A process in which the spent nuclear fuel is reprocessed and the usable elements inside it are used as fuel. The spent fuel (incorrectly “burnt”) removed from a nuclear reactor still contains fissile isotopes of uranium and plutonium. They can be separated in a reprocessing plant and used in the production of new fuel, for example of MOX type. The remaining highly radioactive waste is then concentrated and vitrified. This process is called the closed fuel cycle. When the spent fuel is not reprocessed, we talk about an open fuel cycle.