The organic phase exiting the codecontamination cycle contains most of the actinides (U, Pu, and Np) initially present in the dissolved fuel. Some resid­ual fission products, notably Tc and Zr, are also present in the organic phase. The next step of the process (referring to Fig. 6.2) is partitioning, which involves the selective stripping of Pu and U from the loaded organic in two complementary steps. The first is Pu partitioning and the later is U stripping.

Plutonium partitioning

The first operation in the partitioning cycle involves selective plutonium back-extraction or stripping from the loaded organic phase. This is accom­plished by the reduction of extractable Pu(IV) to the inextractable Pu(III) oxidation state. A variety of reducing agents have historically been used for the reduction and concomitant partitioning of plutonium, the most promi­nent being: (a) uranous cation, U(IV), (b) hydroxylamine nitrate (HAN), or (c) ferrous sulphamate, Fe(NH2SO3)2. Of these reducing agents, the later, ferrous sulphamate, is no longer used in modern PUREX plants and will not be further discussed. Hydroxlyamine nitrate is used in some instances in the downstream Pu purification cycle and will be further described sub­sequently. The reductant universally used in modern facilities for Pu parti­tioning is U(IV). However, to explain the logic behind the process diagram, it is once again convenient to regress to a brief review of the pertinent process chemistry.