Multispecies random walk simulations in radial symmetry: model concept, benchmark, and application to HTO, 22Na and 36Cl diffusion in clay
Shuo Meng, Wilfried Pfingsten, 2017. "Multispecies random walk simulations in radial symmetry: model concept, benchmark, and application to HTO, 22Na and 36Cl diffusion in clay", Radioactive Waste Confinement: Clays in Natural and Engineered Barriers, S. Norris, J. Bruno, M. van Geet, E. Verhoef
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Modelling of radionuclide transport in clay can be simplified and accelerated by exploiting radial symmetry of laboratory experiments or nuclear waste repositories design. Consequently, the multispecies reactive transport code MCOTAC has been extended to radial symmetry, exploiting the advantages of its random walk transport description. Random walk in radial r-symmetry is mimicked by two-dimensional (2D) random walk projected to a radial coordinate for geochemical calculations. This guarantees fast 2D(x, y) transport calculations, together with complex geochemical calculations in 1D(r) only. The new model concept has been benchmarked for simple geochemical systems and applied to laboratory diffusion experiments with HTO, 22Na and 36Cl in Opalinus Clay using higher spatial resolution for modelling than reported in the literature. This makes it possible to distinguish between the transport properties of the sample and the filters, and yields more careful determination of radionuclide transport parameters: for example, there was a difference of a factor of 2 between the diffusion coefficient and log Kd sorption coefficient in the case of 22Na compared to the former best-fit analysis.
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It is internationally accepted that the safest and most sustainable option for managing radioactive waste is geological disposal, utilizing both engineering and geology to isolate the waste and contain the radioactivity.
This Special Publication contains 25 scientific studies presented at the 6th conference on ‘Clays in natural and engineered barriers for radioactive waste confinement’ held in Brussels, Belgium in 2015. The conference and this resulting volume cover many of the aspects of clay characterization and behaviour considered at various temporal and spatial scales relevant to the confinement of radionuclides in clay, from basic phenomenological process descriptions to the global understanding of performance and safety at repository and geological scales.
The papers in this volume consider research into argillaceous media under the following topic areas: large-scale geological characterization; general strategy for clay-based disposal systems; geomechanics; mass transfer; bentonite evolution and gas transfer.
The collection of different topics presented in this Special Publication demonstrates the diversity of geological repository research.