A process used to recover spent nuclear fuel from nuclear reactors by solvent extraction for recycling and fabrication into new fuels. An abbreviation for plutonium and uranium recovery by extraction, it is a hydrometallurgical process that was developed in the late 1940s in which spent nuclear fuel is initially dissolved in refluxing nitric acid. Solvent extraction is then used to extract the uranium and plutonium from the fission product (p. 306) containing nitric acid solution into a kerosene diluent containing the solvent tri-butyl phosphate (TBP). Final separation of the uranium from the plutonium involves manipulation of the plutonium oxidation state. It superseded the earlier butex process, which used solvent extraction of spent nuclear fuel by solvent extraction from a nitric acid solution using diethylene-glycol dibutyl ether as the solvent. See nuclear reprocessing.