Bentonite re-saturation: different conceptual models – similar mathematical descriptions
Klaus-Peter Kröhn, 2017. "Bentonite re-saturation: different conceptual models – similar mathematical descriptions", Radioactive Waste Confinement: Clays in Natural and Engineered Barriers, S. Norris, J. Bruno, M. van Geet, E. Verhoef
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Thermohydromechanically (THM) coupled models based on two-phase flow formulations for the hydraulics are presently the most established tools for simulating bentonite re-saturation. Alternatively, the extended vapour diffusion (EVD) model, involving basically diffusion of water vapour and of hydrated water, has been developed specifically for this purpose. These two model concepts are obviously based on entirely different hydraulic processes.
However, applying some acceptable simplifications to the underlying mathematical models leads to an identical formulation for the two approaches that looks formally like Fick’s second law with a saturation-dependent ‘diffusion coefficient’. Concerning the hydraulics, the THM codes hence state a mathematical problem that is similar to the one resulting from the EVD model.
A subsequent quantitative comparison of the two hydraulic approaches requires only the respective diffusion coefficients to be compared. This has been done on the basis of benchmark calculations for two different water-uptake tests with compacted MX-80 at largely the same conditions. Even for variations in temperature and dry density, the resulting diffusion coefficients for the two approaches showed remarkable agreement.
Different physical models have thus led to a similar mathematical problem. The mathematics indicates a diffusion-like process, although the exact nature of this process still remains to be determined.
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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.