Tritium

Tritium is a radioactive isotope of the chemical element Hydrogen (the symbol is H in the Periodic Table).

Description
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 Pinch Fusion Reactors.

Tritium is extremely rare on Earth, but it abounds in deep space. Areas like the Lambda Serpentis system possess large amounts 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.

The Spartan-III Beta Company was sent to neutralize a Covenant tritium refinery on Pegasi Delta, in 2545 with all but two losing their lives in the process.