Isotope of hydrogen , chemical symbol written as 3 H or T, with atomic number 1 but atomic weight approximately 3.
Its nucleus contains one proton and two neutron s. Tritium is radioactive (see radioactivity ), with a half-life of 12.32 years. Its occurrence in natural water in an amount 10 -18 that of ordinary hydrogen is believed to be due to the action of cosmic ray s. Some tritium is used in self-luminous materials (e.g., for watch dials) and as a radioactive tracer in chemical and biochemical studies. deuterium and tritium at high temperatures releases enormous amounts of energy . Such reactions have been used in nuclear weapon s and experimental power reactors. See also heavy water .