Hethree is Helium-3, an isotope of helium with three nucleons : two protons and one neutron, most helium has four nucleons : two protons and two neutrons. It's used as fuel for fusion reactors. These reactors require no radiation shielding because the reaction produces no dangerous neutron radiation. Reactors that use common hydrogen produce much more energy, but also dangerous amounts of neutron radiation. Hethree is usually fused with more hethree, which produces very clean energy. Deuterium can also be used, but during fusion some of the deuterium will fuse with more deuterium, releasing neutron radiation, though the amount released is much less than with a hydrogen reactor. Hethree reactors can be made small enough to power ground vehicles and are used in jet engines. When used is jet engines, they are the source of heat and no "fuel" is burned, other than very small amounts used in the reactor. Since no fuel is oxidized, these jet engines can be used in any atmosphere, whereas a hydrogen powered jet requires an atmosphere with substantial amounts of oxygen.