Decay and the Replenishment Requirement
At ~5.5% loss per year, a tritium inventory halves in about 12 years; holding a level means a permanent make-up supply, not a single delivery.
- Half-life
- ~12.3 years
- Annual loss
- ~5.5%/yr
- Half-inventory
- ~12 years
- Quarter-inventory
- ~25 years
The arithmetic of decay
Radioactive decay follows an exponential law. For tritium, the decay constant corresponds to a half-life of ~12.3 years, which is why an unattended inventory falls to about half its starting mass in a little over a decade and to roughly a quarter in about 25 years. On a yearly basis the loss is close to 5.5% of whatever is present.
Two consequences follow. First, any requirement expressed as a level — a quantity that must be on hand — implies a continuous production rate just to stand still. Second, the make-up rate scales with the size of the level being held: larger inventories decay faster in absolute terms and need more make-up each year.
From decay to a production target
If a fixed level L must be maintained, the annual make-up is approximately 5.5% of L, plus whatever is consumed or lost in processing. A domestic source sized to that make-up rate keeps the level stable; a source below it means the level erodes year over year regardless of how full it started.
- Decay never pauses — storage does not stop the clock.
- Make-up demand is roughly proportional to the level held.
- A source below the decay rate cannot hold the inventory.
Compounding works against inventory
The exponential form has a second consequence worth stating plainly: shortfalls compound. A year of under-supply does not merely miss its target; it leaves a smaller base that next year's decay and any withdrawals draw down further. Recovering a level after it has slipped therefore takes more than steady-state make-up, because the source must both offset ongoing decay and rebuild what was lost. Planners size domestic production to the steady-state rate and hold modest buffers precisely to avoid entering that harder recovery regime in the first place.
This is the core reason a standing domestic production capability matters more for tritium than for a stable isotope: the requirement regenerates itself every year through physics, not policy.