Tritium (³H)

contains two neutrons in its nucleus and is radioactive with a 12.3-year half-life, which is continuously formed in the upper atmosphere due to cosmic rays.  It is can also be made in a lab from Lithium-6 in a nuclear reactor. Tritium is also used in hydrogen bombs. It is very rare (about 1 in every 1,018 atoms) and is formed in the environment by cosmic ray bombardment. Most tritium is manufactured by bombarding Li with neutrons. Tritium is used in thermonuclear weapons and experimental fusion reactors.