Media playback is unsupported on your device Media caption Paul Henley visits the building site in Cadarache, southern France

The International Thermonuclear Experimental Reactor (Iter) is no quick fix for the world's energy problems. Not even its staunchest supporters would argue that.

The holy grail of carbon-neutral energy production - fusing, as opposed to splitting, atoms - has already been chased for decades by scientists who see its potential.

The current project in Cadarache, in the southern French countryside, is already one of the biggest building sites in Europe and the latest incarnation of a deal signed by Presidents Ronald Reagan and Mikhail Gorbachev in the Cold War days of 1985.

The key problem, however, is in the title.

Iter is an experiment, a gamble. And at a current cost of 15bn euros ($20bn; £13bn), it is a very expensive one.

So eyebrows were raised in Brussels this month, when the go-ahead was given to use EU money to cover Iter's latest overspend of £1.3bn euros.

'Risky project'

Robert Jan Smits, director general of Research and Innovation at the European Commission in Brussels, says he understands the sensitivity.

Image caption David Campbell is undeterred by the prospect of the experiment not working

"I admit it is an unfortunate time to be asking the Council, the member states and the European Parliament for this money," he says.

"In the middle of a big economic crisis, there could not have been a worse moment. But this project is so crucial for Europe and for the whole world that we simply had to bite the bullet and try to find an agreement.

"Of course, it is an extremely risky project and we do not know the outcome. There are a lot of scientists who are 100% convinced that it will work. But it will only be through building the machine and making it operational that we will know.

"But looking at the current and future need for energy, we cannot afford not to explore this option."

'Limitless source'

On the 192ha (474 acres) site in Provence, David Campbell surveys the teams of workers with pride. He is head of physics for Iter.

"I have been working on the design for about 15 years," he says, "so to be here and to see the concrete being finally poured is very exciting."

If this cost is taken from the European budget supporting science, then it is terrible because it is a huge amount and it would cut our budgets drastically Sebastien Balibar, France National Council for Scientific Research

What Iter aims to do is demonstrate that nuclear fusion can eventually be commercially viable.

At no stage will the reactor under construction provide power to the French national grid, or anyone else's. That outcome is an optimistic one and still a long way off.

But for now, there is international co-operation on an almost unprecedented scale. As well as the European Union, Russia, the United States, Japan, China, India and South Korea are all contributing in kind with products and expertise.

Their combined skills are supposed to create a reactor which will heat plasma - a giant flame rotating inside a magnetic field - to a temperature 10 times hotter than the sun.

So far, smaller projects have managed to produce only a fraction of the power that has had to be put in to create the reaction. The difference, at Cadarache, is scale.

Mr Campbell is undeterred by the prospect of his experiment not working.

"It depends on exactly what you mean by 'not working'," he says.

"We have spent the past 30 to 40 years learning about how to produce fusion power. We have already done it on a small scale. So we think we have already got all the elements together.

Image caption There are several approaches to nuclear fusion - but is society investing enough in them?

"The attractive thing about fusion is that there is an almost limitless source of fuel. Both the main ingredients are heavy isotopes of hydrogen bred inside the reactor from lithium. And in the earth's crust there are very extensive reserves of lithium.

"Plus, it is an inherently safe form of energy. If anything goes wrong with the reaction inside the vessel, it shuts down. And there is no long-term nuclear waste produced. It is our aim to build the reactor from materials which can be recycled into more reactors after a hundred years".

Money worries

Anti-nuclear campaigners are quick to dispute his claims that fusion can be risk-free, especially given that radioactive waste will be produced, albeit with a relatively short life of around 150 years.

But even among supporters of conventional nuclear power, there are objections.

Sebastien Balibar, head of research at the France National Council for Scientific Research in Paris, doubts that the requisite technology to make the experiment work is anywhere near existing.

"I agree that to control fusion is an interesting challenge," he says, "but it is a long-term applied research project.

"If indeed it does work then it would provide a big source of clean energy. And if its budget is paid by the industrial sector - the oil companies or by the industry ministries of various countries - then that is perfectly alright.

"15bn euros is actually about the same as the annual profits of one of the big oil companies. But if this cost is taken from the European budget supporting science, then it is terrible because it is a huge amount and it would cut our budgets drastically. And I cannot agree with this," Mr Balibar says.

The momentum of the Iter project is already such, however, that it is likely to outlive its critics.

Indeed, if a fully viable - and commercial - nuclear fusion reactor ever exists, it is questionable whether even its most youthful present-day supporters will still be around to see it.