Articles techniques et présentations

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 thermo-hydro-mechanically (THM) coupled process of bentonite saturation is commonly based on two-phase flow. As an alternative to the formulations in these THM models, a thermo-hydraulically coupled saturation model for confined conditions (as expected in a repository) has been developed and realised for 1D-problems in the FORTRAN code VIPER. In order to enhance the inherently limited range of possible applications of this code, an own interface, based on the VIPER equations has been developed using the COMSOL® Physics Builder. An example for the user interface of the Physics Builder is shown in figure 1. After including the VIPER interface in the COMSOL® plugins, it could be used like a COMSOL® interface.
At first, a COMSOL® model based solely on the VIPER interface has been developed that matches an isothermal water uptake test performed at GRS as well as the earlier performed simulations with code VIPER very well, shown in figure 2. 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 Heat Transfer in Porous Media interface that is included in COMSOL Multiphysics® has been realised. Because COMSOL’s own interfaces cannot be accessed within the Physics Builder, the coupling was prepared in the VIPER interface, so that it had to be implemented manually in the final model. The new coupling has been checked on the basis of 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. A technical illustration of the experiment setup is shown in figure 3. Wetting of the bentonite buffer was provided by the quite strongly water conductive host rock. Measured and simulated temperature evolution as well as wetting dynamics in the buffer were found to match satisfyingly well. A comparison between the measured and calculated humidity evolution for one point is shown in figure 4. 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 demonstrated in this context. However, a number of further enhancements of the new model are conceivable as a follow-up such as additional boundary conditions for water vapour or swelling into free space.