Tritium (or, symbol or, also known as Hydrogen-3)'s a radioactive isotope of hydrogen. The nucleus of tritium (sometimes called a triton) contains one proton and two neutrons, whereas the nucleus of protium (the most abundant hydrogen isotope) contains one proton and no neutrons. Decay While tritium has several different experimentally-determined values of its half-life, the NIST recommends 4,500±8 days (approximately 12.33 years). It decays into helium-3 by the reaction All atomic nucle… (More on Tritium)