Helium-3 is a non-radioactive isotope of Helium, the second element in the known periodic table. It is used in Hellion for fusion reactions, effectively as engines like primitive vehicles used to convert chemical energy into electrical energy. Helium-3 can be used in low temperature nuclear reactions by combining with natural Helium to splice off neutrons and generate approximately 5MeV per reaction, or it can be combined with Deuterium and Tritium (isotopes of Hydrogen) for higher yield reactions at 17.5MeV per reaction.
Helium-3 is refined from regolith, or top layer soil and stone, typically found on asteroids. By processing regolith in a refinery, you can recover Helium-3 for the reactor fuel.