The purpose of the plutonium purification cycle is to complete Pu decon­tamination and to concentrate the plutonium stream prior to the conversion step to the solid oxide. The plutonium entering this step is in the Pu(III) oxidation state, and as a prerequisite to extraction, the plutonium must undergo reoxidation to Pu(IV). This oxidation is typically accomplished by sparging the solution with nitrous vapors (in essence, the chemical reaction in Eq. 6.7), and subsequently with air to remove excess nitrous acid. Subsequent to this reoxidation step, the plutonium extraction and back — extraction operations both result in increased concentration. It should be noted that such concentration of the plutonium stream may be enhanced by providing recycle of part of the production stream to the head end of the cycle.

Flowsheets have been developed using uranous cation, U(IV), as the reductant for Pu partitioning in the purification cycle. However, reductive stripping in the Pu purification cycle is sometimes performed using hydrox — ylamine nitrate (HAN) as the reductant. Hydroxylamine nitrate, although not as effective as U(IV) for Pu reduction, is considered better suited than U(IV) for plutonium cycles involving higher plutonium concentrations. Furthermore, the re-addition of uranium back into the Pu purification cycle seems counter-intuitive since that negates the desired separation obtained in the initial partitioning cycle. Hydroxylamine nitrate (HAN) can readily reduce Pu(IV) to Pu(III) as follows:

4Pu4+ + 2NH3OH+ ^ 4Pu3+ + N2O + H2O + 6H+ 6.10

Furthermore, HAN will also scavenge HNO2 at low acidities, up to ~1 to 1.5 M HNO3:

NH3OHNO3 + HNO2 ^ N2O + HNO3 + 2H2O 6.11

The additional nitrate available from HAN also provides a salting effect at low acidity which partly compensates to help keep uranium in the organic phase. HAN is not required in large excess and it can be readily destroyed by heating the solution to >60 °C before the subsequent plutonium finishing steps.

Even with the use of HAN as the primary Pu reductant, purification flowsheets can include a “plutonium barrier”, using hydrazine stabilized U(IV) to complete plutonium stripping from the solvent, prior to the solvent regeneration operation. In the La Hague plants, HAN is used as the reductant in the Pu purification cycles: UP-3 uses the HAN flowsheet in pulsed columns; UP-2 uses HAN in centrifugal contactors. In the UP-2 plant, the Pu product from the first (codecontamination) cycle requires no additional decontamination from U; additionally, the introduction of a single reductant (HAN) is much simpler than adding several as would be the case for uranous nitrate. The Pu product from UP-2 is reportedly quite pure with <5000 ppm total impurities, <0.1 pCi P-у activity, and <100 ppm U (Baron 2008).

Uranium purification cycle

A fraction of the neptunium coextracted with uranium, and plutonium follows the uranium stream through the process. The uranium stripping operation in the partitioning step is operated such that the uranium product stream is at a relatively low concentration in dilute nitric acid. This stream is concentrated in evaporators prior to treatment in the uranium purifica­tion cycle. This concentration increase is necessary if the extraction opera­tion is to be carried out with favorable organic to aqueous flow ratios. During the concentration step, neptunium is oxidized to Np(V) and Np(VI) and the acid concentration is increased in the aqueous solution fed to the U purification cycle. In order to complete uranium purification from Np, hydrazine nitrate is introduced into the extraction step to reduce extract­able Np(VI) to the inextractable Np(V) species. The uranium stripping operation is then similar to that carried out in the first cycle.