French regulator puts ITER tokamak welding on hold

March 3, 2022, 7:01AMNuclear News
Approval from French regulator ASN is required before ITER vacuum vessel welding can begin. (Photo: ITER)

In a February 28 article posted on the ITER Organization website, Gilles Perrier, head of ITER’s Safety and Quality Department, addressed the decision by French nuclear safety regulator ASN (Autorité de sûreté nucléaire) to delay the anticipated February 1 release of a preset tokamak assembly “hold point.”

ASN has requested clarifications and information from ITER Organization staff, including additional modeling and analysis on specific safety-related issues. Following receipt of the material, ASN will permit welding of the first sections of the vacuum vessel for what is planned as the world’s largest magnetic confinement fusion device.

“ITER takes these ASN concerns very seriously,” Perrier wrote. “Experts are vigorously preparing the additional information and analysis required, with the goal of providing key answers to the ASN during the second quarter of 2022.” Perrier added that ITER director general Bernard Bigot “is hopeful that this will allow ASN sufficient time to consider the additional information and release the hold point without impacting the ITER assembly schedule.”

Planned hold points: In 2012, ASN validated the proposed ITER design and signed a decree authorizing the international fusion project. ASN has put certain “hold points” in place as part of the regulatory process, requiring ITER to demonstrate that its safety-relevant buildings, civil structures, systems, and components conform to the approved design. For a hold point to be lifted, ITER must demonstrate the associated safety elements and receive ASN's approval.

The hold point under review was established by ASN in November 2013. “Under this hold point, ITER cannot begin to weld the first two sections of the vacuum vessel together in the Tokamak pit—an assembly step considered irreversible—until certain safety aspects related to the B2 slab (a 120-meter-long, 80-meter-wide, and more than 1.5-meter-thick mass of reinforced concrete supporting the Tokamak Complex Building) have been validated,” Perrier explained. “First, ITER must demonstrate the as-built safety performance of this B2 slab. Second, ITER's radiological maps, which calculate the shielding effects of concrete and steel barriers that contain the radiation from the machine, must demonstrate that radiation levels will be safe wherever humans are present without requiring extra shielding material.”

Specific issues: “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,” Perrier said.

Status and schedule: According to Perrier, the ASN action has not affected the ITER assembly schedule to date. The first full sector subassembly—including the double-walled vacuum vessel sector, the thermal shield, and two toroidal field coils—has been completed and will be transferred to the tokamak pit in the coming weeks, and the fabrication of the second subassembly has begun. “It will take several months to complete, and still more time before it is inserted into the tokamak pit and preparations can be made for welding,” Perrier said.

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