Quantification of uranium diffusion and sorption within a geochemical gradient in the Opalinus Clay on the host rock scale

In Claystones, the storage concept for spent nuclear fuel mainly consisting of uranium is based among others on the isolation of the radionuclides within the effective containment zone due to the high retention capacity of the host rock. The transport properties are typically determined in laboratory experiments representing the host rock and for defined geochemical conditions. However, previous numerical studies have shown for the Swiss Opalinus Clay, that sorption of uranium and with that the migration are highly affected by variations in the composition of the porewater resulting from mineralogical heterogeneities. The hydro-geological system of the Opalinus Clay is characterized by a 210 m thick, low permeable section embedded between aquifers. The porewater components of the low permeable section show an asymmetric geochemical gradient towards the aquifers due to diffusive exchange over several million years between them. By modelling the past history of the porewater based on multi-component diffusion simulations, we quantify uranium migration for one million years considering the changing geochemical conditions as a result of the hydro-geological system. Our results show, that uranium is retained within the theoretical effective containment zone and adjacent aquifers are not reached. Consequently, a clay rock with a thickness > 100 m in a geochemical and mineralogical framework like the Opalinus Clay would be a suitable host rock for the storage of uranium from spent fuel.