Tritium

Tritium is a hydrogen isotope. The nucleus of tritium contains one proton and two neutrons, whereas a hydrogen nucleus consists of just one proton. It's [atomic weight]? is 3.

It is radioactive and has a half life of approximately 11 years.

Tritium figures prominately in studies of nuclear reactions, especially nuclear fusion due to it's fvorable reaction cross section and high energy yield. Please explain.

Small amounts used with phosphors for self illuminating dials, especially watches.