Saturation of barrier materials under thermal gradient

- Research in context of engineered barriers for nuclear waste repositories.
- Two sealing materials: bentonite pellets and 65/35 sand/bentonite mixture.
- Effect of heating/hydration on temperature and humidity distribution analysed.
- Temperature depends on geometry and thermal conductivity of sealing and cell materials.
- Water redistribution more restricted in pellets (lower gas and liquid permeability).

Two different sealing materials-a granulate of MX-80 bentonite pellets (B) and a 65/35 sand/MX-80 bentonite mixture (S/B)-were tested in cylindrical cells to analyse the effect of heating and hydration on the spatial distribution of temperature and humidity inside them. The 50-cm long columns were heated on their base to 140°C while Pearson water was supplied through their upper surface at a very low pressure. The aim of the tests was to simulate in the laboratory the conditions of the two barrier materials used in a large-scale in situ test (the HE-E, performed at the Mont Terri Underground Laboratory), reproducing the engineered barrier of an underground repository of nuclear waste. The S/B cell was dismantled after three years of operation, whereas cell B has been running for four years.The temperature distribution was very dependent on the geometry of the experimental setup and on the thermal conductivities, not only of the sealing materials, but also of the different components of the cells. The changes in thermal conductivity of the sealing materials because of the changes in water content affected the heat conduction and hence the temperatures inside the columns. Heat dissipation through the bottom of the cells seems to be a relevant process.The two materials have very different gas and liquid permeabilities, as well as water retention capacity, and this conditioned the water redistribution in the vapour phase triggered by heating as well as the liquid water intake, which were both much more restricted in the bentonite pellets. Convection cells probably formed near the heater, since evidences of soluble species transport and their precipitation in the area were found.The good consistency of column S/B upon dismantling indicated that, despite the low percentage of bentonite in the mixture and the low average dry density, the bentonite was able to swell and give the mixture a cohesive texture, even though the high salinity of the saturation water probably reduced the swelling capacity and increased the hydraulic conductivity of the material.