Permeation Control
Material choices, coatings, and negative-pressure design slow tritium migration through walls to keep it inside the loop.
Tritium's defining engineering challenge is permeation: as the smallest atom, it can diffuse through hot metal walls that would contain ordinary gases. Controlling permeation is how a fusion plant keeps its fuel where it belongs and keeps routine emissions low.
How permeation is fought
- Permeation-barrier coatings: oxide and ceramic layers that sharply reduce diffusion.
- Material selection: alloys with low tritium solubility and diffusivity.
- Temperature management: permeation rises with temperature, so hot boundaries are minimized.
- Double walls with sweep gas: intercept and recover permeated tritium.
- Negative pressure: keep the driving pressure gradient pointed inward.
Permeation control has two payoffs. It keeps tritium inside the fuel loop, conserving a scarce material, and it reduces the amount that reaches the outer barriers, lowering routine emissions and the worst-case source term. Because the breeder breeds tritium in the blanket at a ratio treated as a design lever (1.1 / 1.5 / 1.8), permeation from hot breeding and extraction systems is a specific design focus.
Verification
Barrier performance is confirmed by permeation testing and by the monitoring that would reveal any breakthrough. Permeation control is the upstream complement to detection: minimize migration first, then measure to confirm. See confinement barriers.
The burner (Aegis / MetroVolt) largely sidesteps this concern by not relying on a large tritium inventory.