Investigation of uncertainties in the re-saturation model of bentonite as near-field buffer in a spent fuel repository in granite

Bentonite plays an important role in the design of underground repositories for toxic and radioactive waste. The hydrophilic properties of bentonite appear to be ideally suited for minimising any contact of water with the waste. However, the behaviour of bentonite during re-saturation is a very complex phenomenon which may be influenced by hydraulic, mechanical, thermal and chemical processes. A short discussion of the microstructure of bentonite provides the base for a sound conceptual model of re-saturation. This model does not compare satisfyingly with the already existing numerical simulation codes, everyone of which is based on unsaturated flow or two-phase flow in the pore space. Thus, new conceptual models were developed at GRS for the re-saturation of bentonite. In accompanying experiments a data base for checking the new re-saturation models as well as the already existing models was created. The experiments show a high relevance of re-saturation via water vapour. Without further experiments it cannot be excluded that the main water transport mechanism is vapour diffusion in the pore space and not flow of liquid water. This would call the applicability of two-phase flow based codes to complex physical situations into question. (authors)