BURIED beneath the blizzard of announcements that will pour from the G8 summit will be a much-awaited decision - the winner of the race to build the International Tokamak Experimental Reactor (Iter), one of the world's most sophisticated energy generators.

The pounds 3 billion project is intended to pool the world's best scientific resources to prove once and for all that electricity can be produced by nuclear fusion. More than 60 years have passed since scientists first claimed a clean, safe and inexhaustible form of electricity could be produced by exploiting the same processes that power the sun.

Leading scientists, including the government's chief scientific adviser, David King, believe that within 30 or 40 years Iter could unlock a carbon-free energy future. 'If successful, [Iter] will deliver what could be the world's most important energy source over the next millennium. Its impact will be bigger than landing the first man on the moon,' he said.

The announcement will also finally end the year-long diplomatic tug-of-war over whether the EU or Japan should host the main experimental site, in which the Americans and South Koreans backed Japan's bid, and the Russians and Chinese favoured Cadarache in southern France. France is expected to get the nod.

Nuclear fusion works by heating a large volume of gas, containing deuterium, found in sea water, and tritium, derived from lithium, to 100 million degrees centigrade, 10 times the temperature of the sun.

This causes the atoms to collide and fuse, releasing enormous amounts of energy and leaving only helium as waste. King says that the 'the lithium from one laptop battery and deuterium from a bath of water would generate enough energy to cover the needs of a UK citizen for seven years'.

The only hitch is that the walls of the reactor will become radioactive over time, though the radioactivity would die away within 100 years. The biggest existing fusion reactor is the Joint European Torus (Jet) at the UK Atomic Energy facility at Culham, Oxfordshire. It has produced 16 megawatts of fusion power, though the enormous amounts of energy required to do it means Jet uses more energy than it produces.

Iter, at 10 storeys high, will be twice as big as Jet, but should produce 500 megawatts of power, 10 times its input, the ratio needed for a viable power plant. Professor Sir Chris Llewellyn Smith, director of the Jet project, said last week: 'Iter is almost certainly going to achieve its goal.' He pointed out that a location for the accompanying testing facility, the International Fusion Materials Irradiation Facility, has not been established, but if it is built at the same time, 'in 25 to 30 years we could have prototypes putting electrical power into the grid'.

The first generation of fusion power stations could produce 10 gigawatts by 2050 worldwide and 1 tetrawatt by 2100, several times more than today's conventional nuclear power.

It is a pity that it has taken this long to push ahead with it, he said. 'Jet's been running since 1983. We could have got on with Iter 10 to 15 years ago. But back in the 1980s people were happy to burn coal forever.'

Robert May, president of the Royal Society, says: 'The fact that [fusion] has always been 20 years over the horizon for the last 50 years does not deter me from believing that it is a really important, hopeful thing.'

Professor Ian Fells, chairman of the New and Renewable Energy Centre in Northumberland, and former energy adviser to the European Parliament, lays 50/50 odds on Iter's chances of producing electricity - which he says makes the bet worthwhile.

'The difference between understanding the physics and demonstrating it works and then turning it into a power station is unbelievably difficult,' he warns. He adds that unless Iter can demonstrate early on that it can produce electricity, there is a danger it could be overtaken by a new generation of breeder reactors the Russians have been developing that use uranium 60 times more efficiently than thermonuclear reactors - though with 100 times the plutonium.

Fells believes technologies such as wind power and solar will only ever account for 20 per cent of the global energy mix: 'As the world goes on and global warming takes control we have to do something, and only large-scale carbon-free energy sources such as Iter or the breeder reactors are going to do it.'

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