In Cadarache, in the south of France, workers are slotting together vast steel sectors with sub‑millimetre precision, hoping to prove that fusion energy can leave the realm of theory and move toward industrial reality.
Third giant module drops into place in France
On 25 November 2025, at the ITER site near Aix‑en‑Provence, a several‑hundred‑tonne steel sector began its slow descent into a reinforced concrete pit. Engineers watched every movement on screens and through windows, ready to halt the lift if any measurement drifted by more than a fraction of a millimetre.
This massive piece is vacuum vessel module no. 5, one of nine wedge‑shaped sectors that will form the doughnut‑shaped heart of the ITER fusion reactor, known as a tokamak. It now sits alongside modules 6 and 7, which were installed earlier in 2025.
ITER’s third installed vacuum vessel module means roughly one‑third of the machine’s central chamber is now physically in place.
The operation may look like a slow industrial ballet, but it marks a key step for the most ambitious energy experiment ever attempted: a 30‑metre‑high, 30‑metre‑wide device designed to confine plasma at 150 million degrees and show that fusion can generate more power than it consumes.
How ITER’s “steel doughnut” is built
The tokamak’s vacuum vessel is a torus, a hollow ring similar to a giant metal doughnut. Instead of being built in one piece, it is divided into nine sectors that resemble curved slices of a colossal cake. Each of these pieces is wildly complex.
Every vacuum vessel module includes:
- a section of the double‑walled chamber where plasma will be confined
- two huge superconducting magnets known as poloidal field coils
- a thermal shield to keep extreme cold and extreme heat separated
- interfaces for dozens of pipes, cables and diagnostic systems
The modules must interlock like a perfectly engineered ring. There is almost no spare room. Engineers are stacking them from the bottom upwards, following a predefined sequence that leaves little room for improvisation.
A precision choreography on overhead cranes
Before a module can enter the tokamak pit, it passes through a dedicated cleaning building. There, workers remove every speck of dust, because even small contaminants can cause trouble in an ultra‑high‑vacuum system.
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From there, the sector is moved into the assembly hall. Overhead cranes take over, lifting the steel giant several metres above the floor. Teams on the ground and in control rooms adjust its position millimetre by millimetre.
During installation, tolerances sit in the range of tenths of a millimetre, despite the components weighing several hundred tonnes.
The module is then lowered into the central pit, guided by laser trackers and reference markers. It must align precisely with the sectors already installed, leaving space for welding, insulation and the eventual installation of internal components such as the divertor and shielding blocks.
An industrial symphony with global players
ITER is often described as an “energy CERN” because it brings together partners from across the planet. That mix is visible at every step of the vacuum vessel assembly.
A consortium centred on China’s CNPE and several Chinese institutes, working with French nuclear firm Framatome, oversees core tasks such as cryostat assembly, magnet feeding systems and integration of the modules into the pit.
Italian company SIMIC S.p.A. shares responsibility for positioning each sector and making the mechanical connections between them. Indian engineering giant Larsen & Toubro handles ultraprecise welding of the vessel “windows” and interfaces. US‑based Westinghouse will carry out the final welding that locks all nine modules into a sealed ring.
Every element is bespoke. Components are machined to micron‑level precision. That makes each successful lift feel less like routine construction and more like a unique technical performance.
Where ITER stands: three down, six to go
With module no. 5 now sitting alongside its neighbours, roughly a third of the vacuum vessel’s circumference is installed. The remaining six sectors will follow at a pace of one every two to three months, stretching into 2026.
| Module | Installation date | Status |
|---|---|---|
| Module no. 7 | April 2025 | Installed |
| Module no. 6 | June 2025 | Installed |
| Module no. 5 | 25 November 2025 | Installed |
| Modules no. 1–4 & 8–9 | 2026 (planned) | Pending |
Once all nine modules are in place, teams will start the painstaking process of final welding and leak testing. Only when the vessel is fully sealed can the internal components be installed and the huge vacuum pumps switched on.
A schedule under pressure
ITER’s timeline has shifted repeatedly since ground was broken in 2010. The giant machine was once expected to generate its first “plasma shot” in 2025. Those hopes faded as design changes, supply chain issues and the Covid‑19 pandemic piled up.
The current roadmap aims for vacuum commissioning and early test operations around 2028–2029. The first plasma, using ordinary hydrogen heated to about 100 million degrees, is now targeted for around 2030.
A later phase, in the mid‑2030s, will use a mix of deuterium and tritium — heavy forms of hydrogen — to test whether ITER can achieve its headline goal: producing more fusion power than the plant consumes in heating and control systems.
If successful, ITER aims to show that a future commercial reactor could generate several times more energy than it needs to keep the plasma burning.
Why fusion at ITER matters beyond this one machine
The stakes extend far beyond a single facility in southern France. Fusion promises low‑carbon power without long‑lived, high‑level waste and without the meltdown risk associated with conventional fission reactors.
That does not mean fusion is risk‑free. Tritium is radioactive and must be handled carefully. The machine’s inner walls will become activated by neutron bombardment and will eventually need managed disposal. The project also locks in tens of billions of euros and decades of work before commercial plants can even be considered.
Yet governments and private investors see a potential payoff that could reshape energy systems. If fusion works at scale, it could provide steady, controllable output to complement intermittent renewables such as wind and solar. It could also power industrial processes that need high heat, including hydrogen production and some forms of heavy manufacturing.
Key concepts behind the machine
What a tokamak actually does
A tokamak confines a plasma — an ionised gas of charged particles — using powerful magnetic fields. In ITER’s case, that plasma will swirl inside the vacuum vessel at temperatures hotter than the centre of the Sun.
The vacuum vessel’s job is not to hold the pressure, as a boiler would, but to provide a perfectly sealed, ultra‑clean environment in which the magnets can do their work. Any leak or impurity could cool parts of the plasma, destabilise the magnetic field and shut the reaction down.
Fusion itself happens when light nuclei, such as deuterium and tritium, slam into each other and fuse, releasing energy in the form of fast neutrons and heat. ITER will not convert that heat into electricity. It is a physics and engineering demonstrator, not a power plant. Future reactors would surround the vacuum vessel with blankets that capture the neutron energy and turn it into steam.
Why the engineering is so unforgiving
The vacuum vessel modules must survive extreme conditions for years at a time. The inner surfaces will face constant neutron bombardment. The outer surfaces sit close to magnets cooled to near absolute zero. The structure will expand and contract as temperatures shift and magnetic fields ramp up and down.
That mix calls for exotic steels, precise welding techniques and extensive non‑destructive testing. A single misaligned sector might not only compromise performance but delay the schedule for months while engineers cut, re‑weld and re‑qualify the affected parts.
What comes next after the ninth module
Once the final module slots into place and the welding teams complete their long shift, attention will turn to the machine’s “organs”. The divertor — a complex structure at the bottom of the vessel — will handle exhaust gases and impurities. Shielding blocks will protect structures from radiation. Heating systems using radio waves and high‑energy beams will inject power into the plasma.
Engineers will run vacuum tests, bake the vessel to drive out water and gases, and gradually bring magnets and cryogenic systems online. Early pulses will be short and cautious, designed to probe the machine’s behaviour before longer, hotter shots begin.
For now, the quiet satisfaction on site comes from something more basic: three giant pieces of a 30‑metre steel ring now sit locked together in a pit in Provence. For a project that aims to show that fusion energy can work at scale, that concrete progress matters at least as much as any computer simulation or design milestone.
