Tritium

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Tritium is the radioactive hydrogen isotope that supplies half of the deuterium-tritium fuel, and it is the constraint that most coverage of fusion omits. It does not occur naturally in useful quantities, it decays with a half-life of about 12.3 years — roughly 5.5% of any stockpile lost per year — and the only significant civil source is heavy-water CANDU fission reactors in Canada, which produce it as a by-product at a rate of order half a kilogram per reactor per year. Published assessments put the world civil stockpile at roughly 25 kilograms, projected to peak and then decline as CANDU units retire and as the inventory decays and is consumed. Estimates of consumption for a gigawatt-scale plant run to the order of a hundred kilograms per year, which is several times the entire present world stockpile annually. This is why breeding is not optional, and why a fusion economy cannot be started from mined or stored fuel.

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