Deuterium

Deuterium is an isotope of the chemical element Hydrogen. Its nucleus contains one proton and one neutrons, as opposed to Tritium, which contains two protons, and Protium, the most common form of Hydrogen, which contains no neutrons. Tritium amd Deuterium produce a great deal of energy when reacted together, so they are a primary fuel source for Covenant starship reactors.

Deuterium has a natural abundance on planet surfaces, such as on Earth and Pegasi Delta. The Covenant build refineries to refine this material for use in their ships .