Deuterium is an isotope of hydrogen with two nucleons : one proton and one neutron, most hydrogen has one nucleon : a proton. Deuterium is used in fusion reactors. With practical proton-proton fusion common hydrogen is most often fused with more hydrogen. Fusing deuterium with common hydrogen produces no neutron radiation, but still requires substantial shielding due to the gamma ray radiation released. It is sometimes used in small reactors and fused with lithium because the reaction produces no neutron radiation. It can be fused with hethree, but some neutron radiation is released, though far less than that release by hydrogen reactors. Hydrogen reactors, despite producing large amounts of dangerous radiation, are widely used due to their immense power output.