The vacuum vessel provides the high vacuum boundary for the plasma and the first line of confinement against radioactive releases within the vessel. The vessel cooling also provides decay heat removal by natural water convection for all the vessel and in-vessel components even when other in-vessel cooling is not working. The vessel also provides in-built attachment points for the blanket and divertor, and anchors (and partly cools) the port plugs.

The vacuum vessel is divided toroidally into 9 sectors welded at the central plane of alternate ports during assembly in the tokamak pit ("field welds"). It is an all-welded structure made from stainless steel and with a double, ribbed shell. The total thickness of this structure is typically in the range of 0.3-0.8 m. The space between the shells is filled with water and an array of plate inserts for neutron shielding. Some of the rib material, on the outboard side of the plasma in the shadow of the TF coils, is ferromagnetic, to reduce toroidal field ripple and its consequent trapped plasma particles.

Feedthroughs for the blanket cooling manifolds enter through "chimneys" either side of the upper ports and partway along their length. The vessel coolant enters at the bottom and leaves at the top, and two independent circuits are used, each able to handle the cooling load alone.

Should coolant leak into the vessel from in-vessel components, steam will be generated which, if unchecked, would result in the vessel having to withstand high pressures. To avoid this, provision is made for the connection of a large tank part-filled with water. This vacuum vessel pressure suppression system is automatically linked to the vessel by burst disks as the pressure rises above the preset limit of 2 atmospheres.

Cutaway at equatorial port level showing vacuum vessel pressure suppresion system

Key challenges in the manufacturing of such a large vessel are to join together manageable pieces to very small overall assembly tolerances, particularly considering the presence of field welds, the assembly of different plate orientations to ensure relatively high electrical toroidal resistance of the vessel as a whole, and the integration of ports and their extensions into the vessel. For this purpose the Vacuum Vessel Sector Project (L-3) was conceived and executed.

Further design details can be found in the Technical Basis .