Geochemical study of uranium mobility in tertiary argillaceous system at Ruprechtov site, Czech Republic

The natural analogue study at the Ruprechtov site is aimed to investigate and understand the behaviour of natural radioelements in plastic clay formation. Uranium is present predominantly in U(+IV) form, either in secondary uranium minerals (uraninite, ningyoite) or in detrital immobile phases (monazite, rhabdophane, xenotime, zircon) and in hardly characterisable non-crystalline forms as well. Process of uranium accumulation probably proceeded via reduction of U(VI) and U(IV) precipitation with involvement of As. Sorption also played important role in U accumulation formation. The sequential extraction experiments were performed in order to specify phases causing uranium retention and accumulation. The isotopic exchange tests with 233U were used to determine the amount of exchangeable uranium. The contradictions in the extraction schema used for the Ruprechtov sediment samples were found. The results prompt careful approach and potential sequential schema modification. (author)