Local vs Net TBR: the Open Reconciliation
A blanket cell can compute TBR 1.8 in isolation, but the as-built machine has ports, gaps, and losses; reconciling local to net TBR is unresolved.
The gap between the cell and the machine
Neutronics simulations often report a local tritium breeding ratio: the breeding computed for an idealized blanket module, fully surrounding the plasma, with no penetrations and simplified structure. That number can reach the 1.8 target. It is not the number that governs the fuel cycle.
The governing quantity is the net TBR of the complete machine, which is lower because real neutrons are lost to effects the ideal cell ignores:
- Ports and penetrations for heating, fueling, diagnostics, and pumping
- Gaps between blanket modules and around the center post
- Parasitic absorption in structural steel, welds, and manifolds
- Absorption and moderation in coolant and tritium-carrier fluids
- Streaming of neutrons out through open lines of sight
Why it is flagged as open
The reconciliation is listed among Hyperion's principal open questions because the magnitude of the penalty — how far net falls below local — is exactly what determines whether the machine is self-sufficient with margin. At the 1.8 target this has not been closed to Kronos's own standard. Reporting a local figure as if it were net would overstate the design, and Kronos declines to do that.
How it gets resolved
Closure requires full-geometry neutronics with realistic ports and structure, blanket coverage accounting, and eventually measured tritium production at FOAK. Until then, net TBR and the product targets that depend on it are stated as contingent design-and-simulation values.
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.