TBR 1.1: the Thin-Margin Point

At TBR 1.1 the blanket breeds barely more tritium than the plasma burns — self-sufficient in principle, but fragile to any real-world loss.

Breakeven with almost no cushion

TBR 1.1 means eleven tritium atoms bred for every ten fusion neutrons — a 10% surplus over the one-for-one the plasma burns. On paper this clears self-sufficiency. In practice it leaves almost no room for the losses that every real blanket incurs.

The concern is that the 10% headroom is smaller than the typical penalty from ports, gaps, structural absorption, coolant, and tritium holdup. A local TBR of 1.1 can become a net TBR below 1.0 once those are counted, at which point the machine consumes more tritium than it makes and cannot sustain its own fuel.

When 1.1 is acceptable

TBR 1.1 is a meaningful design point for studying the lower bound: it defines the minimum breeding performance the blanket must exceed and stresses the loss budget. It is not the target for a foundry expected to supply external offtake or seed new units, because it leaves essentially no surplus for either.

The lesson of the low point

• Self-sufficiency requires margin, not merely TBR > 1.0
• At 1.1, holdup and streaming can erase the surplus
• Offtake and fleet growth need a higher working point

1.1 is therefore studied as a floor to design against, not as the operating goal. The reference target is higher precisely so that net TBR survives the reconciliation.

This page describes a design-and-simulation study, not a built machine. Construction begins Q2 2027; first-of-a-kind (FOAK) first tritium is targeted near 2030. No net-gain claim is made before FOAK.