Uranium/plutonium separation (partition)

Uranium is separated from plutonium using the selective reduction of plutonium to the trivalent state (Fig. 16.8). This is done by adding four-valent uranous nitrate to the aqueous feed to produce the reaction

2Pu4+ + U4+ + 2H2O ^ 2Pu3+ + UO22+ + 4H+

In the trivalent state, plutonium has little ability to make neutral complexes with nitrate ions so that it has a higher affinity for the aqueous phase. The uranium, on the other hand remains in the organic phase in the six-valent state.

This plutonium reduction from valence IV to valence III is a delicate one within a nitric acid solution because plutonium III can be oxidized to plutonium IV by nitric acid through complex reactions where autocatalysis of nitrous acid plays a major role. This is prevented through the addition of hydrazine nitrate, which stabilizes uranous nitrate (U(NO3)4) but prevents parasitic re-oxidation of plutonium by nitrous acid:

HNO2 + NH2NH3+ ^ HN3 + 2H2O + H+

possibly followed by the reaction:

HNO2 + HN3 ^ N2 + 2N2O + H2O

The aqueous phase containing plutonium is scrubbed by a small flow of solvent to remove the fraction of uranium that is stripped with the plutonium (uranium scrubbing). The organic and aqueous phases containing, respectively, uranium and plutonium can be separated using a mixer-settler and a pulsed column. Uranium is then concentrated through evaporation. Uranium and plutonium are then transferred to the purification stages.

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16.8 Separation of uranium and plutonium (Source: AREVA, 2010).