Blanket Coverage and TBR
TBR scales with how completely the blanket surrounds the plasma; ports and the center post open gaps that lower net breeding.
Coverage is a first-order lever
The tritium breeding ratio depends strongly on solid-angle coverage: the fraction of neutrons that actually reach breeding material before being lost. A blanket that surrounds more of the plasma captures more neutrons and breeds more tritium. Any area that is not blanket — a port, a gap, the inboard center post — is area that does not breed.
Spherical tokamaks make this trade sharper. The compact center post that carries the toroidal-field conductors occupies inboard space, and shielding it from neutron damage competes with breeding there. So inboard coverage, which matters a great deal for TBR, is constrained by the machine's geometry.
What eats coverage
- Heating and current-drive ports
- Fueling and pumping penetrations
- Diagnostic lines of sight
- Center-post shielding volume (inboard)
- Gaps between blanket modules for assembly and maintenance
Coverage and the reconciliation
Coverage is precisely why local TBR overstates net TBR. A cell calculation assumes full coverage; the real machine cannot. Recovering breeding lost to ports — by breeding in more locations, minimizing gaps, and adding neutron multiplication near penetrations — is central to closing the net-TBR gate at the 1.8 target.
Coverage is a design and validation trade under active study; the achievable net TBR follows from where it settles.
Construction begins Q2 2027; first-of-a-kind (FOAK) first tritium is targeted near 2030. No net-gain claim is made before FOAK.