He said: "We are talking about a unique project for this planet. The danger linked to the site of the accident is not confined to Ukraine's borders."

Various projects have been proposed since the mid-1990s to replace the "sarcophagus" hastily erected by workers and troops over the reactor in the weeks and months following the April 26, 1986, fire and explosion at the plant.

But agreement on financing, overseen by the European Bank for Reconstruction and Development, was only announced last month.

Dismantling

Donors, mostly foreign governments, have so far contributed 739 million euros.

The new "tomb" will take the form of an arch 105m high, 150m long and 260m across. It will be built onsite and then slid over the fourth reactor, providing conditions for the complete dismantling of its nuclear inventory, most of which is still inside.

The second facility is to house more than 20,000 spent fuel assemblies used by the other three reactors during the plant's 23 years in operation before it was shut down in 2000.

Kris Singh, president of Holtec International, said the site could ultimately house spent fuel from some of Ukraine's 16 working reactors if authorities approved such a move.

Stagnation

Yves-Thibault de Silguy, president of the Vinci firm, which has a stake in Novarka with Bouygues, said the deals would give a green light for plans to rejuvenate the nuclear industry after 20 years of stagnation attributed partly to Chernobyl.

"I am convinced that the resumption of the new development of nuclear plants in the world depends on finding a solution to the Chernobyl case," he said. "And today we have a very important signal for the world."

Estimates of the number of deaths linked to the Chernobyl accident vary widely.

The World Health Organisation puts the number at 9,000, while the environmental group Greenpeace predicts an eventual death toll of 93,000.

About 200,000 residents were evacuated from Ukraine alone, though the accident hit neighbouring Belarus especially hard. Experts are still studying the long-term health effects.