Immobilisation and Retention Processes of Uranium in Tertiary Argillaceous Sediments (Czech Republic)

摘要

Natural analogues can provide valuable information on timescales that are important for performance/safety assessment of deep geological repositories, specifically in the fields of process understanding and model validation. The Ruprechtov natural analogue study is focused on understanding and quantification of relevant geochemical and hydraulic processes related to uranium (U) mobilisation and behaviour in argillaceous sediments. The Ruprechtov site is situated in the western part of the Czech Republic where Tertiary basins with clay and organic material (coal, lignite) commonly occur, in some cases with high concentrations of U. U accumulation was identified in two separated horizons at the depths of 12-15 and 30-40 m in argillitised volcanodetritic layers with kaolin and granite forming the underlying rocks. It is supposed that granite was the source of the U. The volcanodetritic sediment is highly heterogeneous in mineralogical content even at a small scale. U in sediment is present in the form of secondary phosphorous minerals (ningyoite) and also associated with organo-clay complexes and Fe hydrous oxides. There is an indication that U accumulation can also be present in the underlying kaolin. Two borehole monitoring systems with bladder pumps were established to allow sampling of undisturbed groundwater. Interpreting acquired data, it is supposed that U was mobilised under near-surface oxidising conditions with subsequent immobilisation in downgradient reduced conditions. Minor supply due to in situ dissolution of U-bearing minerals or from discrete tectonic zones in underlying granite could also contribute to the accumulation of U. Future investigations will be focused on site-scale characterisation (connectivity of permeable horizons) integrated with a laboratory migration/retention experimental programme and transport/ geochemical modelling.