Tritium

Tritium, chemical symbol T or 3H is an isotope of the element hydrogen which has a nucleus containing one proton and two neutrons (i.e., atomic weight 3.016). A compound containing tritium is said to be tritiated.

It has a half-life of 12.3 years and emits a very weak beta particle, which non-penetrating with a maximum energy of 18.6 keV and an average of 5.7 keV.

Most tritium is produced in nuclear reactors, although some can be formed by cosmic rays.

Nuclear weapons
Its major use is in neutron generators and tritium boosting for fission devices, and, mixed with deuterium, as fuel for fusion devices. Replacement of decayed tritium is one of the major drivers of maintenance for nuclear weapons.

Lighting
Tritium is used in a number of lighting applications, such as exit signs and gunsights, where reliable lighting is needed without an electrical power source.