Identification of uranium enrichment scenarios by multi-method characterisation of immobile uranium phases

We investigate natural uranium occurrences as analogues for uranium migration and immobilisation in the post-operational phase of a radioactive waste repository. These investigations are aimed at gaining insight into the behaviour of uranium in a complex natural system. We characterise the immobile uranium phase and trace elements distributions in argillaceous uranium-rich samples from the Ruprechtov site, Czech Republic, applying a combination of different analytical methods. We use wet chemistry to determine the distribution of U(IV) and U(VI), sequential extraction to characterise different uranium phases, and 234 U/238U-activity ratios to correlate results between U(IV) and U(VI) distributions and the various uranium phases. Most of the uranium was determined to occur in a very long-term stable, tetravalent phase. Results from chemical methods are in good agreement with the results from spectroscopic methods. U(IV) mineral phases are identified by SEM-EDX spectroscopy and synchrotron-based μ-EXAFS. Electron-microprobe analysis confirmed that uraninite is newly formed and not a relictic phase from the altered granite. Correlation of uranium with As(V) located on thin As-rich layers on pyrite surfaces determined by confocal μ-XRF supports element correlations obtained by the sequential extraction. The key processes involved in uranium immobilisation in the argillaceous layers have been identified and can be used to reconstruct the geological history at the site.