France's nuclear fusion reactor. Credit:ITER When ITER hums up to full power in 20 years time, a few grams of gas will swirl into an airless magnetic cage inside a huge metal doughnut, heat up to 150 million degrees (ten times hotter than the centre of the sun), and fuse together. At that moment, that doughnut will either be the world's first working nuclear fusion power generator, or the biggest white elephant in the history of science. If it's the former – as those who are building it insist it will be - it could change the global economy and geopolitics as we know it. The people who work here, true believers all, will tell you nuclear fusion will end wars, solve global warming and make you a nice cup of tea afterwards.

A fusion generator cannot – simply cannot - melt down like the current nuclear fission reactors. There is no chance of a Fusion Chernobyl or Fusion Fukushima. Its fuel is 'heavy hydrogen', plentiful in seawater (seawater!) and a few kilograms could power a small city for a year. Fusion emits no greenhouse gases. Its waste is balloon-ready helium. It sounds like science fiction because it is. Most sci-fi worlds run on fusion - except the dystopias. Iron Man has an ITER in his chest. Iron Man. Credit:Paramount Pictures But generating power from fusion is the hardest problem science and engineering have ever tackled. We've been trying for 70 years. As one scientist tells me, the hardest problem before this was flight, and "we made flight happen by having a good look at birds". But the only fusion generators in the universe are stars. We can't use nature's cheat sheet.

And that's not even the biggest problem ITER faces. This project has teetered on the precipice of failure many times from sheer managerial incompetence, the bickering of nations and a disconnect between the dreams of scientists and the practicalities of building a bloody big, expensive, nuclear power plant. It's a money sponge with a timeline that recedes like a man chasing his shadow. At one point, an international observer joked they'd make better progress just by doing nothing, because each year of work pushed the 'switch-on' date back more than a year. And now Australia is on board. At the end of September ITER signed a deal with Australia's nuclear science research organisation ANSTO, under which Australian researchers will take part in the project (Australia didn't pay for the privilege). Welcome aboard. Get ready for the ride. In a way we were already there. The Scottish burr of the head of the science program at ITER, David Campbell, disguises 14 years in Sydney at high school and university – and his wife is Australian.

Now 63, he's been on the ITER project for 20 years. He will be long-retired when they turn it on – though he hopes to be at the party. But, he says, this is common to fusion researchers – "you have to try and do your bit and then hand over to the next generation". Campbell is a hardcore physics boffin, not a salesman. Over an hour and a half I learn a lot about 'plasma confinement parameters', about the tricky art of keeping the hottest place in the universe stable, about his career-long battle with the substance he is trying to tame: not smooth and cooperative but a turbulent, writhing beast that flings itself against its cage. nick miller story on nuclear fusion.? We know fusion works, Campbell says: "It's the process that powers the sun." It's what makes stars suddenly appear every time a lot of hydrogen draws near. It's implied in Einstein's famous equation. Smoosh two atoms together hard enough, hot enough, and they fuse into a new element just a little lighter than its predecessors' weight combined. That lost weight is multiplied by the speed of light squared into a massive amount of energy.

That's the principle. The trick is the smooshing. You must somehow confine heat that would melt any material container. Stars use gravity. On Earth, the method ITER will use (there are others, but none so advanced) was invented by the Soviets in the 1950s. The fuel gas – deuterium – is put inside a metal tube called a Tokamak and powerful magnetic fields squeeze it into a plasma where fusion happens. From there it's standard power generation: heat, water, turbines. Smaller Tokamaks worked well enough. But at that scale the extra bits and pieces – power and cooling for the superconductor magnets, mostly – cost more energy than the fusion generated. Fusion was a lossmaker, and has stubbornly remained so for 70 years. ITER, however, should (around 2035, after many, cautious, incremental tests) put out 500 megawatts, ten times the power input. That's proof of concept for a power plant, and the rest is economics – does it pay its way (it should, the fuel is dirt cheap) and recoup its cost (maybe more dicey).

Campbell is "pretty confident" ITER will do what it says on the tin. Fifty years of engineering R&D are in ITER. Many, many problems were solved to get here. They know how to build a Tokamak. And they've finally cracked the physics. "There's lots of things going on in these plasmas," he says. It's so hard exactly because the payoff is so big: "you've got free energy", Campbell says. Picture a snooker ball balanced on top of a pointy mountain. The slightest nudge and that ball goes hurtling downward. Free energy. The plasmas in a Tokamak misbehave. The edge of the plasma flares out, scarring the generator's interior. Or the whole plasma goes unstable, what's known as a 'disruption' – suddenly dropping from 100 million degrees to 'just' 10,000, emitting a tsunami of heat and magnetic energy. Smaller Tokamaks have literally jumped in the air. ITER could melt from the inside out.

Plasma, like the blood it's named after, is a mixture of two fluids. And when you have fluids you have turbulence, which is one of the least-understood phenomena in the physical world. Famed physicist Werner Heisenberg started his career trying to figure out turbulence, then gave up and invented quantum mechanics instead. According to a (possibly apocryphal) story he said when he died: "I would ask God two questions: 'Why quantum mechanics, and why turbulence?' I think he will have an answer for the former." But while turbulence can't quite, yet, be solved with maths, it can be managed. Containers can be modelled and iterated (think a winged keel in a water tank). The plasma's electromagnetic cage can be flexed, folded or spiked within microseconds to counter instabilities, or at least mitigate their effects. This is where the Australians can help. The team at ANU have a new way to monitor the flow of plasma in great detail, to help spot problems. "It's a very novel technique and it's something we've wanted to do for a long time," Campbell says. "We've not been able to find another way of doing it, so the collaboration promises to be very profitable. It's a very neat technique, very creative physics, and it looks like it really does work." Despite the optimism and excitement of the scientists, ITER has nearly failed already – for reasons far from physics.

Johannes Schwemmer, director of F4E, the European Union organisation responsible for Europe's huge contribution to ITER, smiles when I ask him if he's happy with progress, and gives a very German reply. "What is happy? I mean the project is in dire straits and we are improving tremendously. Should I be happy? "I would say it's fun to fix it but we also have some way to go. So I think it has passed through its worst time and deepest period of despair. But there is still some way to go." Another person close to the project tells me that "for years we saw very little progress ... We saw a flat (site) and the cranes stopped moving at 3 in the afternoon. We became extremely concerned." Before 2013 the management were "constantly perfecting the machine … and never getting anywhere", they say. "Fusion's always been a hard sell because it tends to be expensive. (ITER) is between 10 and 25 billion dollars."

Another problem, they say, was a crippling project structure. ITER sets the design but the seven partners, Europe, Japan, the US, Russia, China, India and Korea bear the cost. Each design change triggered six months of haggling over the bill. The very internationalism that made the project possible was making it unworkable. But under new director-general Bernard Bigot, ITER is coming back on track. There is a reserve fund for design tweaks. The project has a timeline that it now even sometimes beats. nick miller story on nuclear fusion.? "We could not do it in a rush, we need to do it very seriously," Bigot says. Bigot has the look of an absent-minded professor but the neat, orthogonal piles of papers in his office speak to a precise, ordered mind.

He believes fusion can play the role of reliable, baseline power alongside renewables as the energy source of the indefinite future. "This is a unique opportunity to demonstrate the potential of fusion as an alternative for long-term energy supply. "We need to drastically reduce the amount of fossil fuels we use to avoid spoiling the climate, the environment. We need to have a clear answer (on fusion) before the century is over. So that is why, despite the cost, (the partners) are considering it is still a very worthwhile project to be in." Vladimir Vlasenkov is a cheeky, personable Russian scientist, now 75, who played a key role in developing Tokamak technology in the USSR. He's now observing ITER on behalf of his government. "It can possibly change the world but it turned out it's not very easy," Vlasenkov says, proud, grinning. "I thought that as soon as we have other countries in the world, as soon as we combined our efforts everything would be easy.

"But they bring not only their money, but their problems." He is confident the science is sound. Quietly, passionately, he hopes ITER will generate power. "If it works it brings a lot to mankind," he says. "We eliminate our main fights for oil and gas and so on – and this is the key point for our military interaction in the world now. So we can eliminate that." ITER has always been about more than science. It was born in a nuclear disarmament deal between Reagan and Gorbachev. It has survived its members fighting wars hot, cold and tepid. In last year's nuclear pact with Iran, potential ITER membership was a 'carrot' that helped seal the deal. Inside the international nuclear fusion project ITER. Magnet construction. Photo: Nick Miller Credit:Nick Miller

"This has to be about idealism," says Steven Cowley, one of fusion's biggest experts and advocates. "Towards the end of the century we don't know where the hell our energy's going to come from, and while renewables will do some of that they won't do all of that. "We can't go on burning fossil fuels until we cook everybody on the planet." But beyond that, he says, "it's fun" "When we started (on fusion) after the Second World War … I think we didn't understand quite how hard it was going to be. Finally after 60, 70 years of trying we are there. "Creating a plasma that's burning for the first time in this device will be one for the history books. When ITER comes online all the world's attention will be on it. It's the most important experiment of the 21st century."

On my way out of ITER, the automatic doors open without question, and close neatly behind me.