International negotiations are under way to choose a site for the world's first experimental nuclear fusion reactor. This would harness energy created in the same way that the sun radiates heat and light.
A shortlist of four sites - in France, Spain, Canada and Japan - has been drawn up and assessed by the specialists for the project, ITER, named after "the way" in Latin.
Member governments approved further talks at a meeting last month in St Petersburg, Russia, where the US government formally announced that it wanted to rejoin the $5 billion (£3.14 billion) programme.
Washington withdrew from the global scheme in 1998, balking at its then $10 billion cost. But the remaining members - the European Union, Russia, Japan and Canada - have since slashed this by half and the US has informally offered to provide 10 per cent of the funds.
The US's decision has been welcomed by EU research commissioner Philippe Busquin. He said: "This is now a truly global project," noting that China, too, told the meeting that it wanted to join the international fusion consortium. Construction of the reactor could begin in 2006 and it could be operational by 2014.
Further negotiations will be held in Vienna from May 20-21, when governments will try to agree what costs they will shoulder, depending on the choice of the site.
The four sites on the shortlist are Clarington, Ontario, in Canada; Cadarache, Provence, in France; Vandellos, Catalonia, in Spain; and, Rokkasho-mura in northern Honshu, Japan.