Uranium leaving the partitioning step in the organic phase is back extracted to the aqueous phase by 0.01 M HN03. It is then purified by one or more additional cycles of solvent extraction by TBP, while plutonium is kept in the inextractable trivalent state. To purify this uranium sufficiently to permit its use as feed to a UF6 plant, the U. S. DOE requires that the total beta-gamma activity be less than twice that of aged natural uranium and that the alpha activity be less than 1500 disintegrations per minute per gram of uranium, corresponding roughly to a plutonium-uranium ratio less than і X 10"8. To meet these strict specifications, a final cleanup step is usually needed. The first Purex plants passed the concentrated uranyl nitrate solution through a silica gel bed, which adsorbs fission products, primarily zirconium and niobium. A recent, more versatile process [B4, SI], developed in Italy, removes zirconium, niobium, and tetravalent neptunium and plutonium from aqueous nitrate solution by batch extraction with a 0.4 M solution of oleyl hydroxamic acid in 20 v/o octyl alcohol, 80 v/o n-dodecane. Distribution coefficients for these contaminants in this solvent are very high. When the solvent becomes too contaminated it is regenerated by washing with aqueous oxalic and nitric acids.