The European Union has chosen France as its preferred location for a nuclear reactor that scientists hope will revolutionise world power production.
ITER - NUCLEAR FUSION PROJECT
Project estimated to cost 10bn euros and will run for 35 years
It will produce the first sustained fusion reactions
Final stage before full prototype of commercial reactor is built
It will cost billions to build the fusion machine which releases energy in a similar way to the Sun's furnaces.
Scientists say the new reactor will be the first to give out a lot more power than it consumes on initial ignition.
International partners in the immense engineering project include Canada, the US, China, Japan, Russia and Korea.
A final decision on the siting of the International Thermonuclear Experimental Reactor (Iter) should come in December at a meeting of officials involved in its planning.
The EU candidate, Cadarache, in southeastern France, is likely face stiff competition from Rokkasho in Japan.
The plant, wherever it is constructed, is expected to generate thousands of jobs.
Spain had initially put forward its own choice of Vandellos but then fell in line with its EU partners when research ministers agreed it could host the administrative headquarters for the European arm of the Iter project.
Europe believes it stands a good chance of hosting the fusion plant.
A recent report, chaired by Sir David King, chief scientific adviser to the UK Government, said "either (European) site would be likely to win the international site selection".
The Iter project is the latest stage in the decades-long quest to develop fusion power.
In conventional nuclear power plants, heavy atoms are split to release energy. But in a fusion reactor, energy is harnessed by forcing the nuclei of light atoms together - the same process that takes place at the core of the Sun and makes it shine.
Advocates say commercial fusion plants of the future could be cheap to run and environmentally friendly, with much less radioactive waste produced.
However, developing the necessary technology is proving very expensive and time-consuming.
To use fusion reactions as an energy source, it is necessary to heat a gas to temperatures exceeding 100 million Celsius - many times hotter than the centre of the Sun. At these temperatures, the gas becomes a plasma.
The plasma particles, from deuterium and tritium, then fuse to form helium and high speed neutrons.
A commercial power station will use the heat generated by the energetic neutrons, slowed down by a blanket of denser material (lithium), to generate electricity.
All of this is a huge engineering challenge, however. The only way to contain a super-hot plasma is with magnetic fields. No material vessel could do the job, and it is partly the design of these magnets that has taken so much work.
On the other hand, the fuels used are virtually inexhaustible.
Deuterium and tritium are both isotopes of hydrogen. Deuterium is extracted from water and tritium is manufactured from a light metal, lithium, which is found all over the world.
One kilogram would produce the same amount of energy as 10,000,000 kilograms of fossil fuel.
Iter would be the world's largest international cooperative research and development project after the International Space Station.
Its goal will be to produce 500 megawatts of fusion power for 500 seconds or longer during each individual fusion experiment and in doing so demonstrate essential technologies for a commercial reactor.