Credible estimates of the bill are impossible right now, they cautioned, given that engineers at the Fukushima No. 1 plant are still in emergency mode and data about damage and radiation remain sketchy.

But assuming the plant is stabilised, many years of work lie ahead and the final tab will be billions of dollars, they said.

Never before has any country had to deal with the aftermath of partial meltdowns occurring simultaneously in three reactors and highly radioactive spent fuel rods lying in two crippled cooling tanks.

"We have to assume that Japan will have a long-term issue of managing the impacts," the head of Nuclear Safety Authority (ASN), Andre-Claude Lacoste, said on Monday.

"It's a problem that Japan will have to deal with for decades and decades to come."

David Lochbaum, who worked for 17 years as a nuclear engineer and heads nuclear safety at the Union of Concerned Scientists (UCS), a respected US NGO, said satellite images showed major damage to several reactor buildings but damage to the nuclear fuel inside was unclear.

"That determines how bad or how costly the cleanup will be," he said. "It's going to be high, I just don't know how high."

He observed, though: "They basically had on March 10 a multi-billion-dollar asset -- on March 11 they had a multi-billion-dollar liability."

On the plus side, sources ruled out a comparison with the 1986 Chernobyl disaster, whose impact was worsened because the reactor exploded, showering parts of Belarus, Ukraine and Russia with radioactive debris.

In its first two decades alone, Chernobyl cost "hundreds of billions of dollars", according to a 2005 estimate by the Chernobyl Forum, which gathers seven UN agencies, the World Bank and the governments of Belarus, Russia and Ukraine.

Barring any such catastrophe, the cleanup at Fukushima will be a lot easier than Chernobyl, for the fuel will have stayed contained in the reactor vessels, said Malcolm Grimston at London's Chatham House thinktank.

That makes it more analagous to the 1979 accident at Three Mile Island in Pennsylvania, where there was a partial meltdown -- but where there was no problem from spent fuel rods, nor any major radioactivity release.

"The key issue at Fukushima is the spent fuel rods," Grimston said.

"You take these rods out, put them in functioning fuel ponds for a while to cool, then take them away for reprocessing, and that probably removes well over 99.9 percent of the radioactivity from the site."

He added: "It's easy to remove the spent fuel rods. You can do that with a crane. But the question is whether they have the technology that can protect you from doses (of radiation) while you are removing it, can transport it safely and then have a facility at the site that can deal with it.

"We have no precedents for the cost of something like this."

As for the three damaged reactor cores, which may house molten slag seething with radioactivity, Japan will have two options, both of them massively expensive.

The first is to take the reactors apart and reprocess or store the fuel, a delicate business that took 10 years and cost around a billion dollars in the case of Three Mile Island.

Alternatively, it could build a Chernobyl-style concrete "sarcophagus" to entomb the reactors for effectively forever, or at least until the cores have cooled sufficiently for some future technology to deal with them.

Intended as a temporary structure, the sarcophagus thrown up around Chernobyl is showing dangerous signs of wear.

A second containment vessel, designed to last 100 years, is in the works, at a cost of at least 870 million euros (1.23 billion dollars), of which only a fraction has so far been pledged.

Another long-term question is contamination of land beyond the plant itself.

Radioactive "hotspots" from Chernobyl were found as far as Norway and Ireland, requiring restrictions on milk and beef raised on affected farmland that in some cases lasted decades.

Ukraine keeps a 30-kilometre (18-mile) exclusion zone around Chernobyl in place 25 years after the disaster, and the town of Pripyat, which had a population of 49,500 people before the disaster, remains empty.

Ten towns around Fukushima, lying in a 20-km (12-mile) evacuation zone, have been emptied of some 200,000 people.

Could they, too, become ghost towns?

This will depend on contamination by heavy metals, notably caesium, which was the long-term problem after Chernobyl, said Grimston.

Based on current information, "you could be looking at some contamination up to 10 kilometres (six miles) from the plant, but I don't think you're going to see a series of Pripyats," he said.

"I would think that they (the inhabitants of the evacuation zone) will be back."

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