The role of interfaces in the bentonite barrier of a nuclear waste repository on gas transport

The FEBEX in situ test provided bentonite samples that had been submitted to the conditions of the engineered barrier of a nuclear waste repository for 18 years. These samples can be considered quite evolved from the microstructural point of view (aged, matured) when compared with samples prepared in the laboratory under shorter and more usual time scales. The barrier, composed of bentonite blocks, was hydrated with granitic groundwater under natural conditions while it was submitted to the thermal gradient generated by a heater mimicking the waste canister. Some of the samples were drilled between two bentonite blocks, therefore they were crossed along by an interface. The gas permeability of samples with and without interface was tested in the laboratory under different triaxial boundary conditions. Samples with an interface drilled in the inner part of the barrier (i.e. closer to the heater and consequently drier) had higher gas permeability than samples of similar accessible void ratio (related to dry density and water content) with no interface, and it was necessary to apply higher confining pressures to reduce or suppress gas flow in them. Both observations point to the interface as a preferential pathway for gas flow in this kind of samples. In contrast, wetter samples drilled along interfaces of the external part of the barrier (which had very low accessible void ratio, because of the high saturation), had permeabilities similar to those corresponding to the same accessible void ratio in the reference, untreated bentonite. This would prove the healing of the interfaces between blocks as a result of full saturation. The importance of the testing boundary conditions, particularly with respect to confinement, on gas transport processes was also highlighted