Separation of low levels of actinides by selective oxidation/reduction and co-precipitation with neodymium fluoride

Abstract

A systematic study of separating the actinides from each other in 1 M hydrochloric acid media has been carried out using selective oxidation/reduction processes followed by coprecipitation with neodymium fluoride. We have optimized two such procedures, one with bromate and another with permanganate, for the sequential separation of Am, Pu, Np, and U isotopes. The first procedure involves oxidation of Pu, Np, and U to +6 state in 1 M HCl media at 85° C with 30% NaBrO₃ and separation from trivalent Am by collecting the latter on the first NdF₃ coprecipitated source. Plutonium is then reduced and converted to +4 oxidation state with 40% NaNO₂ at 85°C, while Np and U are kept oxidized with additional bromate in 50–70°C hot solution, thus separating Pu by collection on a second NdF₃ source. At this stage, Np present in the filtrate is reduced with hydroxylamine hydrochloride and separated from U by collecting on a third source. Subsequently, U is reduced with 30% TiCl₃ and co-precipitated on a final source. The second procedure, which employs KMnO₄ in 1 M HCl media at 60–85°C for oxidizing Pu, Np, and U, and separating from Am, produced MnO₂ which is collected along with Am on the coprecipitated NdF₃. This MnO₂ is dissolved on the filter itself with 1 mL of acidified 1.5% H₂O₂ without any degradation of the α-spectra. After evaporating the filtrate to destroy H₂O₂, Pu, Np, and U are separated by following steps similar to those in the bromate procedure. The recoveries of the actinides with both procedurés are >99%. The decontamination factors are between 10³ and 10⁴. The precision and accuracy of measurements, as expressed by the relative standard deviation of replicate analyses, are within 5%. Absolute detection limits for a one-day count on a 600 mm² detector at 32% counting efficiency and 450 mm² detector at 27% counting efficiency are about 2.7×10⁻⁴ and 3.2×10⁻⁴ Bq, respectively. These procedures have been applied to the analysis of actinides in environmental samples.