The Purex process uses a mixture of tributyl phosphate (TBP) and a hydrocarbon diluent to extract uranyl nitrate and tetravalent plutonium nitrate from an aqueous solution containing nitric acid. The Purex process was suggested by the discovery of Warf [W2] in 1949 that tetravalent cerium nitrate could be separated from the nitrates of trivalent rare earths by solvent extraction with TBP. The Purex process was developed by the Knolls Atomic Power Laboratory of the General Electric Company and carried through the pilot-plant stage at Oak Ridge National Laboratory from 1950 to 1952. It was adopted by E. I. duPont de Nemours and Company for the Savannah River plutonium-production plant that company built for the U. S. AEC at Aiken, South Carolina, where the Purex process was put into operation in November 1954. Its success there led to replacement of the Redox process by the Purex process by the General Electric Company at Hanford in January 1956. The Purex process was used in a plant owned by Nuclear Fuel Services, Inc., which that company operated at West Valley, New York, from 1966 to 1972. The plant was designed to reprocess 1 MT/day of irradiated, slightly enriched uranium fuel. It also reprocessed irradiated thorium and irradiated plutonium, with appropriate flow-sheet modifications [R8]. This plant was noteworthy for being the only one to reprocess fuel from privately owned nuclear power plants in the United States.

Although the West Valley plant met all safety and environmental requirements in effect when it first went into operation, in the 1970s the plant was required to meet increasingly strict licensing requirements on permissible radioactive effluents and resistance to ground motion in an earthquake. It was also required to provide facilities for converting plutonium nitrate to oxide and for solidifying high-level acid wastes instead of neutralizing them and storing as liquid. Because of the high cost of retrofitting the plant to meet these later requirements and because its capacity was by then too small to permit it to compete with a larger plant under construction by Allied-General Nuclear Services (Sec. 4.14), the West Valley plant was permanently shut down in 1976 [N8].

A solvent extraction process similar to Purex using TBP was developed by the Commissariat a l’Energie Atomique [Gl] for use in the French plutonium separation plant at Marcoule. Since then, the Purex process has replaced the Butex process at Windscale [W3], has been used in the Soviet Union [SI 1], India [S7], and Germany [S3], and by now is the universal choice for separation of uranium and plutonium from fission products in irradiated slightly enriched uranium. Fuel from the liquid-metal fast-breeder reactor (LMFBR) is also reprocessed by the Purex process, with modifications to accommodate the higher concentrations of plutonium and fission products.

The Purex process has four significant advantages over the Redox process: (1) Waste volumes can be made much lower, as the nitric acid used as salting agent can be removed by evaporation. (2) The solvent, TBP, is less volatile and less flammable than hexone. (3) TBP is more stable against attack by nitric acid. (4) Operating costs are lower.

The Purex process will be described in considerable detail in Sec. 4.