Jacques Repussard, head of France's Institute for Radiological Protection and Nuclear Safety (IRSN), said a radioactive plume would extend from "several dozen kilometres" around the plant to several hundred kilometres "in the coming days".

But he said the plume would have no consequences for health in Tokyo, 250 kilometres to the south-east.

Britain's Chief Scientific Officer, Professor John Beddington, told the British embassy in Tokyo that even in the worst-case scenario, an explosion following a meltdown would only be serious for the local area.

During the meltdown, the nuclear material would fall to the bottom of the containment structure and react with the concrete and other materials.

A man holds his dog as they wait to be scanned for radiation exposure at a temporary scanning centre for residents living close to the quake-damaged Fukushima Dai-ichi nuclear power plant. Photo: AP

"In this reasonable worst case you get an explosion. You get some radioactive material going up to about 500 metres up into the air. Now, that's really serious, but it's serious again for the local area. It's not serious for elsewhere. Even if you get a combination of that explosion it would only have nuclear material going in to the air up to about 500 metres.

"If you then couple that with the worst possible weather situation [that is] prevailing weather taking radioactive material in the direction of Greater Tokyo and you had maybe rainfall, which would bring the radioactive material down, [the question is] do we have a problem? The answer is unequivocally no. Absolutely no issue. The problems are within 30 kilometres of the reactor."

When Chernobyl exploded in 1986, it sent dangerous materials into the upper atmosphere for a long period of time.

A woman holds her dog as they are scanned for radiation near Fukushima Dai-ichi nuclear power plant. Photo: AP

At the Fukushima plant, the explosive potential within the six reactors is easing with time, experts said.

"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."

Professor Beddington said the problems with Chernobyl were exacerbated by people "continuing to drink the water and continuing to eat vegetables".

"When Chernobyl had a massive fire at the graphite core, material was going up not just 500 metres but to 30,000 feet. It was lasting not for the odd hour or so but lasted months, and that was putting nuclear radioactive material up into the upper atmosphere for a very long period of time.

"But even in the case of Chernobyl, the exclusion zone that they had was about 30 kilometres. And in that exclusion zone, outside that, there is no evidence whatsoever to indicate people had problems from the radiation. The problems with Chernobyl were people were continuing to drink the water, continuing to eat vegetables and so on and that was where the problems came from. That's not going to be the case here."

Professor Javier Dies, head of Nuclear Engineering at the Polytechnic University of Barcelona in Spain, said: "The situation may recede or deteriorate and lead to a massive radiation leak to the atmosphere.

"As things stand, this cannot be ruled out."

Malcolm Crick, Secretary of the UN Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) said Fukushima was "a serious situation but it's too early to say" what the worst outcome could be.

A key warning sign would be if plumes of caesium are emitted from the plant, according to Richard Barrett, a former US Nuclear Regulatory Commission member who led the regulator's emergency response centre.

"When caesium comes out, that means your temperatures have reached the 4000- and 5000-degree level," he said.

After that, the next step would produce "transuranics" such as strontium, plutonium and americium.

"If you begin to see those things then it may be an indication that the accident has proceeded to the next step," he said. "The core would have melted through the bottom of the vessel ... and it is on the floor of the reactor cavity."

Long term scenarios

The long-term scenarios are coloured by many unknowns concerning the rate and type of radioactive release.

Mr Repussard, who was speaking at a French parliamentary committee meeting, said there could eventually be a "strongly contaminated zone" extending up to 60 kilometres around Fukushima, beyond which "there will be measurable impacts but not dramatic impacts".

Didier Champion, who overseas environment and intervention issues at the IRSN, said Fukushima would probably generate "more local impacts than were seen for Chernobyl".

"The downside is that contamination is likely to be more concentrated inside a 10- to 20-kilometre zone. The upside is less contamination over a larger area," he said.

Mr Beranek said that heavy pollution from cesium could make some areas of Japan near the plant uninhabitable, at least for decades, as happened around Chernobyl.

Dr Crick 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.

People pick up about 2.4 millisieverts a year in background radiation.

Main sources of contamination

Longevity of highly toxic nuclear waste is a major problem.

The likely contaminants from Fukushima are iodine-131 and caesium-137, with half-lives of eight days and 30 years, respectively.

The half-life is the number of years required for any amount of a radioactive element to decompose by half. Typically, such elements remain hazardous for a period 10 times their half-lives.

Radioactive iodine and caesium are carcinogenic and pose a threat to health, whether directly in the polluted air or water or indirectly through the food chain.

Radioactive iodine is highly volatile and disperses easily into the air in the form of vapour plumes such as those generated by the disabled Fukushima reactors.

It becomes a hazard when it settles on water sources or crops, such as leafy vegetables, that are later ingested by livestock or humans.

Since iodine-131 has a half-life of eight days, it decays completely within a matter of months.

Everyone is exposed to limited, and apparently harmless, levels of caesium-137 as a result of atmospheric testing of nuclear weapons during the 1950s and 1960s, although much of that contamination has since decayed.

The Chernobyl reactor blast also generated caesium pollution.

Tainted soil can become airborne as dust, resulting in exposure to internal organs when breathed in, according to the US Environmental Protection Agency. Drinking the element in contaminated water also exposes living tissue.

In plants, the element is first absorbed by leaves, then roots. It also concentrates easily in mushrooms and wild birds.

Clean-up

Cleaning up radioactive sites is massively costly, time-consuming and dangerous.

Contaminated soil can be removed and buried in lined landfills, but doing so does not eliminate the waste. Incineration in special furnaces renders toxic elements harmless, but is expensive and technically challenging.

Soil can also be "washed" by mixing it with solvents that transform the contaminants into a liquid form that must then be disposed of.

Water can be filtered using synthesised cloth that traps solid particles, but the technique works well only with low levels of contamination.

Demineralisation through electrodialysis - to remove radioactive ions - has also proved effective, as has boiling water to separate out contaminants as solids.

There are several methods for sealing off failed reactors which continue to generate radiation long after they have been shut down.

A concrete and steel sarcophagus was built to entomb the Chernobyl reactor, but the makeshift structure now has cracks and is being replaced.

SOURCES: US Environmental Protection Agency, UN Food and Agriculture Organisation, French Institute for Radiological Protection and Nuclear Safety

AFP, Reuters with Glenda Kwek