attack on Iran's uranium refining plants) but I'd think it would be limited to any nearby communities," said Edwin Lyman, a nuclear expert at the Union of Concerned Scientists in Washington.
The Vienna-based IAEA and Iran failed on Friday to strike a deal aimed at allaying concerns about Tehran's nuclear program. Diplomatic sources say Iran has installed many more uranium enrichment centrifuges at Fordow, a fortified underground site and a likely target in any attack.
Bellicose rhetoric from some Israeli politicians has fanned speculation that Israel might hit Iran's nuclear sites before the November U.S. presidential vote. Washington has said there is still time for diplomatic pressure to work, but it might be drawn into any war between the two Middle East foes.
Most experts contacted said that Israel would probably not target the Bushehr nuclear reactor on Iran's Gulf coast, which started providing electricity to the grid last September. Such an attack could release a Fukushima-style radioactive plume that could spread to the entire region - including Israel.
"An attack against Bushehr nuclear power plant would probably be a violation of international law," Blix said.
Attacks on Iran's other nuclear sites - such as the Natanz and Fordow enrichment plants and a uranium conversion facility east of the city of Isfahan - may have a localized health and environmental impact on a similar scale caused by the bombing of Iraqi nuclear sites Tuwaitha and Al Qaim in the Gulf Wars.
"Uranium is a very heavy metal, chemically and physically," so it would not be transported far on the wind if Iranian enrichment facilities were attacked, said Malcolm Grimston, of Imperial College, London.
"It is about as poisonous as lead ... the issue would be in the immediate area trying to prevent people from ingesting it for its chemical poisonous properties," he said.
Uranium before it is introduced into a nuclear power plant is also much less radioactive than fissile reactor material.
"It is not like a reactor where you got the volatile fission products - the iodines and caesiums - which can be carried in principle all around the world by wind," Grimston said.
Iraqi plants have not become global bywords for disaster, unlike the 1986 Chernobyl reactor explosion in what is now Ukraine and the Fukushima Daiichi plant in Japan that suffered a meltdown after an earthquake and tsunami last year.
"The health effects (in Iraq) were very localized," said Robert Kelley, of the Stockholm International Peace Research Institute and a former director of IAEA inspections in Iraq.
Others say health risks linger in Iraq, and estimates of long-term health risks near the sites are difficult because of a lack of monitoring of cancer rates.
"In Tuwaitha, they have never seen full decontamination," said Mike Townsley of environmental group Greenpeace. He and colleagues found a ruptured container of raw uranium "yellowcake" near the plant in 2003.
About 1,000 people live near the Tuwaitha reactor complex south of Baghdad, the former site of Saddam Hussein's nuclear research program destroyed by U.S.-led forces in 1991 and 2003. Al Qaim, where uranium was extracted at a fertilizer factory, was bombed in 1991.
Paul Sullivan, a professor of economics and adjunct professor of security studies at Georgetown University in the United States, said there were double standards in judging risks.
"If there were a chance of an attack on such facilities in France, Germany, the U.S., Japan and the like there would be constant and very loud cries about the potential environmental and human health impacts," he said.
Iran says it needs to refine uranium as a fuel for nuclear power. But extra refinement can make uranium for a bomb.
The other main way to build a bomb is to use plutonium, from the waste of spent nuclear fuel rods from power plants. But experts say Bushehr is ill-designed for such uses, and that would also require a separate reprocessing plant.
"Iran's plutonium program is thought to be less advanced than its uranium program," said Karl Dewey, a nuclear analyst at IHS Jane's in England.
Any attack on Bushehr, perhaps to cripple nearby buildings without rupturing the reactor, would involve big risks, he said.
The extent of the fallout from any strike on the reactor would depend on what capacity level it had operated on and for how long, experts say. An IAEA report in May said it operated at 75 percent of its power after being shut down in January.
Israel would probably want to destroy the Arak heavy water research reactor, which is not yet online but which experts say is more suited to producing plutonium than Bushehr.
The United Nations said in 2005 that the main impact of the 1986 disaster at Chernobyl would be up to 4,000 thyroid cancer deaths. About 30 people died at the plant, mainly from radiation exposure. Some environmentalists project far more deaths.
A Stanford University study in July estimated that radiation from Fukushima Daiichi might eventually cause anywhere from 15 to 1,300 deaths.
Radiation poisoning can cause nausea, vomiting, diarrhea, headaches and fatigue in lower doses. In bigger doses, it can cause burns, hemorrhages, cancer and death.
Radiation can also damage plants and animals, poisoning food for human consumption. A type of butterfly near Fukushima has been found with high rates of mutation, such as deformed wings and eyes.
Part of the risks of enrichment is that the process involves heating uranium to a gas form, Dewey said.
The process frees uranium hexafluoride (UF6), which is both toxic and radioactive and can cause kidney damage. When UF6 comes in contact with moisture it converts to uranyl fluoride and toxic hydrofluoric acid, in a gas form.
Among accidents, in 1986 the rupture of a cylinder at a uranium enrichment facility run by Sequoyah Fuels Corp. in the United States released a cloud of UF6, killing one worker and injuring 31 others. None of the 31 suffered lasting kidney damage.
(Additional reporting by Marcus George in Dubai; Editing by Pravin Char)