The separation is based upon hydrometallurgical techniques of liquid-liquid extraction in which material is transferred between two immiscible liquids: aqueous and organic phases. Extraction consists in passing the materials to be separated from the aqueous phase to the organic phase (solvent). The reverse

operation is called re-extraction. These operations are the core of the PUREX process, Fig. 16.6, whose main steps are:

• uranium and plutonium separation from fission products by extracting jointly uranium and plutonium from the clarified aqueous solution (Fig. 16.7)

• separation of uranium and plutonium (U/Pu partition)

The nitric acid solution that contains the nuclear material is mixed with the solvent (TBP diluted in a C12 alkane with a formula close to kerosene). TBP is selected for its very low miscibility in aqueous solution, its selectivity with regards to uranium and plutonium, its resistance to hydrolysis in a nitric acid environment, its resistance to radiolysis and its ability to be regenerated.

Uranium and plutonium migrate to the solvent while fission products stay within the aqueous phase (nitric acid). Settling and decanting then allows the two phases to be separated. This operation is repeated several times. The fission products that remain dissolved in the nitric acid are then concentrated through evaporation and stored in tanks prior to vitrification. Traces of fission products and minor actinides are still present in the solvent containing uranium and plutonium; they are removed through additional washings with nitric acid.