As described in Chap. 3, fuel discharged from a nuclear reactor after irradiation to the end of its useful life still contains most of the fertile material (238U or thorium) that was present in the fuel when charged, appreciable concentrations of valuable fissile nuclides (235 U, plutonium, and/or 233U) and large amounts of radioactive, neutron-absorbing fission products. The principal objectives of reprocessing are (1) to recover uranium and plutonium, and thorium if present, for reuse as nuclear fuels; (2) to remove radioactive and neutron-absorbing fission products from them; and (3) to convert the radioactive constituents of spent fuel into forms suitable for safe, long-term storage. There may be some interest in recovering individual fission products such as 90Sr or 137Cs for use as radiation sources or in recovering by-product transuranic elements such as neptunium, americium, or curium.