The ITER nuclear fusion project has installed the sixth of nine massive tokamak sector modules at its facility in southern France.
Announced on Monday, the milestone puts two-thirds of the reactor’s core in place and brings construction nearly six months ahead of its current schedule.
Crews lowered the module into the assembly pit at Saint-Paul-lez-Durance on July 28. The operation followed roughly 30 hours of preparation, lifting and positioning involving more than 100 workers.
Additionally, the installation marks a notable turnaround for a project better known for delays and escalating costs. ITER’s director-general acknowledged earlier this year that the project’s cost has climbed beyond €22 billion.
ITER aims to demonstrate whether nuclear fusion can generate energy at a scale relevant to future power plants. Fusion releases energy by combining light atomic nuclei, rather than splitting heavy atoms as conventional nuclear reactors do.
The project uses a tokamak, a doughnut-shaped machine that confines extremely hot plasma using powerful magnetic fields. Those magnets keep the plasma away from the reactor walls while fusion reactions occur inside it.
However, ITER will not generate electricity for the power grid. Instead, researchers designed it to demonstrate the technologies and physics required for future commercial fusion reactors.
The newly installed sector forms a 40-degree wedge of the tokamak’s eventual plasma chamber. At its centre sits a double-walled stainless steel vacuum vessel sector weighing about 440 tonnes.
Thermal shielding surrounds the vessel, while two D-shaped superconducting magnetic coils sit along its sides. Together, the assembled sector module weighs approximately 1,100 tonnes.
Furthermore, the lifting equipment pushes the total suspended load close to 1,400 tonnes. ITER compares an individual vacuum vessel sector to a jumbo airliner.
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Crews began operation in late July
The complete module weighs roughly two-and-a-half times as much. Four crane hooks carried the structure through the Assembly Hall and into the 30-metre-deep tokamak pit.
Crews began the operation early on July 27 with a briefing before moving the module. Subsequently, they guided it onto its supports the following afternoon.
The roughly 30-hour operation also represented an improvement over previous installations. Earlier sector moves typically required between 36 and 48 hours from initial checks through final positioning.
More than 100 ITER employees and contractors worked in shifts during the latest operation. The module carries the designation sector module #1 despite arriving sixth.
ITER numbers sectors according to their positions around the tokamak rather than their installation order. Crews also lowered this module backwards, placing its outboard edge into the pit first.
Meanwhile, the installation completed another international contribution to the reactor. Korea manufactured four of ITER’s nine vacuum vessel sectors, including the latest one.
Europe is responsible for manufacturing the remaining five sectors. ITER held Korean Fusion Day on July 22 to mark Korea’s final sector contribution.
Moving structures this large requires continuous measurements rather than visual guidance alone. Components within each sector module also have different centres of gravity.
Consequently, load distribution changes as cranes progressively pick up different parts of the assembly. Poor control could allow closely packed components to make contact.
Engineers therefore operate a dedicated monitoring station throughout each descent. Eight strain gauges surround each rod supporting the superconducting coils.
Load cells monitor bracing forces, while lasers measure clearances between components. In addition, inclination sensors track whether the suspended module begins tilting.
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Continuous monitoring remains essential
The monitoring system records measurements ten times every second. A single installation consequently generates millions of data points for engineers to follow.
Jordi Utges, a mechanical engineer with contractor Arial Industries, said continuous monitoring remains essential throughout the operation. Utges has worked at the control desk during all six successful module installations.
His team began installing sensors for the latest lift in May. Additionally, engineers pay particularly close attention during the final 500 millimetres of the descent.
Crews can hold the module about half a metre above its supports for hours. Metrology teams then verify its alignment before authorizing the final movement.
The recent pace contrasts sharply with problems that halted assembly four years ago. Inspectors discovered cracks in thermal shield cooling pipes during 2022.
They also identified dimensional problems along welding bevels on the vacuum vessel sectors. All nine sectors suffered from the manufacturing problems to varying degrees.
Consequently, crews removed the module already sitting inside the pit and dismantled it for repairs. ITER stopped sector assembly in September 2022.
Seven thermal shield sets subsequently went to India for repairs. Korea manufactured two replacement sets from scratch.
Those problems contributed to ITER adopting a revised project baseline in 2024. The schedule now targets first plasma in 2034 and deuterium-tritium fusion operations in 2039.
However, module installation restarted in April 2025 and has accelerated since then. Crews have installed six modules in roughly 15 months.
Teams have also improved positioning accuracy as they gained experience. Early installations encountered offsets of approximately 100 millimetres between neighbouring modules.
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Installation expected by middle of 2027
More recent work has reduced those gaps to around 10 millimetres. Furthermore, crews have moved modules past some obstacles with only 0.4 millimetres of clearance.
Sergio Orlandi, head of the ITER Construction Project, said crews cannot simply repeat identical procedures for each module. Each installation presents different positioning, lifting and engineering challenges.
The current schedule called for the ninth module to arrive in December 2027. ITER now expects to complete that installation around the middle of 2027.
However, that improvement applies to the revised 2024 construction baseline rather than ITER’s original timetable. The project remains years behind the schedule envisioned during its earlier development.
Meanwhile, other major components continue arriving and moving into position. The American-built central solenoid completed its final deliveries during the spring.
The enormous pulsed magnet will help drive electrical current through the reactor’s plasma. Five of its six modules have already been stacked at the French facility.
ITER expects crews to lower the seventh tokamak sector module before the end of 2026. After all nine arrive, workers must weld the sectors into a single vacuum-tight chamber.
That completed torus will measure roughly 19 metres across and weigh approximately 5,200 tonnes. Welding and testing the enormous chamber will itself require several years of precision work.
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