Schematic diagram of a tritium atom.

An isotope of hydrogen containing two neutrons and one proton in its nucleus. Tritium is also called super heavy hydrogen and is rare on Earth as it is radioactive with a half-life of about 12.3 years. It occurs in trace amounts in the upper part of the atmosphere, where it is produced by cosmic rays. It is generated artificially as a by-product in nuclear reactors such as CANDU.

The fusion of tritium with deuterium is expected to generate energy in thermonuclear power stations. It is envisaged that for fusion purposes, tritium would be produced by neutron capture in lithium or boron. The so-called tritium breeders will be part of a thermonuclear reactor.