As the ITER tokamak is larger and more demanding than previous experimental machines, it presents considerable challenge for assembly. The most significant issues for assembly are derived from the large size and mass of the components, and associated tight installation tolerances. Use of a high precision optical metrology system is key to a precise assembly of ITER.

Assembly is planned to allow adjustment of tolerances between assembly steps to prevent unacceptable build-up of deviations. The most critical tolerances for assembly are driven by two requirements; minimization of non-axisymmetric error fields of the magnet system, and alignment of the plasma facing components with the toroidal magnetic field. The error fields resulting from magnet construction and location tolerances must be <~10-4 of the toroidal field to allow the correction coil system to adequately compensate. The alignment tolerances on the limiter, divertor, and first wall are driven by the requirement to distribute heat loads to acceptable values, particularly during off-normal plasma events.

The machine is assembled using the two 750 tonne bridge cranes. Using a connecting beam, this dual-crane system allows precise lifting of components weighing up to 1300 tonnes. Approximately 25 lifts over 1000 tonnes are planned.

Specialized tooling is designed for transportation, pre-assembly, and installation of the components as large subassemblies. The largest of these consists of a TF coil, thermal shield segment, and vacuum vessel segment. Each of these 20 subassemblies is pre-assembled on a large tool, rotated into a vertical orientation, and installed with the dual-crane system. Installation tolerances of ± 2mm at the TF coil inboard leg are maintained using customized shims and adjustable guides. The position is verified with the optical metrology system. After installation of the 20 TF coils, the interface with the central solenoid is verified prior to installation.

The most severe requirement of the assembly process is controlling and compensating for weld distortion in the field joints of the vacuum vessel and the blanket backplate. The most critical blanket modules must be aligned with the toroidal field to tolerances of ± 2mm in the most critical locations. These modules are installed after completion of the final backplate welds to allow a post-weld survey of the mounting points.