ITER’s Vacuum Vessel comes together: the journey from factory to tokamak pit

The ITER Tokamak Complex has a tempo of its own. Inside this immense facility precision sets the pace and keeps the project advancing. Assembling the biggest fusion reactor is a delicate job: components larger than a bus are lifted, fitted and inched into place with millimetre accuracy, and new equipment keeps coming in.

This synchronised motion of workers and machinery is steadily transforming the site.  To capture ITER’s progress, all you have to do is glimpse down the tokamak pit, where the Vacuum Vessel is taking shape.

The stainless-steel torus forms the heart of ITER. It will contain the plasma (confined by superconducting magnets), where the fusion reaction will unleash vast amounts of energy. The massive vessel –5,200 tonnes in weight and 19.4 m in diameter– is made up of nine slices, known as sectors. Europe is responsible for five of them.

Fusion for Energy (F4E) partnered with the Ansaldo Nucleare, Westinghouse Electric Italy, and Walter Tosto (AMW) consortium to embark on this manufacturing journey. Now, they are reaching the final stretch: four sectors have been delivered, and the last one will soon join them in ITER.

Once there, the sectors enter the assembly choreography. There are heavy pirouettes and graceful alignments until each sector module finds its mark in the tokamak pit. These steps unfold in parallel and, with experience, their speed is increasing.

Currently, each of Europe’s five sectors is at a different stage of the sequence.

Let’s follow them through it.

Sector #2 – Final touches at the factory

At the Westinghouse workshop, in Monfalcone (Italy), sector 2 is in the last phase of manufacturing. Over 15 companies were part of the supply chain to shape and weld this demanding component. In the last weeks, the teams have been busy with final machining. Using a special milling machine, they are smoothing out its sides to ensure it meets the strict geometry requirements.

Once ready, the component will be placed in protective housing and shipped by sea to the south of France in September. Its delivery will mark a memorable milestone for Europe’s contribution to ITER, closing a project of over a decade.

Sector #3 – Polishing the giant

After its arrival on site in May, metrology experts spent several days inspecting sector 3 inside and out. The component was then rotated to expose its underside and taken into the former cryostat workshop. There, technicians worked on its edges, shaving off excess metal and filling low spots with a bespoke robot. This ensures the joints between sectors match the exact profile and they can be welded together seamlessly. With the metalwork now over, the sector will soon move to the big stage: the Assembly Hall.

Sector #9 – The coil embrace

One flank of the Assembly Hall is occupied by two 22-metre-tall Sector Sub-Assembly Tools. These structures are made to fit each Vacuum Vessel sector with a thermal shield and two Toroidal Field coils. The result is called a sector module, and it weighs about 1,200 tonnes.

Europe’s sector 9 is suspended in one of the tools and about to receive the tight embrace of the second D-shaped magnet. By the end of the year, it will make its move to the tokamak pit –an impressive heavy-lifting operation that can last over 24 hours.

Sector #4 – Welding marathon in the pit

The Vacuum Vessel’s inner surface will be dotted with hundreds of sensors to track ITER’s magnetic fields. Each has a specific place, with their cabling carefully routed through the busy walls. This ‘in-vessel forest’ will soon be planted in Europe’s sector 4.

The sector module sits in the tokamak pit since May. Scaffolding is being set up for welders to reach every inch of the wall. One by one, they will install over 2,000 small supports to which the sensors and cables will be attached. It’s a matter of patience, but also precision: 3D software and laser tools help pinpoint the position, and skilled hands complete the job.

Sector #5 – Touchdown

Sector 5, the first delivered by Europe in 2024, completed its last move this month. It was only a few centimetres, but a big step, nonetheless. The assembly teams landed it on the gravity supports, releasing the load from the overhead beam that held it until then. These foundations, and the concrete crown under it, will bear a weight of around 23,000 tonnes once the device is fully assembled.

The nine sectors will all come to rest on the supports before eventually being welded into one massive torus. From above, the iconic shape can already be made out. But at the Tokamak Complex, the work is far from over — and even in the height of summer, the rhythm does not skip a beat.