Local vs Net Breeding Ratio

A blanket segment can show high local breeding while the whole machine breeds less; the local-versus-net gap is the heart of the TBR reconciliation.

Two different ratios called TBR

Local TBR is the breeding computed for an idealized patch of blanket — ample thickness, optimized lithium and multiplier, no interruptions. Net TBR is what the entire machine actually breeds once every real feature is included: ports for heating and diagnostics, pumping ducts, structural steel that parasitically absorbs neutrons, and the inboard center post that cannot carry a full breeding blanket at all.

A design can report an encouraging local TBR and still fall short on net TBR, because coverage is never complete and every penetration and structural element steals neutrons from breeding. On a compact spherical tokamak the inboard side is especially constrained, which makes the local-to-net penalty larger than in a machine with room for a full inboard blanket.

Why it matters for the 1.8 claim

The reconciliation at TBR 1.8 is fundamentally a local-versus-net question: an idealized calculation may reach 1.8, but the net figure after realistic coverage and losses is what determines self-sufficiency. Keeping the two explicitly separate prevents an optimistic local number from being mistaken for a delivered result — which is why the design flags it rather than reporting a single TBR.

• Local TBR: idealized blanket segment, no interruptions
• Net TBR: whole machine including ports, structure, uncovered center post
• The 1.8 reconciliation is essentially a local-vs-net gap

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