Tungsten as a Plasma-Facing Material

Tungsten armors the surfaces facing the plasma; it resists heat and erosion and activates to mostly short- and medium-lived products.

The surfaces most exposed to the plasma — the first-wall armor and the divertor in the breeder — need a material that survives extreme heat flux and particle bombardment. Tungsten is the standard choice because of its very high melting point, low erosion, and low tritium retention relative to carbon.

Activation behavior

Under 14 MeV neutrons, tungsten transmutes toward rhenium and osmium and produces a set of activation products that are predominantly short- and medium-lived. It is not activation-free — nothing in a neutron flux is — but its profile is compatible with the low-activation strategy, which is one reason tungsten also substitutes for molybdenum inside reduced-activation steels.

• High melting point and thermal conductivity handle the heat flux at the plasma boundary.
• Low sputtering erosion extends component life and reduces plasma contamination.
• Low tritium retention limits the tritium held in plasma-facing surfaces.
• Activation is mostly short- and medium-lived, fitting the overall waste plan.

Because plasma-facing surfaces are eroded during operation, tungsten also enters the waste account as dust and redeposited material, collected and consolidated for recycling. Its short-to-medium-lived activation profile means this dust decays to a manageable class rather than becoming a long-lived stream, keeping the most exposed surface consistent with the overall waste plan.

Tungsten's brittleness and its behavior under combined heat and neutron loading are the open engineering questions, addressed through composites, alloying, and design. For the waste story, the key point is that the most heavily loaded surfaces use a material whose activation does not create a large long-lived burden. These are design-and-simulation choices for machines not yet operating.