Development of a Special COMSOL® Interface Based on an Alternative Model for Bentonite Re-Saturation

Bentonite is a versatile material that is, among other things, envisaged in most countries worldwide for use in radioactive waste repositories to protect the waste canisters against groundwater. The saturation process of bentonite is commonly described with thermo-hydro-mechanically (THM) coupled models. As an alternative to the formulations in these THM models, a thermo-hydraulically coupled saturation model for confined conditions has been developed and realized for 1D-problems in the FORTRAN code VIPER. To enhance the possible range of applications of code VIPER, a new interface has been developed using the COMSOL® Physics Builder. The resulting VIPER interface can be used like a native COMSOL® interface. At first, a COMSOL® model based solely on the VIPER interface has been set up that matches an isothermal water uptake test performed at GRS. After successfully completing this step, further terms in the balance equation as well as equations of state were added which are required for calculating non-isothermal water transport in the bentonite. Furthermore, a full coupling of the VIPER interface with the native Heat Transfer in Porous Media interface has been realized. The new coupling has been checked based on temperature and humidity measurements from the FEBEX in-situ experiment performed at the Grimsel Test Site in Switzerland. The FEBEX experiment was intended to represent the initial phase of storage of heat-producing nuclear waste where canister-like heaters were installed inside compacted bentonite blocks and emplaced in a tunnel in the granitic rock. Wetting of the bentonite buffer was provided by the quite strongly water conducting host rock. Measured and simulated temperature evolution as well as wetting dynamics in the buffer were found to match satisfyingly well. The work presented here allows for the application of the alternative re-saturation concept to a wide range of three-dimensional problems not applicable to the original code VIPER. Also, the process of creating, testing, and implementing an own COMSOL® interface using the Physics Builder is illustrated in this context.