Nuclear fuel refers to materials that are used in nuclear reactors as a source of energy. These fuels typically undergo fission, which is the process by which their nuclei split into smaller fragments, releasing large amounts of heat and radiation. The most common type of nuclear fuel is uranium-235 or plutonium-239, both of which are highly radioactive elements that can sustain a chain reaction when bombarded with neutrons. Nuclear fuel plays an essential role in generating electricity through nuclear power plants and has become increasingly important as a source of clean energy.