Tritium (T) or superheavy hydrogen is an isotope of hydrogen. Its atomic nucleus consists of a proton and two neutrons.
Tritium is the heaviest and the only radioactive version of hydrogen; it has a half-life of 12.3 years. Its radiation – beta radiation, i.e. electrons – is of too low an energy to be capable of penetrating human skin from the outside. It is harmful to living matter when tritium is absorbed in the body by inhalation, eating, drinking or diffusion. Once present in the body, tritium there has an effective biological half-life of about ten days.
Small quantities of tritium are permanently present in the upper atmospheric strata as a result of the high-altitude radiation. Owing to the short half-life, however, the estimated world inventory amounts to just seven kilogrammes.
As fuel component for a future fusion power plant, tritium can be produced directly in the power plant from the element lithium by means of the neutrons produced in the fusion process.