Tritium

Tritium is a radioactive isotope of the chemical element Hydrogen (the symbol is H in the Periodic Table). Its nucleus contains one proton and two neutrons, as opposed to Deuterium, which contains one neutron, and Protium, the most common form of Hydrogen, which contains no neutrons. Tritium produces a great deal of energy when reacted, so it is a primary fuel source for Covenant starship reactors.

Tritium is extremely rare on Earth, but it abounds in deep space. Areas like the Lambda Serpentis system have large nebulae of Tritium gas, as does the planet Pegasi Delta. Covenant ships are equipped to collect Tritium gas from deep space, but it has been known to be harvested and collected from planet surfaces as well.