Blanket.

The blanket protects the vacuum vessel and other machine components from the extreme heat and high-energy neutrons produced by the fusion plasma. Because the vacuum vessel is a permanent structural component of the machine, the blanket must limit neutron and thermal loads to levels compatible with its lifetime.

Blanket Modules

During operation, the inner surface of the vacuum vessel will be exposed to intense neutron radiation, plasma heat loads and transient events such as disruptions. To protect the vessel and the superconducting magnets behind it, the inner wall is covered by 440 removable blanket modules that absorb heat and reduce the neutron flux reaching the machine structure.

During later operating phases, selected blanket locations will be fitted with Test Blanket Modules containing lithium-based breeder materials to investigate tritium production for future fusion power plants.

Design

The design varies according to the module’s position in the vacuum vessel, but a typical blanket module measures about 1 × 1.5 metres and weighs up to 4.6 tonnes. Each module consists of a main stainless-steel shield block attached to the vacuum vessel and a detachable first-wall panel that directly faces the plasma. As high-energy neutrons are slowed and absorbed in the shield block, much of their kinetic energy is converted into heat. This heat is removed by pressurised water flowing through cooling channels in the blanket modules at about 4 MPa and 70 °C.

Actively Cooled Blanket

ITER will be the first fusion device to operate with an actively cooled blanket. Its cooling system is designed to remove up to about 736 MW of thermal power. In a future fusion power plant, heat absorbed in the blanket would be transferred to a power-conversion system and used to generate electricity.

Plasma-Facing Components

First-wall panel prototype.

Each shield block is attached to the vacuum vesse and carries a detachable first-wall panel mounted on its front surface. Together, the 440 first-wall panels cover about 600 m² of the inner surface of the vessel.

There are two types of first-wall panels. The 215 normal-heat-flux panels are designed for surface heat loads of up to 2 MW/m², while 225 enhanced-heat-flux panels are designed for loads of up to 4.7 MW/m². Their plasma-facing surfaces will be armoured with tungsten, which is bonded to a copper-alloy heat sink mounted on a stainless-steel structure.

ITER originally planned to use beryllium armour on the first wall, but in 2023 the reference material was changed to tungsten. Tungsten offers much greater resistance to erosion and high heat loads and is considered more relevant to future fusion power plants. The change also reduces concerns associated with beryllium toxicity, dust production and tritium retention, although tungsten impurities must be carefully controlled because even small concentrations can increase radiation losses from the plasma.

First Wall Panels

Blanket shield block.

The first-wall panels are built from shaped “fingers” attached to a structural beam that forms the backbone of each panel and contains the cooling-water channels. The panels are designed for remote handling and replacement if necessary. With the change from beryllium to tungsten armour, a complete replacement of the first wall during ITER operation is no longer expected to be necessary.