Nuclear fuel removed from a reactor after it has undergone a fission reaction. During fission, the amount of fissile material, such as uranium or plutonium, in the fuel has been reduced. At the same time, the content of fission products, which are not usable or even inhibit the fission reaction by absorbing neutrons, has increased. Such fuel is no longer energetically usable and must be replaced in the reactor by fresh fuel. Spent fuel is highly radioactive due to the fission products. The intense radioactive decay that takes place within it causes it to heat up. To ensure cooling, the spent fuel is usually stored in the spent fuel pool after removal from the reactor. The water layer serves as both a coolant and a shield. Spent fuel can be further stored in interim storage, sent for reprocessing, or stored in a deep geological repository.