After a series of set backs the international project is back on track, say scientists, giving tentative hope for a major new source of clean power by 2025

An international project to generate energy from nuclear fusion has reached a key milestone, with half of the infrastructure required now built.

Bernard Bigot, the director-general of the International Thermonuclear Experimental Reactor (Iter), the main facility of which is based in southern France, said the completion of half of the project meant the effort was back on track, after a series of difficulties. This would mean that power could be produced from the experimental site from 2025.

Q&A What is nuclear fusion and how will it work in the Iter project? Show Hide The International Thermonuclear Experimental Reactor (Iter) aims to replicate the workings of our solar system’s chief source of energy: the sun. Nuclear fusion reactions taking place billions of times a second at the sun’s core generate enough energy to power the Earth many times over, and within the last century the secrets governing such power have been uncovered by physicists. What physicists have been unable to do so far, is to replicate the extreme conditions needed for the reactions to take place, in such a way that their energy could be harnessed usefully on Earth. Doing so requires absolute control over conditions including: a structure that can contain the reactions - in the case of Iter, a doughnut-shaped vessel that can divert the volatile reactions away from the metal casing; superconducting magnets to draw the plasma into the areas required; temperatures in the millions of degrees; and a method of generating usable electricity from the raw energy unleashed.

The scientists involved say they are now halfway through creating the structure required, and should be able to provide proof that their methods work by the end of 2025.

The effort to bring nuclear fusion power closer to operation is backed by some of the world’s biggest developed and emerging economies, including the EU, the US, China, India, Japan, Korea and Russia. However, a review of the long-running project in 2013 found problems with its running and organisation. This led to the appointment of Bigot, and a reorganisation that subsequent reviews have broadly endorsed.

Fusion power is one of the most sought-after technological goals in the pursuit of clean energy. Nuclear fusion is the natural phenomenon that powers the sun, converting hydrogen into helium atoms through a process that occurs at extreme temperatures.

Replicating that process on earth at sufficient scale could unleash more energy than is likely to be needed by humanity, but the problem is creating the extreme conditions necessary for such reactions to occur, harnessing the resulting energy in a useful way, and controlling the reactions once they have been induced.

For these reasons, fusion power was long ago abandoned by nuclear physicists as a potential source of commercial energy in favour of fission reactors, using processes by which radioactive materials release energy as they are induced to decay.

The Iter project aims to use hydrogen fusion, controlled by large superconducting magnets, to produce massive heat energy which would drive turbines – in a similar way to the coal-fired and gas-fired power stations of today – that would produce electricity. This would produce power free from carbon emissions, and potentially at low cost, if the technology can be made to work at a large scale.

For instance, according to Iter scientists, an amount of hydrogen the size of a pineapple could be used to produce as much energy as 10,000 tonnes of coal.

However, while fusion has been the subject of intermittent scientific research projects since the 1940s, no means has yet been found to make it work in controllable conditions at the scale needed to produce the power of a fossil fuel power station. The extreme conditions driving the sun require temperatures of millions of degrees, and mimicking those conditions has proved elusive.

Iter has been described as the most complex scientific endeavour in human history. The project requires hydrogen plasma to be heated to 150 million C – 10 times hotter than at the sun’s core. A doughnut-shaped reactor called a tokamak would be surrounded by giant magnets that take the superheated plasma away from the metal walls of the container. This requires the magnets to be cooled to -269C.

Getting to this stage of the project has already required remarkable feats of engineering, such as the manufacturer of more than 100,000km of niobium-tin superconducting strands of metal, produced by nine suppliers over seven years, to make the magnets.

Bigot said the milestone of building half of the project indicated that the rest of the project was now technically feasible.

However, there are still political difficulties. One is that US president Donald Trump’s administration is notably cooler on clean energy research than its predecessor, and the US budgetary contribution, of nearly 10%, or more than $1bn, is now partially in doubt. The EU is providing 45% of the cost of the project, with the rest provided by the other main partners.

Bigot, currently in Washington DC holding meetings to try to break the impasse, told the Guardian he was confident the conflict could be resolved.

He said the project was on track to reach “first plasma” in December 2025, proving the concept. If successful, the Iter machine would produce 500MW of power, enough to study a self-heating plasma, which has never been produced in a controlled way on earth. That could in turn lead to the development of power plants to harness the energy produced.

Google is also now working on fusion power, in the first major private sector try-out of the technology. This summer, the internet giant announced it had formed a partnership with Tri Alpha Energy, backed by Microsoft co-founder Paul Allen, to generate new computer algorithms that could help test the concepts behind nuclear fusion engineering efforts.