Tritium ( or , from Ancient Greek τρίτος (trítos) 'third') or hydrogen-3 (symbol T or 3H) is a rare and radioactive isotope of hydrogen. The nucleus of tritium (sometimes called a triton) contains one proton and two neutrons, whereas the nucleus of the common isotope hydrogen-1 (protium) contains just one proton, and that of hydrogen-2 (deuterium) contains one proton and one neutron.
Naturally occurring tritium is extremely rare on Earth. The atmosphere has only trace amounts, formed by the interaction of its gases with cosmic rays. It can be artificially produced by irradiating lithium metal or lithium-bearing ceramic pebbles in a nuclear reactor, and is a low-abundance byproduct in normal operations of nuclear reactors.
Tritium is used as the energy source in radioluminescent lights for watches, numerous instruments and tools, and even novelty items such as self-illuminating key chains. It is used in a medical and scientific setting as a radioactive tracer. Tritium is also used as a nuclear fusion fuel, along with more abundant deuterium, in tokamak reactors and in hydrogen bombs.