The Tritium Breeding Ratio
The ratio of tritium produced in the blanket to tritium consumed in the plasma, which must exceed one for D-T self-sufficiency.
Closing the fuel cycle
A D-T fusion plant burns tritium, which does not occur naturally in useful amounts. It must be bred by capturing fusion neutrons in a lithium-bearing blanket. The tritium breeding ratio (TBR) measures whether enough is made:
TBR = (tritium atoms bred per second) / (tritium atoms burned per second)Self-sufficiency requires TBR greater than one, with margin above one to cover radioactive decay, processing losses, and the startup inventory of future plants. A typical target is around 1.05 to 1.15 net, though design TBRs are set higher to allow for losses.
The breeding reactions
- Li-6 + n -> T + He-4 (exothermic, favored by slow neutrons)
- Li-7 + n -> T + He-4 + n (endothermic, also multiplies neutrons)
- Neutron multipliers (beryllium, lead) boost the neutron count via (n,2n) reactions
Because each fusion produces only one neutron and blanket coverage is never complete (ports, structure absorb neutrons), neutron multiplication and lithium-6 enrichment are used to push the local breeding above one.
How it is computed
TBR is calculated with neutron-transport codes (Monte-Carlo, for example MCNP, or deterministic transport) that track fusion neutrons through the detailed blanket geometry and materials, tallying tritium-producing reactions against the fusion neutron source. The result is highly sensitive to blanket thickness, coverage fraction, and material composition.
Kronos context
The Hyperion breeder is designed around tritium breeding, with a design tritium breeding ratio of 1.8, chosen to provide strong margin for coverage losses, processing, and building fuel inventory for a fleet. Achieving self-sufficiency (net TBR above one after all losses) is a defining requirement of any D-T system, and it is evaluated with detailed neutronics as part of the design study.