The blanket covers the interior surfaces of the vacuum vessel, providing shielding to the vessel and the superconducting magnets from the heat and neutron fluxes of the fusion reaction. The neutrons are slowed down in the blanket where their kinetic energy is transformed into heat energy and collected by the coolants. In a fusion power plant, this energy will be used for electrical power production.

For purposes of maintenance on the interior of the vacuum vessel, the blanket wall is modular. It consists of 440 individual segments, each measuring 1x1.5 metres and weighing up to 4.6 tons. Each segment has a detachable first wall which directly faces the plasma and removes the plasma heat load, and a semi-permanent blanket shield dedicated to the neutron shielding.

The ITER blanket is one of the most critical and technically challenging components in ITER: together with the divertor it directly faces the hot plasma. Because of its unique physical properties, beryllium has been chosen as the element to cover the first wall. The rest of the blanket shield will be made of high-strength copper and stainless steel.

At a later stage of the ITER project, test breeding modules will be used to test materials for tritium breeding concepts. A future fusion power plant producing large amounts of power will be required to breed all of its own tritium. ITER will test this essential concept of tritium self-sustainment.