In the preceding we have discussed the extraction of a single component from the aqueous feed. The selection is possible because of the large differences between the values of D for this component and other species like fission products. In general, with TBP, uranium and plutonium have high distribution

coefficient values and are co-extracted. Further separation between plutonium and uranium is thus required. In general this is done first by redissolving uranium and plutonium from the organic phase into an aqueous phase with low nitric acid concentration. Then plutonium is reduced from the tetravalent to the trivalent state with a mild reductant such as U4+ which does not reduce the hexavalent uranium. The trivalent plutonium cannot be dissolved back into an organic solvent, so that a new dissolution leaves the plutonium in the aqueous phase while uranium dissolves in the organic phase. In general, in order to increase the purity of the recovered uranium and plutonium, addi­tional scrubbing is necessary. These different steps, as typically implemented in the Purex process, are shown in figure 9.6.