An Alternative Equation-Based Model in COMSOL Multiphysics® for Bentonite Re-Saturation

Bentonite is a versatile clayey material that is, among other things, envisaged in designs for radioactive waste repositories as a geotechnical protection of the waste canisters against groundwater. The thermo-hydraulic-mechanically (THM) coupled process of bentonite re-saturation is commonly simulated using a two-phase flow and a mechanical stress formulation like the Barcelona Basic Model for unsaturated clayey soils. As an alternative to the numerically demanding THM-formulations, a thermo-hydraulic saturation model for confined conditions (as expected in a repository) based on vapour flow in the pore space has been developed. The conceptual model was experimentally realised in the one-dimensional FORTRAN-code VIPER. While being successfully used, the range of possible applications is inherently limited. The underlying partial differential equations for isothermal water uptake have therefore been transferred to COMSOL Multiphysics® in form of an equation-based model. The non-linear balance equation was implemented using the coefficient form of the PDE interface. It was linearized by taking the solution from the previous timestep to calculate the parameters that depend on the primary variable. Subsequently, a model was set up to match earlier calculations with code VIPER concerning a 1D laboratory test of water uptake. Validation of the new implementation in COMSOL was therefore based on a code comparison as well as on matching model results with the real data. Based on this model, a simple application was developed to provide a reference for subsequent modelling of multi-dimensional problems of the same type. A 2D- and a 3D-model expanding the 1D-problem to two and three dimensions was successfully tested that way. A last 3D-model of a rod with a quadratic cross-section demonstrates the influence of limited space for water uptake on the uptake dynamics.