Nuclear fuel

Nuclear fuel is a material that can be consumed to take over nuclear energy, by analogy to chemical fuel that is burned for energy. Nuclear fuels are the most dense sources of energy available. 
Most nuclear fuels contain heavy "fissile" elements which to undergo a nuclear fission chain reaction in a nuclear reactor. The most common nuclear fuels are uranium and plutonium but not all nuclear fuels are used in reactors. Some of them power radioisotope thermoelectric generators.