Abstract
Multi-component (MC) diffusion simulations enable a process based and more precise approach 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 chemical species in the porewater with the diffuse double layer (DDL) adhering clay mineral surfaces. We studied the shaly, sandy and carbonate-rich facies of the Opalinus Clay. High clay contents dominate diffusion and sorption of uranium. The MC simulations show shorter diffusion lengths than the SC models due to anion exclusion from the DDL. This hampers diffusion of the predominant species CaUO2(CO3)32−. On the one side, species concentrations and ionic strengths of the porewater and on the other side surface charge of the clay minerals control the composition and behaviour of the DDL. For some instances, it amplifies the diffusion of uranium. We developed a workflow to transfer computationally intensive MC simulations to SC models via calibrated effective diffusion and distribution coefficients. Simulations for one million years depict maximum uranium diffusion lengths between 10 m and 35 m. With respect to the minimum requirement of a thickness of 100 m, the Opalinus Clay seems to be a suitable host rock for nuclear waste repositories.
Highlights
For the safe storage of especially highly radioactive waste, emplacement in deep subsurface geological repositories is favoured worldwide to ensure the protection of human and nature from the potential radiation exposure of the waste packages for periods of up to one million years [1]
MC diffusion simulations were performed for two model concepts (MC4Clay and MC2Clay ), which differ in the clay minerals considered for sorption, to identify the effect of the clay mineral composition on the migration lengths
In the shaly and carbonate-rich facies, uranium migrated less far into the Opalinus Clay in the MC simulations compared to SC, whereas, in the sandy facies, it was opposite
Summary
For the safe storage of especially highly radioactive waste, emplacement in deep subsurface geological repositories is favoured worldwide to ensure the protection of human and nature from the potential radiation exposure of the waste packages for periods of up to one million years [1]. Diffusion is the primary transport process in intact formations [2,3,4]. Another important and positive aspect of claystones with regard to the storage of nuclear waste is the high sorption capacity retarding the potential migration of radionuclides. The diffusive transport through the host rock is quantified with numerical simulations usually using Fick’s law [2]. According to this method, only one diffusion coefficient is used for all species, major and minor ions as well as all radionuclides, in the same way. The surfaces of clay minerals are charged due to their internal structure and contact to the external porewater
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