The 440 blanket modules that completely cover the inner walls of the vacuum vessel
protect the steel structure and the superconducting toroidal field magnets
from the heat and high-energy neutrons produced by the fusion reactions. As the neutrons are slowed in the blanket, their kinetic energy is transformed into heat energy and collected by the water coolant. In a fusion power plant, this energy will be used for electrical power production.
Each blanket module measures 1 x 1.5 metres and weighs up to 4.6 tonnes. Over 180 design variants exist (related to the position of the modules in the vacuum vessel), but all have a detachable first wall that directly faces the plasma and removes the plasma heat load, and a main shield block that is designed for neutron shielding. The blanket modules also provide passageways for diagnostic viewing systems and plasma heating systems.
The ITER blanket, which covers a surface of 600 m², is one of the most critical and technically challenging components in ITER: together with the divertor
it directly faces the hot plasma. Due to its unique physical properties (low plasma contamination, low fuel retention), beryllium has been chosen as the element to cover the first wall. The rest of the blanket modules will be made of high-strength copper and stainless steel.
ITER will be the first fusion device to operate with an actively cooled blanket. The cooling water—injected at 4 MPa and 70 °C—is designed to remove up to 736 MW of thermal power.
During later stages of ITER operation, some of the blanket modules will be replaced with specialized modules 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.