The Cryostat is a large, stainless steel structure surrounding the Vacuum Vessel and superconducting Magnets, providing a super-cool, vacuum environment. It is made up of two concentric walls connected by horizontal and vertical ribs. The space between the walls is filled with Helium gas at slightly above one atmosphere that acts as thermal barrier. The Cryostat is 31 metres tall and 36.5 metres wide.

The Cryostat has many openings, some as large as four metres in diametre, which provide access to the Vacuum Vessel for Cooling systems, Magnet feeders, auxiliary Heating, Diagnostics, and the removal of Blanket and Divertor parts. Large bellows are used between the Cryostat and the Vacuum Vessel to allow for thermal contraction and expansion in the structures. The Cryostat is completely surrounded by a concrete layer known as the bioshield. Above the Cryostat, the bioshield is two metres thick.