tritium (trĭtˈēəm) [key], radioactive isotope of hydrogen with mass number 3. The tritium nucleus, called a triton, contains one proton and two neutrons. It has a half-life of 12.5 years and decays by beta-particle emission. The symbol is T or 3H. It is one form of heavy hydrogen, the other being deuterium. It is usually produced in nuclear reactors as a byproduct of the irradiation of lithium. Its current major use is to increase the yield of thermonuclear devices. The U.S. Department of Energy has a production reactor in Savannah, Georgia for this purpose. In the future, vast amounts of tritium will fuel experiments in fusion research. Canada, Europe and Japan have extensive programs underway to develop physics-based, as opposed to mechanical based, production procedures to generate the volumes necessary to proceed with these experiments.