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The size and weight of the major components, the tiny tolerances and careful handling required for the assembly of huge and unique systems, the diversity of manufacturers, the tight schedule, complex interfaces ... all of these elements combine to make the assembly of the ITER machine an engineering and logistics challenge of enormous proportions.

Assembly is planned "bottom-up." Beginning with the heaviest single component—the 1,250-tonne cryostat base—Assembly Phase I operations will continue with the installation of lower cryostat components; the nine large, 40° vacuum vessel sectors (pre-assembled with thermal shield panels and a pair of toroidal field coils); the components at the top of the machine; and finally the cryostat lid.

The largest components will enter the Assembly Hall on self-propelled transporters via the Cleaning Facility, which operates as an airlock between the Hall and the outside environment.

Cleanliness requirements in the pre-assembly and assembly zones requires the components to be "unpacked" and cleaned with compressed air, pressurized demineralized water, or specialty detergents before the large doors of the Assembly Hall open to admit the transporters. There, the overhead crane system takes charge of the loads to transfer them to a temporary laydown area, or to a zone reserved for sub-assembly activities. In the order determined by assembly sequences, the components (or component assemblies) will be transferred by crane to the Tokamak Building and lowered into the Tokamak assembly pit.

During machine assembly, the Assembly Hall and the Tokamak Building will form a continuous working space.


Accurate alignment, particularly of the magnet system and in-vessel components, is essential to the successful operation of the machine. Dimensional control will be critical to ensuring that tolerances are adhered to, and to recording the "as-built" status of the machine, which will be directly compared against ITER's Computer-Aided-Design (CAD) models in order to correct eventual deviations in alignment before they accumulate. Positional tolerances for the largest components, including the magnet coils and the vacuum vessel, are as low as 2 mm. (See related article here.)

Vacuum vessel welding

Welding the ITER vacuum vessel and ports inside of the assembly pit will require a little over two years, an estimated 200 technicians, and a host of customized techniques and tools. The welding tools will have to manoeuvre the complex geometry of the sectors and ports and reach areas that are not directly accessible to operators and where visibility is restricted. The robots will also be confined to welding from inside the vessel and ports, as the pre-installed vacuum vessel thermal shield makes access to the exterior surfaces impossible. Extensive trials on mockups have been performed to quantify and plan for shrinkage, and to qualify tools, processes, and inspection techniques. (See related article here.)

Ultra-high and high vacuum
Machine assembly contracts

TAC1: cryostat and cryostat thermal shield; magnet feeders; the central solenoid, poloidal field and correction coil magnets; and cooling structures and instrumentation.

  • Awarded in 2019 to the CNPE Consortium (China Nuclear Power Engineering; China Nuclear Industry 23 Construction Company Ltd.; Southwestern Institute of Physics; Institute of Plasma Physics, Chinese Academy of Sciences ASIPP; and Framatome)
TAC2: main vessel and ports, sector sub-assembly with toroidal field coils and vacuum vessel thermal shielding, and welding

  • Awarded in 2019 to the DYNAMIC SNC consortium (Ansaldo Nucleare; Endel Engie; Orys Group ORTEC; SIMIC; Ansaldo Energia; and Leading Metal Mechanic Solutions SL).

ITER will host one of the largest and the most complex high vacuum systems ever built. To ensure correct machine performance, all connections associated with ultra-high vacuum or high vacuum components must be 100 percent leak-tight. Vacuum lines, helium lines, pipes, valves, seals, joints, and/or flanges have all gone through pre-qualification, while some of the largest vacuum components—for example, the seals manufactured for the vacuum vessel's 50+ large ports—are being tested on dedicated test rigs. During in-pit assembly, only qualified welding procedures, carried out by certified professionals, will be employed, and the quality of all vacuum-related welds will be verified by surface and volumetric non-destructive examination. Full-scale leak testing is planned during the 12-month integrated commissioning period before First Plasma, during which the large vacuum chambers—the vacuum vessel and the cryostat—will be evacuated and leak tested. In parallel, the ITER Organization is developing a set of specialized tools and technologies for the localization and detection of leaks smaller than the width of hair divided by one million.

French nuclear regulations

As the nuclear operator of the ITER installation, the ITER Organization has an obligation to ensure that safety and security standards are implemented and enforced throughout construction, manufacturing, assembly, and operation in compliance with the Host country's safety and security regulations. All activities related to a safety-important or protection-important component or system (for example ITER's first confinement barrier, the vacuum vessel) must be performed correctly and documented as verified through regular surveillance and inspection.


Once delivered to the site, the interfacing elements for a number of large components such as the ITER blanket and the divertor have to be customized to comply with precise alignment requirements. Assembly contractors will have to use dimensional control and reverse engineering to plot out this customization, and must provide the workshop space, tooling and expertise to carry out the work on short timescales. (See related article here.)


The ITER Organization and its assembly partners will be managing a large volume of assembly activities—such as major lifting operations, handling/positioning, mechanical fixation, welding, cabling, pipe work, metrology, non-destructive examination, and leak testing—in the extremely crowded environment of the Tokamak pit and the Tokamak Complex. Close coordination between the activities of the assembly contractors and the teams completing building works contracts in the Tokamak Complex is crucial.


Throughout Assembly Phase I, the ITER Organization will rely heavily on the on-time delivery of components and systems from the seven Domestic Agencies. Any delay related to fabrication or transport risks disrupting the carefully planned assembly sequences, especially for time-critical components such as the magnets or the vacuum vessel sectors.

Machine Assembly Facts:

Assembly of the core machine: March 2020-December 2024
Engineering work packages for machine assembly: 1,200
Machine weight: 23,000 tonnes
Cryostat dimensions: 28.5 m (diameter) x 29 m (height)
Number of components (estimated): 1,000,000
Heaviest single component: cryostat base (1,250 tonnes)
Tallest single component: central solenoid (18 metres, with structure)
Superconducting magnets: three major systems, 10,000 tonnes
In-pit welding of the vacuum vessel: 50 km of weld joints, 25 tonnes of welding wire