Mr Potocnik (2nd r) was joined by politicians and scientists

In the company of French ministers and fusion scientists, he used his trip to Cadarache to salute the co-operation which made the endeavour possible.

Iter, as the reactor is known, is the most expensive joint scientific venture after the International Space Station.

Europe won the right to host �6.6bn project over tough rival Japan.

"Iter is not just a large international research project but has great importance for this region, for the EU and for the whole world," Mr Potocnik said during his visit.

"We've seen with our visit to Tore-Supra (fusion facility at Cadarache) today how far we've come already in fusion research. I am optimistic that Iter will allow us to go even further down the road to safe, clean, abundant energy."

Clean energy

Cadarache is situated about 60km (40 miles) from Marseille in the Provence-Alpes-Cote d'Azur region. Cadarache currently hosts Tore-Supra, one of the existing European centres for fusion research.

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

Nuclear fusion taps energy from reactions like those that heat the Sun. It is seen as a cleaner approach to power production than nuclear fission and fossil fuels.

Officials from the six-party Iter consortium signed the deal in Moscow on Tuesday last week to site the experimental reactor in southern France. Their announcement ended more than 18 months of discussions and deal-making - in which Mr Potocnik was heavily involved.

The European Union, the United States, Russia, Japan, South Korea and China are partners in the project.

Japan withdrew its bid, after a deal was worked out for the "runner-up" to receive a generous concessions package.

According to the package, Japan will get 20% of the project's 200 research posts while providing only 10% of the expenses, and host a related materials research facility - of which half the construction costs will be shouldered by the EU.

Also present during Mr Potocnik's visit to Cadarache were the French minister for Research, Francois Goulard, former European Commissioner for Research, Philippe Busquin; and former French ministers for research, Francois d'Aubert and Claude Haigner�; as well as the chief administrator of the French Atomic Energy Commission, Alain Bugat; and the High Commissioner for Atomic Energy, Bernard Bigot.