The world’s biggest nuclear fusion project has entered its five-year assembly phase.
After this is finished, the facility will be able to start generating the super-hot “plasma” required for fusion power.
The £18.2bn (€20bn; $23.5bn) facility has been under construction in Cadarache, southern France.
Advocates say fusion could be a source of clean, unlimited power that would help tackle the climate crisis.
Iter is a collaboration between China, the European Union, India, Japan, South Korea, Russia and the US. All members share in the cost of construction.
Current nuclear energy relies on fission, where a heavy chemical element is split to produce lighter ones.
Nuclear fusion, on the other hand, works by combining two light elements to make a heavier one.
This releases vast amounts of energy with very little radioactivity.
Iter will confine hot plasma within a structure called a tokamak in order to control fusion reactions.
The project will aim to help demonstrate whether fusion can be commercially viable. France’s President Emmanuel Macron said the effort would unite countries around a common good.
President Macron said: “Iter is clearly an act of confidence in the future. The greatest advances in history have always proceeded from daring bets, from journeys fraught with difficulty.
“At the start it always seems that the obstacles will be greater than the will to create and progress. Iter belongs to this spirit of discovery, of ambition, with the idea that, thanks to science, tomorrow may indeed be better than yesterday.”
But fusion power has its sceptics. Making it commercially viable has been difficult because scientists have struggled to get enough energy out of the reactions.
Advocates believe Iter can overcome the technical hurdles and that, given the planetary challenges being faced, fusion is worth the expense and effort.
The UK is a member of the Iter project but is set to fall out, because the British government withdrew from a key treaty as part of Brexit. The UK could stay only if a new way is found to maintain its involvement by the end of the Brexit transition.
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