Tritium (seembol T or 3H, an aa kent as hydrogen-3) is a radioactiveisotope o hydrogen. The nucleus o tritium (whiles cried a triton) conteens ane proton an twa neutrons, whauras the nucleus o protium (bi far the maist abundant hydrogen isotope) conteens ane proton an na neutrons. Naiturally occurrin tritium is extremely rare on Yird, whaur trace amoonts are formed bi the interaction o the atmosphere wi cosmic rays. It can be produced bi irradiatin lithium metal or lithium bearin ceramic pebbles in a nuclear reactor. Tritium is uised as a radioactive tracer, in radioluminescent licht soorces for watches an instruments, an, alang wi deuterium, as a fuel for nuclear fusion reactions wi applications in energy generation an wappens. The name o this isotope is derived frae the Greek wird τρίτος (trítos) meanin "third".