Separation techniques for low-level determination of actinides in soil samples

The separation methods for soil samples applied at PSI are based on extraction chromatography and ion exchange. After sample leaching, the actinides are pre-concentrated via precipitation using oxalic acid. Besides the classical separation methods applying the extraction chromatographic resins U/TEVA™ (for U, Th), TRU™ (Pu, Am), new methods were recently implemented to increase the radiochemical recovery of particularly trivalent Am and Cm. These methods do not require initial reduction of Pu(IV) to Pu(III) but stabilize Pu on the tetravalent oxidation state using a mixture of NaNO₂/H₂O₂ in strong acidic medium. The Pu-fraction is then fixed along with Th onto Dowex AG 1-X2 anion exchanger resin. Th is eluted via complexation with 10 M HCl, Pu via reduction with HI. The fractions of Am+Cm and U are loaded onto DGA™ resin. This resin shows extraordinary high distribution coefficients (k′-values) exceeding 10⁴ (for Am) in strong nitric acid medium. The separation between U and Am is obtained quantitatively by decreasing the HNO₃ concentration from 3 to 0.25 M (stripping of the U-fraction) while Am can be easily eluted thereafter using 0.25 M HCl as complexation compound.