Isotope of hydrogen, chemical symbol written as 3H or T, with atomic number 1 but atomic weight approximately 3. Its nucleus contains one proton and two neutrons. Tritium is radioactive (see radioactivity), with a half-life of 12.32 years. Its occurrence in natural water in an amount 1018 that of ordinary hydrogen is believed to be due to the action of cosmic rays. Some tritium is used in self-luminous materials (e.g., for watch dials) and as a radioactive tracer in chemical and biochemical studies. Nuclear fusion of deuterium and tritium at high temperatures releases enormous amounts of energy. Such reactions have been used in nuclear weapons and experimental power reactors. See also heavy water.
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