Plutonium in the aqueous phase leaving partitioning contains around 1 percent of the feed uranium, an uncertain fraction of the feed neptunium, and fission products. This plutonium is usually purified by two additional cycles of solvent extraction. In each, plutonium is made tetravalent and, in the A contactor, is extracted by TBP, together with the uranium, thus separating it from most of the fission products and neptunium, here mostly pentavalent. In the В contactor, the plutonium is stripped selectively from uranium into the aqueous phase, either by use of 0.35 M HN03 or by reducing plutonium to the trivalent state prior to stripping. In some plants, final plutonium purification is by anion exchange from nitrate solution. Some others use precipitation as plutonium oxalate with oxalic acid. Optimum conditions are reported [S23, p. 449] to be as follows: HN03, 1.5 to 4.5 M; H2C204, 0.05 to 0.15 M. A problem in plutonium purification systems is heating and radiolysis of solvent or resin owing to the high alpha activity.

Many reprocessing plants are required to convert purified nitrates containing plutonium to oxides before shipment. Conversion processes are described in Chap. 9, Sec. 4.7.