Stress corrosion cracking and welding nonconformities are behind ITER’s fresh delays

December 5, 2022, 3:00PMNuclear News
A total of about 23 kilometers (about 14 miles) of piping are welded to the surface of the thermal shield panels. The piping on a vacuum vessel thermal shield panel is clearly visible in this photo. (Photo: ITER Organization)

The ITER Organization is working on a new baseline schedule for the magnetic confinement fusion experiment launched in 1985 and now under construction in southern France. First plasma was scheduled for December 2025 and deuterium-tritium operations for 2035 under a schedule approved in November 2016 that will soon be shelved. In addition to impacts from COVID-19 delays and uncertainty resulting from Russia’s war in Ukraine, ITER leaders must now factor in repair time for “component challenges.”

A November 21 press release issued by the ITER Organization frames it this way: “From the first stages of fabrication to the final insertion in the tokamak pit, component challenges are a constant and familiar companion. Sometimes, however, in the midst of ordinary, almost daily issues, a concern of a larger dimension arises—one which demands in-depth examination, creativity in devising corrective actions, and time and budget to repair. Two-and-a-half years into its machine assembly phase, ITER is facing a concern of this nature: defects have been identified in two key tokamak components, the thermal shields and the vacuum vessel sectors.”

Leaking shields: The vacuum vessel thermal shields and the cryostat thermal shields are both actively cooled silver-plated elements with intricate piping to carry helium to help insulate the superconducting magnet system. Each of ITER’s nine vacuum vessel modules—one of which has already been installed in the machine assembly pit—has a thermal shield panel positioned between the vacuum vessel sector and the two associated toroidal field coils. The circular thermal shield, about 20 meters in diameter, also lines the inner wall of the cryostat as a heat barrier.

During helium tests in November 2021, staff detected leaks on an element of the vacuum vessel thermal shield. According to the ITER Organization, “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 residues in some small areas near the pipe welds.”

The working groups concluded that the thermal shield panels were cleaned with hydrochloric acid before receiving their silver plating and then coated with nickel. During that process, chlorine residues were trapped near the pipe welds. The resulting stress corrosion cracking left cracks up to 2.2 millimeters deep in the pipes, according to the ITER Organization.

Investigative techniques revealed cracks in thermal shield cooling pipes such as the ones pictured here. At left, the crack is 1.5 mm deep. At right, it is 2.2 mm deep and crosses the full width of the pipe. (Image: ITER Organization)

Welding-induced nonconformities: A separate problem was caused by “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 three vacuum vessel sectors that have already been delivered to the ITER construction site, the welding of the component's 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 nonconformities 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.”

Early in 2022, the French nuclear safety regulator ASN (Autorité de sûreté nucléaire) effectively put ITER tokamak welding on hold by delaying the anticipated February 1 release of a preset tokamak assembly “hold point.” Vacuum vessel nonconformities were among the concerns raised at that time. According to Gilles Perrier, head of ITER’s Safety and Quality Department, “ASN has asked for more analysis to validate the vacuum vessel welding process to account for some limited dimensional nonconformities the ITER Organization has identified at the interface between the two vacuum vessel sectors now on the ITER site, which require adjustments to the robotic welding procedure.”

Mitigation: “The risk is too high, and the consequences of a leaking thermal shield panel during operation are too dire. We must assume the problem is extensive,” said ITER director general Pietro Barabaschi. “Dealing with it in the pit on the module that has already been assembled would be enormously difficult. This means we have to lift out the installed module and disassemble it in order to proceed with the repairs. We are exploring different possibilities, from on-site repair to remanufacturing in an outside facility, possibly with different pipe attachment options. But there is no question about the need to replace the cooling pipes.

“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,” Barabaschi added. “If there is one ‘good thing’ about this situation, it is that it is happening at a moment we can fix it.”

The ITER Organization’s November 21 announcement of those repair plans followed a November 16–17 meeting of the ITER Council convened to review the performance of the ITER project. The 31st first meeting of the ITER Council was the first meeting led by Barabaschi, who was selected as the new director-general of the ITER Organization in September after the death in May of Bernard Bigot, the previous director general.

According to a November 17 press release issued after the meeting, “The director general has started an assessment of the ITER Project, which has identified several areas to address further.” Despite ongoing construction, deliveries, and installations, “The council noted the need to address a number of concerns related to first-of-a-kind components. . . . The council urged the director general to assess the impacts of these setbacks and commence the necessary repair work as soon as possible. In addition, the council urged the ITER Organization and domestic agencies to work together to ensure an appropriate project-wide quality culture to prevent any recurrence of such issues.”

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