The number of tritium atoms produced in the breeding blanket per tritium atom consumed in the fusion reaction — a ratio that must exceed unity for a D-T fusion plant to sustain its own fuel supply.
The deuterium-tritium (D-T) fusion reaction produces 14.1 MeV neutrons and 3.5 MeV alpha particles. Deuterium is abundant in seawater, but tritium is radioactive (half-life 12.3 years) with no significant natural supply. Global inventories — produced as a byproduct in heavy-water fission reactors — total only about 25 kg and are declining.1 A 1,000 MW fusion plant consumes roughly 55 kg of tritium per full-power year. Self-sufficiency is therefore not optional: it is an existence condition for commercial D-T fusion.
A breeding blanket surrounding the plasma captures fusion neutrons and drives them into lithium-bearing materials. Two reactions produce tritium:
6Li + n → T + 4He + 4.8 MeV (thermal neutrons)
7Li + n → T + 4He + n′ − 2.5 MeV (fast neutrons)
The 7Li reaction is endothermic but releases a secondary neutron, which can breed additional tritium. Neutron multipliers such as beryllium or lead are incorporated to push TBR above 1.2
No breeding blanket has been tested under fusion-relevant 14.1 MeV neutron conditions at reactor scale. ITER will host Test Blanket Modules (TBMs) to provide the first experimental data, but these will cover only a small fraction of the first-wall area.4 Neutronics simulations using Monte Carlo codes predict TBR with quoted uncertainties of 2–5 %, yet nuclear data cross-section uncertainties — particularly for 7Li(n,n′α)T — can shift calculated TBR by several percent, potentially crossing the margin between self-sufficiency and tritium shortfall.
The distinction between local TBR (computed over the full spherical solid angle of ideal blanket coverage) and global TBR (accounting for real-geometry gaps, ports, and penetrations) is critical. A blanket concept reporting local TBR = 1.40 may yield global TBR below 1.10 once engineering reality is included.