Nuclear fuel that has been permanently discharged from a reactor after it has been irradiated. Typically, spent nuclear fuel is measured in terms of either the number of discharged fuel assemblies or the quantity of discharged fuel mass. The latter is measured either in metric tons of heavy metal (i.e., only the heavy-metal content of the spent nuclear fuel is considered) or in metric tons of initial heavy metal (essentially, the initial heavy-metal mass of the fuel before irradiation). The difference between these two quantities is the weight of the fission products produced during irradiation.

Nuclear reactor fuel that has been used to the extent that it can no longer effectively sustain a chain reaction.

(1) Fuel that has been withdrawn from a nuclear reactor following irradiation, has undergone at least one year's decay since being used as a source of energy in a power reactor, and has not been chemically separated into its constituent elements by reprocessing. Spent nuclear fuel includes the special nuclear material, by-product material, source material, and other radioactive materials associated with fuel assemblies. (2) Fuel that has been permanently withdrawn from a nuclear reactor following irradiation, but has not been processed to remove its constituent elements. (Does not include material categorized as waste that has been removed from the DOE accountability system in accordance with DOE 5633.3A, 5633.4 and 5633.5.) Synonym: depleted fuel. Synonym: spent fuel.