Simulation of diffusive uranium transport and sorption processes in the Opalinus Clay


Simulation of diffusive uranium transport and sorption processes in the Opalinus Clay

Hennig, T.; Stockmann, M.; Kühn, M.

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 100m-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 26m in both, the sandy and the carbonate-rich facies, whereas in the shaly facies 16m was the maximum. The main species was the anionic complex CaUO2(CO3)3-2 and hence the effect of anion exclusion was taken into account. This further reduced the migration distances by 30%. The concentrations of calcium and Carbonates reflected by the set pCO2 determine speciation and activity of uranium due to the formation of ternary uranyl complexes and consequently the sorption behaviour. Our simulation results showed, that 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.

Keywords: Reactive transport; facies; heterogeneity; carbonate; PHREEQC; Mont Terri; speciation

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Publ.-Id: 30898