The main vessel and its internal components have a number of functions:

• to attenuate and absorb neutron energy and flux from the plasma to a level tolerable for the magnets and surrounding equipment, as well as to allow personnel access near the machine a short time after plasma shutdown;
• to provide an ultra-high-quality vacuum to allow the plasma to operate;
• to divert helium from the fusion reactions plus impurities coming from the surroundings to locations where the power and particles can be unloaded and pumped away;
• to allow access for plasma heating, fuelling, and diagnosis, and to permit testing of power-reactor-relevant blankets and materials in special test modules, as well as remote maintenance of all in-vessel components;
• to act as the first line of defence to confine any coolant leaks so that radioactive materials are not spread outside the plant.

The blanket absorbs heat radiating from the plasma as well as providing neutron shielding. The divertor exhausts the flow of energy from charged particles produced in the fusion reactions and removes helium and other impurities. Port plugs access the plasma for heating, diagnostics, reactor blanket testing, and remote maintenance. All these are attached in various ways to the vacuum vessel which can remove decay heat from itself and all in-vessel components to the environment by the passive process of natural water convection even when all other cooling systems are not working.

To see a short animation of a flight into the vessel during assembly, click here.