Each D+T→He4+n reaction releases 17.6 MeV, so a reactor of power P consumes tritium at a fixed mass rate. A breeding blanket (Li-6/Li-7) regenerates a fraction TBR×efficiency of every atom burned; the rest must come from external stock.
dI/dt = burn·(TBR·η - 1) - λI
λ = ln2 / 12.32 yr (T half-life)
TBR·η > 1 means the plant breeds more fuel than it burns (self-sufficient); below 1 the inventory drains and must be topped up from an external source.