Key components to be repaired

In November 2021, helium tests detected leaks on an element of the vacuum vessel thermal shield that had been delivered a year and a half earlier. Working groups with experts from different ITER partners were rapidly established to investigate, and they were able to identify the root cause for the leaks—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 residues¹ in some small areas near the pipe welds. This reaction had generated what is called "stress corrosion cracking" and, over time, cracks up to 2.2 millimetres deep had developed in the pipes. A crucial question therefore arose: was the issue a one-off problem, limited to the elements examined, or was it systemic, affecting all thermal shield components?

Whereas this thermal shield issue is now thought to be essentially due to a weakness in the design of the cooling pipe attachment, which made it impossible to fully wash away the chloride residue, vacuum vessel sector issues stem from a more common industrial fabrication difficulty: the distortion that invariably occurs in welding processes, compounded by the overall complexity of the ITER vacuum vessel sectors.

In the case of the three vacuum vessel sectors that have already been delivered, the welding of the component's four individual segments caused deviations from nominal dimensions that were more substantial than the specified limit in different locations on the component's outer shell. 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.

Despite ongoing coactivity in the assembly pit's restricted space and particle contamination risks, solutions were initially contemplated to execute repairs in situ, without extracting the vacuum vessel module from its present position. "The thermal shield issue has now changed the perspective," says the ITER Director-General. "As we need to disassemble the module to fix the thermal shield piping, the question of whether or not to repair the vacuum vessel sector in the pit becomes irrelevant. We have no other solution but to remove it."

As repair strategies for both components are being refined, schedule scenarios established, and costs estimated, the present sequence of vacuum vessel assembly is now on hold. "We know what we need to do, we know ways to do it, and we are of course very much aware of the consequences as far as schedule and cost are concerned—and they will not be insignificant."

¹ Prior to receiving their silver plating, thermal shield panels are cleaned with hydrochloric acid and then coated with nickel. It is during this process that chlorine residues were trapped in tiny pockets near the pipe welds and slowly corroded the component's material.