tritium inventory is the total quantity of tritium a facility holds at a given moment — in storage, in process loops, and embedded in materials — a stock that fusion plants are designed to keep as small as possible because the isotope is scarce, radioactive, and regulated.
Inventory is a stock where the 📝tritium breeding ratio (TBR) is a flow, and the two are easily conflated: a plant with a healthy breeding ratio can still be starved if too much of its 📝tritium sits trapped in pipework, storage beds, and 📝blanket materials rather than circulating as fuel. Minimizing inventory serves three masters at once. Scarcity: the world's usable tritium, mostly from Canadian heavy-water fission reactors, amounts to only tens of kilograms, and it decays at about 5.5 percent per year, so idle stock literally evaporates. Safety and regulation: site licenses cap how much tritium a facility may hold, and a smaller inventory means a smaller worst-case release. Economics: tritium is among the most expensive substances on Earth, priced per gram in the tens of thousands of dollars.
The startup inventory question follows directly: a first-generation power plant needs kilograms of tritium on hand before its 📝breeding blanket begins replacing what it burns, and the global stockpile can seed only a limited number of plants — one reason fast, efficient fuel-cycle design is a gating problem for scaling 📝fusion.