Abstract
Diffusive transport and sorption processes of uranium in the Swiss Opalinus Clay were investigated as a function of partial pressure of carbon dioxide pCO2, varying mineralogy in the facies and associated changes in porewater composition. Simulations were conducted in one-dimensional diffusion models on the 100 m-scale for a time of one million years using a bottom-up approach based on mechanistic surface complexation models as well as cation exchange to quantify sorption. Speciation calculations have shown, uranium is mainly present as U(VI) and must therefore be considered as mobile for in-situ conditions. Uranium migrated up to 26 m in both, the sandy and the carbonate-rich facies, whereas in the shaly facies 16 m was the maximum. The main species was the anionic complex CaUO2(CO3)32−. Hence, anion exclusion was taken into account and further reduced the migration distances by 30 %. The concentrations of calcium and carbonates reflected by the set pCO2 determine speciation and activity of uranium and consequently the sorption behaviour. Our simulation results allow for the first time to prioritize on the far-field scale the governing parameters for diffusion and sorption of uranium and hence outline the sensitivity of the system. Sorption processes are controlled in descending priority by the carbonate and calcium concentrations, pH, pe and the clay mineral content. Therefore, the variation in porewater composition resulting from the heterogeneity of the facies in the Opalinus Clay formation needs to be considered in the assessment of uranium migration in the far field of a potential repository.
Highlights
One of the major challenges for future generations is the long-term storage of nuclear waste
Diffusion and sorption processes of uranium in Opalinus Clay were investigated in one-dimensional diffusion models on the 100 m-scale and for a simulation time of one million years for the first time
Calcu lations were performed in PHREEQC depending on the different mineralogy in the three facies, shaly, sandy and carbonate-rich, and associated changes in porewater composition as a function of partial pressure of carbon dioxide pCO2 in the range of 10− 2 bar and 10− 2.5 bar
Summary
One of the major challenges for future generations is the long-term storage of nuclear waste. The retardation ability of the host rock as natural barrier plays a crucial role in the safety case of a repository. Besides crystalline and salt rock, claystones are favourable host rocks for geological storage of nuclear waste due to low permeability and retardation of transport by sorption. In the Mont Terri region, located in north-western Switzerland, the Opalinus Clay for mation is about 160 m thick and was deposited 180 million years ago by sedimentation in a shallow marine environment of the Jurassic sea (Nagra, 2002a; Pearson et al, 2003). Results prove the applicability of laboratory diffusion coefficients to the formation (Van Loon et al, 2003, 2004; Wersin et al, 2008; Appelo et al, 2010; Leupin et al, 2017)
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