Experimental Approach to Study the Colloid Generation from the Bentonite Barrier to Quantify the Source Term and to Assess its Relevance on the Radionuclide Migration

Abstract

A geological repository for high-level radioactive waste (HLWR) consists on a multi-barrier system, emplaced hundred meters deep in a geological medium. In most of the repository concepts, the waste would be located in metal canisters surrounded by a layer of compacted clay, i.e. bentonite. To guarantee the long-term safety of a repository, all mechanisms that could affect the radionuclide (RN) migration rate must be well defined and quantified. The particular interest of this work lies on the possible contribution of bentonite colloids to RN transport. The first parameter necessary to assess the colloid-mediated transport is the quantification of the bentonite colloid source term. Secondly, it is necessary to define if colloids remain stable in the geochemical conditions of the medium.

Several mechanisms that are basically related to the hydration of the clay can lead to bentonite colloid generation. In the present work the colloid generation is evaluated at laboratory scale under “realistic” conditions, considering static hydration (no flow). To do so, two experimental set-ups were designed with the aim of quantifying the bentonite colloid generation rates. The experimental cells were designed to study the colloid formation in a confined system by introducing compacted bentonite, at different compactation densities, in stainless steel porous filters. The bentonite hydration is facilitated by immersing the confined cells in different electrolytes, from the most favorable conditions (lowest ionic strength) to different groundwaters of interest as aqueous phase. The concentration of bentonite colloids and the average particle size are evaluated as function of time by Photon Correlation Spectroscopy measurements in the aqueous phase.

Preliminary results showed that all the bentonite particles generated have average size in the colloid range, equivalent to that of bentonite colloids prepared in the laboratory, despite the filter porous sizes were hundred times higher. The experimental set up allows performing stability evaluation at the same time and that after months the colloids generated in the lower strength electrolytes remain stable. The configuration allows quantification of the colloid generation rates. The mechanisms responsible of colloid generation are discussed according to the obtained results in different experimental conditions.