One of the cliches of nuclear power research is that a commercial fusion reactor is only ever a few decades away – and always will be. So claims that the technology is on the “brink of being realised” by scientists at the Massachusetts Institute of Technology and a private company should be viewed sceptically. The MIT-led team say they have the “science, speed and scale” for a viable fusion reactor and believe it could be up and running within 15 years, just in time to combat climate change. The MIT scientists are all serious people and perhaps they are within spitting distance of one of science’s holy grails. But no one should hold their breath.

Fusion technology promises an inexhaustible supply of clean, safe power. If it all sounds too good to be true, that’s because it is. For decades scientists struggled to recreate a working sun in their laboratories – little surprise perhaps as they were attempting to fuse atomic nuclei in a superheated soup. Commercial fusion remains a dream. Yet in recent years the impossible became merely improbable and then, it felt almost overnight, technically feasible. For the last decade there has been a flurry of interest –and not a little incredulity –about claims, often made by companies backed by billionaires and run by bold physicists, that market-ready fusion reactors were just around the corner.

There are reasons to want to believe that fusion will one day be powering our lives. The main fuel is a heavy isotope of hydrogen called deuterium which can be extracted from water and therefore is in limitless supply – unlike the uranium used in nuclear fission reactors. But fusion’s science is tricky and the breakthroughs rare. So far there has been no nuclear fusion reaction that has been triggered, continued and self-sustained. Neither has the plasma soup that exists at temperatures found in the stars been magnetically contained. Nor has any research group sparked a fusion reaction that has released more energy than it consumed, one of the main attractions of the technology. Perhaps the most successful fusion reactor has been the JET experiment, so far Europe’s largest fusion device, which ended up in the UK after the SAS stormed a hijacked German airliner in 1977 and Bonn backed the then prime minister Jim Callaghan’s request to host it. JET hasn’t even managed to break even, energy-wise. Its best ever result, in 1997, remains the gold standard for fusion power – but it achieved just 16 MW of output for 25 MW of input.

Hopes for fusion now rest with the International Thermonuclear Experimental Reactor (Iter), a multi-national $20bn effort in France to show that the science can be made to work. Within a decade Iter aims to control a hydrogen bomb-sized atomic reaction for a few minutes. It is a vast undertaking. At its heart is a doughnut-shaped device known as a tokamak that weighs as much as three Eiffel towers. Iter’s size raises a question of how large a “carbon footprint” the site will leave. Like JET, Iter uses a fusion fuel which is a 50-50 mixture of deuterium and a rare hydrogen isotope known as tritium. To make Iter self-sustaining it will have to prove that tritium can be “bred”, a not inconsiderable feat. Iter will also test how “clean” a technology fusion really is. About 80% of a fusion reaction’s energy is released as subatomic particles known as neutrons, which will smash into the exposed reactor components and leave tonnes of radioactive waste. Just how much will be crucial in assessing whether fusion is a dirty process or not.

Iter’s worth is that it is a facility in the real world, where fusion’s promise can be tested. If it turns out to be better than expected then private investment is going to be needed to commercialise a fusion reactor. If it falls short then there must be a realistic rethink of fusion’s potential. After all, the money that has been poured into it could have been spent on cheap solar technology which would allow humanity to be powered by a fusion reactor that’s 150m kilometres away, called the sun.