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Free keywords:
facies; uranium speciation; sorption; reactive transport; heterogeneity; PHREEQC; MontTerri; repository far-field
Abstract:
Multi-component (MC) diffusion simulations enable a process based and more preciseapproach to calculate transport and sorption compared to the commonly used single-component(SC) models following Fick’s law. The MC approach takes into account the interaction of chemicalspecies in the porewater with the diffuse double layer (DDL) adhering clay mineral surfaces. Westudied the shaly, sandy and carbonate-rich facies of the Opalinus Clay. High clay contents dominatediffusion and sorption of uranium. The MC simulations show shorter diffusion lengths than the SCmodels due to anion exclusion from the DDL. This hampers diffusion of the predominant speciesCaUO2(CO3)2−3. On the one side, species concentrations and ionic strengths of the porewater andon the other side surface charge of the clay minerals control the composition and behaviour ofthe DDL. For some instances, it amplifies the diffusion of uranium. We developed a workflow totransfer computationally intensive MC simulations to SC models via calibrated effective diffusionand distribution coefficients. Simulations for one million years depict maximum uranium diffusionlengths between 10 and 35 m. With respect to the minimum requirement of a thickness of 100 m, theOpalinus Clay seems to be a suitable host rock for nuclear waste repositories.
