An idyllic hilltop setting in the Cadarache forest of Provence in the south of France has become the site of an ambitious attempt to harness the nuclear power of the sun and stars.

It is the place where 34 nations representing more than half the world’s population have joined forces in the biggest scientific collaboration on the planet – only the International Space Station is bigger.

The international nuclear fusion project – known as Iter, meaning “the way” in Latin – is designed to demonstrate a new kind of nuclear reactor capable of producing unlimited supplies of cheap, clean, safe and sustainable electricity from atomic fusion.

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If Iter demonstrates that it is possible to build commercially-viable fusion reactors then it could become the experiment that saved the world in a century threatened by climate change and an expected three-fold increase in global energy demand.

This week the project gained final approval for the design of the most technically challenging component – the fusion reactor’s “blanket” that will handle the super-heated nuclear fuel.

The building site in Cadarache has also passed the crucial stage where some 493 seismic bearings – giant concrete and rubber plinths – have been set into the reactor’s deep foundations to protect against possible earthquakes.

Peering over the edge of the huge seismic isolation pit, it is still possible to see some of these bearings before they are covered with a raft of reinforced concrete that will support the massive fusion machine at the heart of the £13bn Iter project.

Click here to see how the Iter Project could produce clean energy

Over the next few years about a million individual components of the highly complex fusion reactor will arrive at the Cadarache site from around the world. They will be assembled like a giant Lego model in a nearby building which has a volume equal to 81 Olympic-sized swimming pools.

Nothing is left to chance in a project that has defied potential Babel-like misunderstandings between the collaborating nations. The design, development and construction of a machine that will attempt to emulate the nuclear fusion reactions of the Sun is proving to be a triumph of diplomacy, as well as science and engineering.

“It is the largest scientific collaboration in the world. In fact, the project is so complex we even had to invent our own currency – known as the Iter Unit of Account – to decide how each country pays its share,” says Carlos Alejaldre, Iter’s deputy director responsible for safety.

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“We’ve passed from the design stage to being a construction project. We will have to show it is safe. If we cannot convince the public that this is safe, I don’t think nuclear fusion will be developed anywhere in the world,” Dr Alejaldre said.

“A Fukushima-like accident is impossible at Iter because the fusion reaction is fundamentally safe. Any disturbance from ideal conditions and the reaction will stop. A runaway nuclear reaction and a core meltdown are simply not possible,” he said.

Conventional nuclear power produces energy by atomic fission – the splitting of the heavy atoms of uranium fuel. This experimental reactor attempts to fuse together the light atoms of hydrogen isotopes and, in the process, to liberate virtually unlimited supplies of clean, safe and sustainable energy.

Nuclear fusion has been a dream since the start of the atomic age. Unlike conventional nuclear-fission power plants, fusion reactors do not produce high-level radioactive waste, cannot be used for military purposes and essentially burn non-toxic fuel derived from water.

Many energy experts believe that nuclear fusion is the only serious, environmentally-friendly way of reliably producing “base-load” electricity 24/7. It is, they argue, the only way of generating industrial-scale quantities of electricity night and day without relying on carbon-intensive fossil fuels or dangerous and dirty conventional nuclear power.

However, the daunting complexity of the Iter project is demonstrated by how long it has taken to reach this early stage of construction – and how much further it still has to go. There is at least another decade of building work and a further decade of testing before the reactor will be allowed to “go nuclear”.

“Every single stage is inspected. Even the specially-prepared concrete cannot be mixed unless a nuclear safety inspector is present. If anything goes wrong with Iter, fusion will be dead,” said a spokesperson for the project.

The roots of the Iter project go back to 1985 when Mikhail Gorbachev, General Secretary of the former Soviet Union, offered his country’s prowess in nuclear fusion as a bargaining chip in the nuclear disarmament talks with the US, which at that time was pursuing its “Stars Wars” defence system.

Gorbachev and President Reagan, with the support of Margaret Thatcher and French President François Mitterand, signed an agreement to cooperate on nuclear fusion using the Russian “tokamak” reactor. This was a revolutionary device that could hold the super-hot fusion fuel by creating a “magnetic bottle” within the reactor’s doughnut-shaped vacuum vessel.

Several experimental tokamak reactors around the world, including one at the Culham Centre for Fusion Energy in Oxfordshire, have shown nuclear fusion is theoretically possible, but the giant tokamak at Iter will be the first to generate more power than it needs to attain the very high temperatures required for nuclear fusion.

The Iter tokamak machine, which is twice the linear size and 10 times the volume of its nearest rival at Culham, will produce temperatures of well over 100 million C – many times hotter than the centre of the Sun.

It is the first experimental fusion reactor to receive a nuclear operating licence because of its power-generating capacity. For every 50 megawatts of electricity it uses, it should generate up to 500mw of power output in the form of heat.

Richard Pitts, a British nuclear physicist working on the project, said that even though Iter has a nuclear operator’s licence and will produce about 10 times as much power as it consumes, the Iter machine will still remain a purely experimental reactor, with no electricity generated for the French national grid. “We’re not building a demonstration industrial reactor. We’re building the first step towards one that does produce electricity for the grid. If we can show that fusion works, a demonstration reactor will be much cheaper to build than Iter,” Dr Pitts said.

A critical phase of the project will be the injection of plasma – the superhot, electrically-charged gases of the atomic fuel – into the reactor’s vacuum chamber. This plasma, a mix of the hydrogen isotopes deuterium and tritium, will drive the nuclear-fusion reaction.

The plasma will be heated to temperatures as high as 300 million C to force the atomic nuclei close enough together to cause them to fuse into helium, a harmless and inert waste product that could be recycled as an important industrial raw material. Giant electromagnets powerful enough to trap an aircraft carrier will contain the plasma within a spinning vortex held by the magnetic bottle of the tokamak reactor.

The original date for “first plasma” was scheduled for November 2020 but delays with the construction and commissioning phases have pushed this back to October 2022 – although some of that lost time has since been clawed back. One of the electromagnetic coils used in the giant magnets, for instance, had to be scrapped after a worker in one of the participating countries left a towel on one of the superconducting cables which then became compressed within a coil. Costly mishaps like this put the entire project behind schedule.

Rem Haange, deputy director-general of the Iter project, said that despite the delays, which are perhaps inevitable with such a huge and complex engineering project, no further problems are envisaged that could threaten the viability of the Iter project. “There are no technical issues any more that will be show-stoppers. We think we’ve overcome all the technical issues,” Dr Haange said.

Although the foundations for the main reactor building are still being laid, there has been a lot of development work off-site in the different member nations – the EU, Russia, US, China, Japan, India and South Korea. More than 90 per cent of the Iter machine’s engineering components, for instance, have now been commissioned.

These components, some the size of small houses, will be shipped by road and sea to Cadarache in the coming years, and the task of putting them together into a working machine will be formidable. Iter will have enough superconducting cabling, for instance, to wrap around the Earth 15 times.

“There are a million parts to the Iter machine and this will be the most complex and technically challenging assembly task. The tokamak reactor is 30 metres tall and consists of 18 toroidal magnetic coils weighing hundreds of tons that will each have to be positioned with a precision of less than two millimetres,” said Brain Machlin, head of Iter’s assembly operation.

As the components of the tokamak arrive in the coming years, Iter engineers will be holding their breaths to make sure the parts fit together perfectly. But even if “first plasma” happens within the next 10 years, it will still be another five years or more before they have the confidence to put radioactive tritium fuel into the vacuum vessel – and go nuclear.

Even if everything goes to plan, the first demonstration power plant using nuclear fusion will not be ready until at least the 2030s, meaning commercial reactors could not realistically be built until the second half of the century.

The long timescales mean nuclear fusion does not often get on the political agenda, unless superpower summitry is involve as it was at the height of the Cold War in 1985. But in the end, the long wait for nuclear fusion, and the experiment to save the world, may prove to be well worth the effort.

Timeline: Chain reaction

1929: Scientists use Einstein’s equation E=mc² to predict release of large amounts of energy by fusing atomic nuclei together.

1939: German-born physicist Hans Bethe, pictured, demonstrates that nuclear fusion powers stars.

1950: Andrei Sakharov and Igor Tamm in the USSR propose a “tokamak” fusion reactor.

1956: Tokamak programme begins in strict secrecy.

1969: Tokamak results declassified, astounding Western scientists.

1973: Design work begins on Joint European Torus (Jet), a tokamak-type reactor in Europe.

1983: Jet completed at Culham, Oxfordshire, on time and to budget.

1985: USSR proposes an international fusion-energy project.

1988: Design work begins for International Thermonuclear Experimental Reactor, later known as simply Iter. 1992: Design phase begins for Iter.

1997: Jet produces 16 megawatts of fusion power, the current world record.

2005: Cadarache, France, chosen as Iter site.

2021-22: “First plasma” scheduled, when ionised gases will be injected into the Iter tokamak.

2027-28: Iter “goes nuclear” with injection of tritium.

2030s: First demonstration fusion reactor to produce electricity for grid.

2050s onwards: First commercial nuclear fusion power plants.