(GRS 269) Code Viper – Theory and Current Status

Compacted air-dry bentonite is foreseen in many repository designs to be utilised as a geotechnical barrier since its hydrophilic and thereby swelling properties let it appear to be a material ideally suited for this purpose. The work reported here concentrates on the re-saturation of a partially saturated (air-dry) bentonite.
 
Numerical simulation of this process is presently based on a thermo-hydro-mechanically (THM) coupled approach. The underlying conceptual model, however, is not entirely consistent with the processes on a micro-scale. Thus an alternative ap-proach was developed at GRS that is more closely related to microstructural phenomena and fits the requirements of a bentonite buffer in a repository as realistically as possible. Based on balance equations that were especially derived for this purpose a numerical code was developed for testing the theory. The code – now called VIPER – was stepwise advanced from physically simple to the expected complex conditions in a repository.
 
This report summarises the theory behind VIPER including the conceptual model, the properties of bentonite relevant for the model, and the development of the related balance equations. Also described are the transfer of the mathematical model into a numerical model and the steps taken to qualify the model for repository relevant condi-tions including the latest effort to extend the applicability of the model to bentonite-sand mixtures. However, since the development of VIPER is not yet finished some questions still remain to be answered.