Work is underway to address defects which were identified in two key tokamak components during assembly of the ITER fusion reactor in France. In a recent interview with AFP ITER director general Pietro Barabaschi said these problems would result in more delays to the project. ITER's previous target was to create the plasma by 2025 but Barabaschi said it would have to be postponed. The two problems were incorrect sizes for the joints of blocks to be welded together for the installation's 19 x 11 metre chamber. The second was traces of corrosion in a thermal shield. However, he points out that such challenges are common to every industrial venture involving first-of-a-kind components. "There is no scandal here," he noted. "Such things happen. I've seen many issues of the kind, and much worse."
Currently, in the Cryostat Workshop, under a vast plastic tent at Saint-Paul-les-Durance in southern France., workers in protective gear are cleaning the surface of a thermal shield panel that has been stripped of its cooling pipes, with the last traces of welds removed. In the Assembly Hall, other workers are assembling the platform used to support the 440-tonne vacuum vessel sectors during the preparation phase prior to their installation in vertical tooling. These activities are part of a programme to define a repair strategy for the Tokamak's thermal shield panels and vacuum vessel sectors.
The cooling pipes that are placed along the surface of the vacuum vessel thermal shield will be removed and replaced Following months of discussion, tests and analysis, a clear picture of the repairs to be executed is now emerging, ITER says. The defects identified in the two components will be fixed. “In-house and external experts have identified the causes and explored different repair options and strategies. Although details remain to be worked out with the contractors that will be chosen to execute the repairs, a clear picture is now emerging.”
The issue with the thermal shield panels is one of "stress corrosion cracking" that is affecting the cooling pipes welded to the component surface. In the course of the manufacturing process, chlorine residues were trapped in tiny pockets near the welds, causing cracks up to 2.2 millimetres deep. The problem was identified on three, yet-uninstalled vacuum vessel thermal shield panels.
The decision was taken to remove and replace all 23 kilometres of cooling pipes from the panels and to choose a solution that would exclude the risk of stress corrosion cracking. However this raised the question of whether removal of the pipes would alter the panels' dimensions, rigidity, or their capacity to accommodate a new set of piping. This is now being investigated.
As for the pipes and their attachment process, different solutions have been explored. "We looked at quite a number of different options," explains ITER Tokamak Assembly Specialist Brian Macklin. "In pipe fabrication, we will use a different steel formulation—more resistant to corrosion. For joining the pipes to the panels we contemplated using clamps rather than welds, but concluded that welding was still best, provided we use a lower-energy process and slightly different welding wire."
Vacuum vessel module No 1 (7), installed in one of the twin sector sub-assembly tools (SSAT-1) is being disassembled in order to prepare for repairs on both the thermal shield segments and vacuum vessel field joints. The toroidal field coils have been rotated out of the preassembly and, by April, all thermal shield segments will have been removed to allow repairs to be executed. Meanwhile, vacuum vessel sector No 8 will be extracted from the vertical tooling and moved to another building to be repaired in a horizontal position. Finally, vacuum vessel module No 6 will be lifted out of the Tokamak pit in late May/early June and installed in the vacated SSAT-2.
"We hope to launch the tender in the first week of February and have a contractor by the end of March, which is very fast by our standards," says Macklin. Discussions with the selected industrial partner will determine whether the components are repaired on site or in an outside facility. According to Barabaschi, there is also a strong possibility that "a few" panels will be re-manufactured. "This option might actually be cheaper than repairing them," he notes, "and in any case I prefer to have a few spares."
ITER management assumes that the crack issue in the cooling pipes could be systemic, affecting all thermal shield components—including the lower cryostat thermal shield, which has already been installed. Due to the difficulty of lifting the component back out of the crowded environment of the assembly pit, it has been decided to leave the original cooling network in place, disconnect it, and attach a new set of pipes to replace it.
The other component issue relates to the dimensional non-conformities discovered in the three vacuum vessel sectors that have already been delivered. (One of these sectors forms the core of the "module" which is already in place in the assembly pit.) The massive D-shaped vacuum vessel sectors are formed from four segments welded together. It was during the welding process that deviations from nominal dimensions occurred in different locations on the sectors' outer shells. These deviations were more important than the specified limit and modified the geometry of the field joints where the sectors are to be welded together in the Tokamak pit, thus compromising the access and operation of the bespoke automated welding tools.
Schematically, valleys must be filled and hills must be shaved in order to restore the geometry to nominal and recover the ability to weld the sectors together. "Machining is best; depositing metal is a bit more risky," says Macklin. "We will need to combine both options, and we are tailoring the solution as best as we can. Repairs will only start once the build-up process has been fully qualified." The quantity of filler material anticipated is approximately 73 kilos for sector No 6 (the one already installed in the assembly pit), 100 kilos for sector No1 (7) and 400 kilos for sector No 8, the most affected of the three.
In view of the importance of the vacuum vessel as the ITER installation's first nuclear safety barrier, an approved notified body, acting on behalf on the French nuclear regulator ASN (Autorité de sûreté nucléaire), is closely involved in the technical discussions on repair options. The weld deposition process is currently being qualified, and the deposited material will be subject to 100% non-destructive examination by either radiography or ultrasonic testing.
The duration and cost of the repairs cannot, at this stage, be precisely estimated. "A substantial amount of time is required, and time always costs money," says Barabaschi. "But we can take advantage of the present situation to reorganise and increase the experimental content of the operational phase and reach full-power operation—our real and final objective—with minimum delay."
The Director of Russia’s ITER Centre Private Institution (part of Rosatom), Anatoly Krasilnikov, says there is no need to dramatise the situation. "This is normal for such a large and unique object. Technical difficulties have arisen before.” Key decisions on scheduling will be made by the ITER council, which meets twice a year, he adds, noting that the situation will finally become clear only by the end of 2023 or in 2024.
