Spent nuclear fuel is the used material that remains after nuclear reactors have generated electricity. It consists mainly of uranium and plutonium, along with various radioactive isotopes that are produced during the fission process. After fuel is used, it is highly radioactive and generates heat, requiring careful handling and storage. Once removed from the reactor, spent nuclear fuel is typically stored in pools of water for several years to cool down and shield radiation. After this initial cooling period, it can be transferred to dry cask storage or reprocessed to recover usable materials, such as plutonium and uranium.