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Problems will require in-depth examination and ‘time and budget’ to repair October 2022 file photo of the 30-metre-deep pit in the tokamak building being prepared for the Iter machine itself. Courtesy Iter. Defects have been identified in two key First-of-a-kind tokamak components for the International Thermonuclear Experimental Reactor (Iter) nuclear fusion plant under construction at Cadarache in southern France, with the €20bn project facing potential delays while repairs are carried out.

Iter said in a project update that the two components are the vacuum vessel thermal shields and the vacuum vessel sectors.

The issues “demand in-depth examination, creativity in devising corrective actions, and time and budget to repair”, Iter said.

The vacuum vessel thermal shields are actively cooled silver-plated elements, 20 mm thick that contribute to thermally insulating the plant’s superconducting magnet system operating at 4K, or minus 269C.

Iter said that in November 2021 helium tests detected a leak on an element of the vacuum vessel thermal weld that had been delivered in 2020. The cause was found to be stress caused by the bending and welding of the cooling fluid pipes to the thermal shield panels “compounded by a slow chemical reaction due to the presence of chlorine residues in some small areas near the pipe welds”.

This had caused stress corrosion cracking and over time, cracks up to 2.2 mm deep had developed in the pipes”, Iter said.

‘It Is Happening At A Moment We Can Fix It’

The issue with the vacuum vessel sector is related to four individual segments that were welded together deviations from nominal dimensions that were “more substantial than the specified limit in different locations on the component’s outer shell”, Iter said. “These dimensional non-conformities modified the geometry of the field joints where the sectors are to be welded together, thus compromising the access and operation of the bespoke automated welding tools”.

Iter director-general Pietro Barabaschi, who took up the post in September following the death of his predecessor Bernard Bigot, said if there is one good thing about this situation, “it is that it is happening at a moment we can fix it”.

He said: “The know-how we are acquiring in dealing with Iter’s first-of-a-kind components will serve others when they launch their own fusion ventures. It is in Iter’s nature and mission, as a unique and ambitious research infrastructure, to go through a whole range of challenges and setbacks during construction.”

Iter said that when building a machine as large and as complex as Iter, difficulties and setbacks “do not come as surprises” – they are “an integral part of manufacturing, assembling and installing first-of-a-kind components”.

Most Complex Engineering Endeavour In History

In July 2020, the Iter project – the biggest of its kind in the world – began its five-year assembly phase. Millions of components will be used to assemble the giant reactor, which will weigh 23,000 tonnes and the project is the most complex engineering endeavour in history. Almost 3,000 tonnes of superconducting magnets, some heavier than a jumbo jet, will be connected by 200km of superconducting cables, all kept at minus-269 Celsius by the world’s largest cryogenic plant.

Europe is contributing almost half of the cost of Iter’s construction. The other members of the venture – the UK, China, India, Japan, South Korea, Russia and the US – are contributing the rest equally.

The fusion industry is essentially trying to replicate on Earth the forces that power the sun, potentially producing limitless low-carbon energy. At extreme temperatures and pressures, atoms collide and “fuse” releasing huge amounts of energy in the process. That means that even small amounts of fuel have a huge amount of intrinsic energy.

In this image of a vacuum vessel module, the thin thermal shield and its cooling pipes are visible, sandwiched between the vacuum vessel proper (right) and one of the associated toroidal field coils (left). Courtesy Iter.

Date: Thursday, 24 November 2022
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