tritium is the radioactive heavy isotope of hydrogen, whose nucleus carries one proton and two neutrons — the scarcer, hotter half of the fuel pair that will power the first generation of 📝fusion power plants.
The name comes from the Greek tritos, "third." Ernest Rutherford, Mark Oliphant, and Paul Harteck first produced the isotope in 1934 by bombarding 📝deuterium with deuterons. Tritium decays by emitting a low-energy beta particle with a half-life of 12.3 years — radiation so weak it cannot penetrate skin, though the isotope must still be handled with care because it behaves chemically like hydrogen and readily becomes tritiated water.
Unlike deuterium, tritium barely exists in nature: cosmic rays create only trace amounts, and the world's usable inventory — measured in tens of kilograms, mostly from Canadian heavy-water fission reactors — could not fuel a fleet of power plants. Fusion's answer is to breed it on site: the neutrons produced by the reaction strike lithium in a blanket surrounding the 📝plasma, splitting it into helium and fresh tritium. A plant that breeds at least as much tritium as it burns — a tritium breeding ratio above one — closes its own fuel cycle, which makes breeding one of the defining engineering problems of commercial fusion.