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Why is tritium radioactive?
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Tritium is radioactive because it is an unstable isotope of hydrogen that possesses an excess of neutrons relative to its number of protons.12
To achieve stability, tritium undergoes beta decay, a process in which one of the neutrons in its nucleus converts into a proton. During this transformation, the nucleus emits an energetic electron (a beta particle) and an antineutrino. This decay process changes the atomic number of the nucleus, effectively transmuting the tritium atom into a stable helium-3 atom. Tritium has a half-life of 12.32 years.21