Construction considerations for the ITER vacuum vessel

The ITER vacuum vessel will be the largest such structure yet designed with a height of 14 m and an outer diameter of 26 m. The vessel must provide a high quality vacuum, high electrical resistivity, and operate at high temperature. The vessel must provide for bakeout, nuclear shielding, support of in-vessel components and access to these. Significant electromagnetic forces act on the vessel especially during a plasma disruption. The vessel is designed as a double walled toroidal shell with poloidal stiffening rings. Construction cost is reduced by fabricating the shell from a series of single curvature plates, 2-4 cm thick, that are fully welded to form a faceted structure. Material selection must consider fabricability, structural properties at temperature and over the life of the machine, and the desire for low activation. Interaction with the selected coolant, especially if it is liquid metal is a consideration. Stress relief operations and the ability to remotely cut and re-weld the vessel are important considerations. Step by step fabrication and assembly sequences were developed and illustrated using computer solid modeling techniques. Final assembly of the vessel at the ITER site considers overall sequence of machine assembly. Final vessel sector weld joint location options include mid TF coil, mid port and just to the side of the ports, which would allow factory fabrication of the more demanding port joint region. Final assembly operations demand that the weight of the vessel be kept low so that the modules can be moved into position for final welding. Nuclear shielding design plays a significant role. The design features solid built-in shield blocks, in difficult to access areas, and bulk shielding using insulated metallic balls, which can be added and removed after the vessel is fully in place. An important part of the design is provision for direction of coolant flow, ensuring adequate thermal control to all regions of the vessel. Port to shell joints consider shielding installation, coolant flow and ease of fabrication. Support of the vessel and the in-vessel components must provide for the thermal expansion experienced while protecting against seismic events. The vessel provides containment for tritium and is important to the overall safety of the facility. Postulated abnormal events must be considered in the design and safety analysis. A set of fabrication development and construction verification mock-up articles and their evaluation is planned prior to the completion of the detail design phase. ITER is in the earliest stages of the design process and today's decisions will form the basis of the detailed design, fabrication and operation.