tritium breeding ratio (TBR) is the ratio of tritium bred in a 📝fusion plant's 📝blanket to tritium burned in its 📝plasma, which must exceed one for the plant to be self-sufficient in fuel.
The number that matters is not one but the margin above it, and the distinction between required and calculated TBR is where the engineering lives. A plant needs surplus tritium to cover what decays in storage — 📝tritium's half-life is 12.3 years — what is lost or held up in processing, and what must be banked to start the next plant, so design targets typically sit in the range of roughly 1.05 to 1.15. Against that requirement stands the calculated TBR from neutronics simulation, which every port, pipe, and structural rib erodes: neutrons absorbed by steel breed nothing. Closing the gap drives choices like 📝lithium-6 enrichment and the addition of a 📝neutron multiplier.
A further distinction separates calculated from demonstrated. No fusion machine has yet measured an integral TBR in an operating 📝breeding blanket; every quoted value is a prediction awaiting hardware. Because each 📝D-T reaction yields exactly one neutron per tritium consumed, TBR above one is only achievable at all through neutron multiplication — making the ratio the single number that decides whether a plant's fuel cycle closes.