Size distribution of FEBEX bentonite colloids upon fast disaggregation in low-ionic strength water

Bentonite colloids generated from the backfill barrier in nuclear waste repositories may act as radionuclide carriers, if they are stable and mobile. Repository scenarios with highly saline groundwater inhibit colloid stability as particles tend to aggregate but, in the time frame of repositories, groundwater conditions may evolve, promoting particle disaggregation and stabilization. The disaggregation of FEBEX bentonite colloids by fast dilution to lower ionic strength was analysed in this study. Time-resolved dynamic light-scattering experiments were carried out to evaluate the kinetics of bentonite colloid aggregation and disaggregation processes in Na+ and Na+-Ca2+ mixed electrolytes of low ionic strength. Attachment and detachment efficiencies were determined.Aggregation is promoted by increasing ionic strength, being more efficient in the presence of divalent cations. Once bentonite colloids are aggregated, a decrease in ionic strength facilitates disaggregation, but the process is not fully reversible as the initial size of the stable bentonite colloids at low ionic strength is not fully recovered. Particle-size distribution and concentration in suspension were analysed on disaggregated samples by single particle-counting measurements. Small colloids were measured in the disaggregated samples but their population was smaller than in the initial stable sample, especially in the presence of Ca2+.

Characterisation of Granite Fractures From the In-Situ FEBEX Experiment (Grimsel, Switzerland): ossible Effects on Bentonite Colloid and Radionuclide Transport

The FEBEX in-situ experiment, installed in 1997 at the Grimsel Test Site (GTS, Switzerland) 400 m depth under the Swiss Alps, simulates a high level radioactive waste repository (HLWR) emplaced in granite. Its initial aim was to study the performance of a bentonite engineered barrier but recently, two new boreholes were drilled in the granite to study the possible bentonite colloid formation and their migration in the granite.This study presents the characterization performed, at the micrometer scale, of the threemain water conductive fractures that were identified on the granite cores extracted from the newboreholes. These fractures are possible pathways for bentonite colloid transport (or retention),may be source of natural colloids and may condition colloid stability. The nuclear ion beamtechniques µ-Particle X-Ray Emission (µPIXE) and Rutherford Backscattering Spectrometry(RBS) were applied for visualizing and quantifying the elemental composition of the fracturessurface and of the surrounding micro-fractures, as support of the bentonite colloid analyses.

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

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.

Effect of colloids on radionuclide migration for performance assessment of HLW disposal in Japan

Colloidal effect is one of the major factors to enhance the migration of radionuclides in groundwater. The experimental and theoretical studies of colloid mobility and colloid-facilitated radionuclide transport for the performance assessment of high-level radioactive waste (HLW) geological disposal is presented in this paper. The major aims of the study are (1) to study the filtration effect on colloids by the engineered barrier system, (2) to study bentonite colloid generation by erosion of the engineered barrier system, and (3) to calculate radionuclide migration with groundwater colloids through fractured rock systems. Alternative coagulants based on prehydrolyzed forms of aluminium or iron can be more effective than the traditional materials in many cases, but their mode of action is not completely understood, especially with regard to the role of charge neutralization and hydroxide precipitation. Basic principles of colloid stability and metal ion hydrolysis are briefly reviewed, and the action of hydrolyzing metal coagulants is then discussed, with some examples from recent experimental studies. Although it is possible to interpret results reasonably well in terms of established ideas, there are still some uncertainties that need to be resolved