ITER’s machine assembly phase began about two and a half years ago. Now, staff are reversing some of that assembly work to make needed repairs. According to a news article published by the ITER Organization on January 9, ITER is “facing challenges common to every industrial venture involving first-of-a-kind components.” Over one year after problems were first detected and less than two months after they were made public in late November, tests and analysis are producing a clearer picture of necessary repairs to the tokamak’s thermal shield panels and vacuum vessel sectors.
“There is no scandal here,” said ITER director general Pietro Barabaschi. “Such things happen. I've seen many issues of the kind, and much worse.”
Stress corrosion cracking: ITER leaders are making the assumption that stress corrosion cracking detected on the cooling pipes of three vacuum vessel thermal shield panels that were awaiting installation could be systemic, affecting all thermal shield components—including some that have already been installed.
“When you find three instances of cracks, it is a red alarm because there could be hundreds of locations where cracks could develop,” said Barabaschi.
The cooling pipes are welded to the component surface, and staff determined that chlorine residues left from the manufacturing process were trapped in tiny pockets near the welds, causing cracks up to 2.2 millimeters deep.
It was decided to remove all 23 kilometers—over 14 miles—of cooling pipes from the panels, clean panel surfaces to remove all traces of previous welds, and fabricate new pipes from a more corrosion-resistant steel. Some panels may be remanufactured as well.
“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,” explained ITER tokamak assembly specialist Brian Macklin. “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.”
Nonconformities: A separate problem was detected with ITER’s D-shaped vacuum vessel sectors, which are among the largest components of the ITER machine—as high as a five-story building and formed from four segments welded together. “Dimensional nonconformities” on the sectors’ outer shells created during the welding process and discovered in the three vacuum vessel sectors that have already been delivered to the ITER site have “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,” according to the ITER Organization.
Restoring that geometry will require machining to remove excess material and welding to deposit material to fill what the ITER Organization describes, schematically, as “valleys.” The quantity of filler material anticipated is not insignificant, according to the organization: approximately 73 kilograms for sector 6 (which is already installed in the assembly pit), 100 kilograms for sector 7, and 400 kilograms for sector 8, the most affected of the three.
“Machining is best; depositing metal is a bit more risky,” said 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.” According to the ITER Organization, the weld deposition process is currently being qualified in cooperation with the French nuclear regulator, and the deposited material will be subject to 100 percent nondestructive examination by either radiography or ultrasonic testing.
The plan: Repairing the three steel vacuum vessel sectors will require complex logistics. Sector 8, presently installed in one of the sector subassembly tools, will be lifted out and placed in a horizontal position on the platform currently being assembled at the opposite end of the assembly hall, then moved to another building where repairs will be executed.
Sector 7 will be repaired in the subassembly tool where it now sits, while sector 6 will take the place of sector 8 on a sector subassembly tool after the module to which it is attached is lifted out of the tokamak pit in late May or early June. Sector 6 will have to be disassembled from its toroidal field coils and thermal shield panels before repair, currently planned for late 2023.
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,” said Barabaschi. “But we can take advantage of the present situation to reorganize and increase the experimental content of the operational phase and reach full-power operation—our real and final objective—with minimum delay.”