Image caption Construction of the Iter reactor will take place at Cadarache in Southern France

The European Union and six member states have reached a deal on the financing and timetable for an experimental nuclear fusion reactor.

An explosion in costs had cast a cloud over the International Thermonuclear Experimental Reactor (Iter).

The project, which is to be based in Cadarache in southern France, aims to harness the same physical process that fuels the Sun.

Additional construction funds will have to come from within the EU's budget.

The extra 1.4bn euros will cover a shortfall in building costs in 2012-13.

Delegates agreed that the overall costs of the project will be almost US $21bn, (16bn euros/£13bn), some three times the original price.

They also agreed a timeline that would see the first plasma experiments in 2019, with a fusion reactor generating significantly more power than it consumed by 2026/27.

'Landmark' day

The organisation also appointed a new director general, Professor Osamu Motojima, who said that he believed the new timeline and budget agreed in Cadarache would make fusion a reality.

The end of ITER would not mean the end of plasma physics or fusion research, just the end of that project which was ill conceived Professor Jacques Treiner, Paris University

"Today is a landmark day for the Iter project, now we are moving into the real construction phase," he said.

Four years ago, the EU, Russia, China, India, Japan, Korea and the US picked Cadarache in the south of France as the location for the experiment that aimed to produce energy from the fusing of light atomic elements, the same process that makes the Sun shine.

But since the science of how to achieve this type of fusion hasn't been settled, the plans for the Iter project have been the subject of several revisions in recent years, each one leading to an increased price tag.

Coupled with the increases in costs for raw materials like steel and cement, the budget for the project has spiralled from around 5bn euros to about 16bn euros, as confirmed today.

The EU has agreed to meet a critical short term shortfall of 1.4bn euros by using money that has been allocated to other research programmes.

But the EU has said it will cap its overall contribution to Iter at 6.6bn euros, leaving the fusion project to find cuts in costs of around 600m euros.

"To meet this we need to reduce the construction costs and also reduce some contingency provisions," said Professor Motojima.

"Cost containment is very important. To get understanding from public we need to reduce costs as much as possible; realising this is my largest responsibility," he observed.

"My basic attitude to realise cost containment is to simplify everything, I propose to simplify a new management structure of the organisation."

Professor Motojima agreed that there would be no more bailout in future.

"It's impossible to look for more money. My expectation is that we can do this within our budgets. I have the prospect we can do this, otherwise I wouldn't accept my new responsibility."

The director of research in nuclear energy at the European Commission, Octavio Quintana Trias, suggested that Wednesday marked a landmark for Iter in coming to terms with reality.

"The most important thing is that the designs which were conceptual in 2001, now are much closer to the reality of the industry. I can guarantee nothing but it is less likely that we will have cost surprises when the designs are much closer to the reality than they were ten years ago."

Mr Quintana Trias said that the while Wednesday's meeting was a major step forward for research, a commercial fusion reactor wouldn't happen until 2040 at the earliest.

But despite the large rise in costs, he added that he believed the investment was worth it.

"If you consider the total costs of the machine even in the worst scenario are the bill we pay for energy for one day in the world, then to get a new energy source for this price I think is worth trying."

Cost concerns

In Europe, some scientists are unhappy with the EU proposal to take funds from unspent budgets to bail Iter out.

In France, a group of physicists - including Nobel prize winner Georges Charpak - have written a letter to the press calling Iter a catastrophe and arguing that it should be shut down.

They suggest that making up the shortfall in Iter's budget is costing France alone the equivalent of 20 years investment in physics and biology.

According to one of the signatories, Professor Jacques Treiner from Paris University, it was time to call a halt to Iter before any more money was spent.

"The end of Iter would not mean the end of plasma physics or fusion research, just the end of that project which was ill conceived," Professor Treiner said.

"At a certain point especially when they say they will take money from other fields to fund this one you have to say, really a clear answer and the answer is no, don't do that."