Spent nuclear fuel:
Spent nuclear fuel, also called used nuclear fuel, is the fuel that has undergone a reactor campaign and is no longer useful for sustaining the nuclear fission chain reaction in a thermal reactor.
Spent fuel discharged from a power reactor contains residual 235U and converted plutonium, as well as fission-product and transuranic wastes.
Spent nuclear fuel is highly radioactive and potentially very harmful. Spent fuel is dangerous because it contains a mixture of fission products, some of which can be long-lived radioactive waste, and also plutonium which is highly toxic. This necessitates remote handling and shielding.