Nuclear fuel is a material that can be burned by nuclear fission or fusion to derive nuclear energy. The various activities associated with the production of electricity from nuclear reactions are referred to collectively as the nuclear fuel cycle. The nuclear fuel cycle starts with the mining of uranium and ends with the disposal of nuclear waste. With the reprocessing of used fuel as an option for nuclear energy, the stages form a true cycle. Uranium is a naturally-occurring element which is enriched in the uranium-235 isotope, before being made into pellets that are loaded into the nuclear fuel assembly.