The blueprint says it all. Pointing at precise locations inside the vacuum vessel sector, hundreds and hundreds of arrows—each with a unique reference number—indicate where the anchoring features for diagnostic sensors, called “bosses,” need to be welded. Down in the tokamak assembly pit, sitting, crouching, or lying flat on the staging platforms inside the vacuum vessel sectors, workers concentrate on their delicate task. One by one, on the inner surface of each sector, they are precisely positioning and welding more than 2,000 bosses for the in-vessel sensors devoted to magnetic field measurement, as well as for sensor cabling. “The magnetic field is the heartbeat of the ITER machine. Monitoring its intensity, geometry and variations is absolutely critical,” explains ITER Magnetic Development Officer Yunxing Ma.Magnetic sensors form “a big family,” with a dozen different types based on five different technologies. Transmitted to the machine’s control systems, the data they collect not only provides precious scientific insight on plasma behaviour, it also enables real-time feedback control of the plasma equilibrium position and the mitigation of plasma disruptions.  One by one, the bosses for magnetic sensors are welded to the vacuum vessel’s inner surface. There are approximately 2,000 such supports per sector. The first phase of diagnostics-related installation activity began five years ago, soon after the first vacuum vessel sector was delivered to ITER. On their outer skin, or “ex-vessel,” every sector on site at ITER (four of which have already been installed in the tokamak pit and two which are being prepared for installation later this year) are now fully equipped with magnetic sensors and instrumentation cables. On their inner skin (“in-vessel”), operations began in January this year. The procedures are the same but the conditions are slightly different: the sectors were resting on the ground in the horizontal position when the ex-vessel components were installed; they are now vertical, inside the tokamak pit, which adds constraints due to restricted workspace and coactivity. Workers, and particularly welders, operate within the constraints of coactivity in a restricted workspace. In order to position each support with the required precision, coordinates are extracted from the machine’s 3D CAD model and fed to a laser pointing device. The laser beam outlines the support’s nominal position, which is marked on the sector’s surface—an innovative technique called “laser templating.” The welder’s skills and experience do the rest and, progressively, small bosses find their assigned place in the dense “in-vessel forest,” amid the much larger support pads onto which the massive shield blocks will be attached. The small bosses for the magnetic sensors progressively find their assigned place in the dense “in-vessel forest,” amid the much larger support pads (yellow) onto which the massive shield blocks will be attached. Once welded, every anchoring feature is submitted to careful endoscopic examination. Beginning in June this year, base plates will be attached to the bosses, followed by connector boxes and eventually by the sensor modules themselves. Phase one of this major operation will last until mid-2027, when all nine sectors are positioned in the tokamak pit. Installation is foreseen to resume in early 2029 (phase two) after all sectors have been welded together.

The X-ray inspection underway on sector #1 is part of a wider set of tools used to verify critical welds. Since 2024, teams at ITER have been repairing the critical “bevel joints” where the vacuum vessel sectors will be joined in the tokamak pit. Once their geometry is restored to nominal by filling the “valleys” or shaving off the “hills,” another task awaits: radiographic testing to ensure the uniformity of the welding and the compliance of the repairs.“We use X-rays to check right down to the heart of these thick metal shells to make sure that the repair is compliant,” says Miguel Dapena-Febrer, ITER’s Radiation Safety Coordinator who oversees the radiographic testing to ensure it is being done in secure conditions in terms of radio protection. “This is a very important process that requires powerful X-ray generators and it needs to be done in very specific conditions.”  Radiographic testing on the vacuum vessel sectors is part of the broader non-destructive testing program to verify welding and other mechanical interventions. The radiographic testing program began in 2016 on the cryostat base and today there are multiple radiographic teams doing as many as 60 radiographic tests a week. Miguel Dapena-Febrer, ITER’s Radiation Safety Coordinator, shows the passive dosimeter (left) and active dosimeter (right) that people involved in radiographic testing wear for safety purposes. The data captured by the passive device can be read afterwards while the active device gives a live reading of the radiation in the environment. Vacuum vessel sector #1 is the fifth sector to undergo radiographic testing. It is a rigorous process that checks the quality of the welding to depths in the tens of millimetres. More than 20 passes can be made over an area that is being tested to ensure the quality of the welding. Two sets of radiographic tests conducted on accessible areas of sector #1 have confirmed the repairs. With the sector positioned vertically in tooling, the final bevel regions are now accessible for testing.To prepare for the tests, protective measures against the ionizing radiation are taken. Radiographic testing is done on nights or weekends to limit the number of people in the vicinity, a vast number of lead-filled bags weighing between 17 and 24 kilograms are draped around the sector to form a protective curtain, and the team doing the testing wears dosimeters to ensure they are not exposed to doses above regulatory limits.Thomas Antonini, the Momentum Construction Manager managing the process, said the positioning of the X-ray generator and the films used to capture the images were an essential part of the process.“All preparations went smoothly and the testing has started successfully,” says Thomas Antonini. “With lessons learned over time on similar operations, we have been able to secure the process, which is good for our assembly schedule.” Just behind the vacuum vessel sector, lead bags weighing between 17 and 24 kilograms (yellow) have been placed on the scaffolding to form a shielding curtain against the X-rays. When radiographic testing on vacuum vessel sectors began, a gammagraph was used and it took about four hours for a film. Now, using an X-ray generator, the process can be completed in one hour. When the welding of the sector modules begins in the tokamak pit, a linear accelerator will be used and the time may be reduced to as little as 10 minutes per film. As a result, radiation protection provisions will have to be strengthened accordingly.Radiographic testing on vacuum vessel sector #1 is scheduled to be concluded by the first week of March.

Another piece of the ITER construction puzzle is arriving. Two days ago in Monfalcone, Italy, European-procured vacuum vessel sector #9 was loaded onto the cargo vessel that will deliver it to the south of France. When sector #9 reaches ITER next month, it will be the seventh vacuum vessel sector received on site. Two additional sectors are already in an advanced stage of completion in the workshops of Europe's Ansaldo Nucleare–Westinghouse–Walter Tosto consortium (AMW); once they arrive later this year, all the components required to form ITER’s torus-shaped plasma chamber will be in the hands of the assembly teams.Vacuum vessel sector fabrication is an exceptionally complex, multi-step industrial process that begins with procuring specialized ITER-grade stainless steel forgings, which are machined into plates. The plates are welded into four large segments, which in turn are assembled into a single sector (see diagram below). In Europe, which was tasked with the procurement of five sectors, the supply chain for vacuum vessel procurement spanned numerous European companies, involved thousands of coordinated tasks, and implicated at least 150 professionals. For sector #9, each of the segments required for its assembly was produced in a different workshop—Equipos Nucleares S.A. (ENSA) in Spain, and Belleli Energy CPE, Walter Tosto, and Westinghouse Mangiarotti in Italy—adding logistical challenges. Source: Fusion for Energy Each sector contains approximately 150 kilometres of welds, adding the risk of distortion and non-conformities to already demanding alignment and tolerance requirements. In sector 9, late-stage inspections identified defects in some joints, requiring invasive “surgical” repairs that involved reopening part of the structure and posed potential schedule delays. The team credits close coordination and established working relationships for the solutions that were eventually deployed that kept the project on schedule. Read the full story on the Fusion for Energy website. Download the vacuum vessel diagram as a poster here.

Since 2019, ITER Japan has been publishing a manga series that brings the ITER project to life for a broad audience. The story follows Japanese student Taiyô Tenna, who discovers ITER while on a world tour after meeting a French student in Aix-en-Provence. His journey continues through an internship experience at ITER, and then as he returns to Japan to learn more about the advanced fusion technologies developed by Japan’s Domestic Agency for the project. In the eighth volume, just released, he has become a staff member at the National Institutes for Quantum Science and Technology (QST), where he learns about ITER's new baseline scope and schedule and the ways in which Japan's flagship fusion device, JT-60SA is participating.Explore all volumes of the ITER manga series on the ITER Japan website or download Volume 8 from ITER's Publication Centre. Editions are available in Japanese, English, French and ... Provençal.