Thermo-hydro-geochemical tests on small cells.

Abstract

In the context of an investigation of the near field for a repository of high-level radioactive waste, the FEBEX Project, a set of laboratory tests has been designed to give a better understanding of the thermo-hydro-mechanical and geochemical behaviour of the compacted bentonite. Small compacted samples of bentonite are heated during variable periods of time, of up to 80 days, under different thermal gradients. The hydration water is either granitic, simulating the conditions of the outer part of the barrier, or saline, which simulates the chemistry of the pore waters inside the bentonite barrier. At the end of the thermo-hydraulic (TH) treatment, a geochemical characterisation is performed in different sections, both of the soluble components and of the solid phase. The microstructure is analysed by means of optical microscopy and determination of the BET surface and pore size distribution. The swelling capacity of the whole sample and its permeability are also checked. The results of the permeability and swelling tests are compared to those obtained in experiments performed under the same conditions with non-treated samples. As a result of hydration, there exists a rapid movement of chloride towards the heater. This ion is progressively excluded of the bentonite after saturation, what makes the bulk salinity of the clay decrease. Anion exclusion should prevent the anion transport once the bentonite is saturated, but the existence of preferential passages is postulated to explain the continuous salinity decrease. This phenomenon makes the salt content in the compacted bentonite very limited, with localised anomalies prior to saturation. All the physicochemical parameters that have been determined are virtually unchanged during the TH treatment. An increase of the hydraulic conductivity after the TH treatment with saline water has been observed, while the swelling capacity of the samples treated with granitic water slightly increases after treatment.