Europe's scientists hope to mimic the power of the sun and create limitless energy on Earth with the help of a £6 billion (US$10.2 billion) experiment in the south of France.
Ministers in Brussels gave the go-ahead on Wednesday for Iter, the world's biggest and most ambitious fusion reactor, at Cadarache near Aix-en-Provence. It will be 10 years in the making and, in its 20-year operating life, researchers will experiment with a kind of slow hydrogen bomb in the hope of extracting vast amounts of clean energy from tiny amounts of heavy water.
Iter will replace Jet, the joint European fusion research project based at Culham in Oxfordshire, north-west of London.
Chris Llewellyn-Smith, head of the UK program, said on Wednesday: "The Iter project will allow a major step towards an inexhaustible source of environmentally friendly power."
Petroleum and coal deliver chemical energy liberated by the breaking of chemical bonds in the form of fire. Nuclear fission of enriched uranium exploits the energy released by the breakdown of a unstable heavy atom to a lighter one. But the "ash" from a fission reaction is radioactive and it stays too hot to handle for thousands of years.
The great prize has always been fusion power: the fusion of two hydrogen atoms to make one of helium, releasing huge quantities of heat. Every second, the sun converts 600 million tonnes of hydrogen into helium and illuminates and warms this planet from 145 million kilometers away.
To do the same on Earth, engineers and physicists have to collect deuterium and tritium -- isotopes of hydrogen -- and heat them to more than 100 million Celsius, many times hotter than the heart of the sun. At these temperatures the heavy hydrogen would become a plasma, a ball of sub-atomic particles which would fuse to become helium and a shower of neutrons and a supply of heat. One kilogram of heavy hydrogen would supply the heat now generated by 10 million kilograms of fossil fuel. There would be no greenhouse gases, no soot, and no long-lived radioactive waste. The oceans contain all the heavy hydrogen such reactors would need.
Fusion power would, in theory, be safe, because the challenge is not to stop a fusion reaction, but to keep it going. But that is the catch. If plasma at that temperature so much as touched anything, it would go out like a light.
The trick is to keep tiny pellets of fuel suspended in a kind of magnetic "bottle" in a sealed chamber.
Then engineers would have to pump blasts of laser fire at the pellets, compressing them to 20 times the density of lead, at which point they would start to behave like tiny stars, releasing a thermonuclear blast of neutrons to heat up a containment wall many meters away.
Fusion's most ardent enthusiasts believe that a viable power plant is 30 years away. Iter is just another stage in the research.
Although the Cadarache site has Brussels' backing, the decision has yet to be confirmed by the other partners in the project. These include Canada, the US, Russia, Japan, South Korea and China. There is one other candidate site -- at Rokkashomura in Japan -- and the final decision could be made in Washington next month.