Experimental Approach to Study the Colloid Generation from the Bentonite Barrier to Quantify the Source Term and to Asse
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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 Ursula Alonso, Tiziana Missana, and Miguel García-Gutiérrez Environmental, CIEMAT, Avda. Complutense 22, Madrid, 28040, Spain ABSTRACT To guarantee the long-term safety of a high-level waste repository, all mechanisms that could affect the radionuclide (RN) migration rate must be well defined and quantified. The particular interest of this study lies on the possible contribution of bentonite colloids, generated at the compacted bentonite barrier, to RN transport. The main parameters necessary to assess the colloid-mediated transport are the source term and the stability behavior in the medium geochemical conditions. In the present work, two experimental set-ups were designed with the aim of quantifying the bentonite colloid generation rates, at laboratory scale and under “realistic” conditions, by static hydration (no flow) of the compacted bentonite, in a confined system. Preliminary results showed that bentonite particles were generated with an average size in the colloid range, equivalent to that of bentonite colloids prepared in the laboratory. At the same time, the experimental set-up allowed performing stability studies which indicated that the colloids generated in the lower strength waters remained stable over months. The possible mechanisms responsible of colloid generation are discussed according to the obtained results in different experimental conditions. INTRODUCTION Colloids may contribute to the radionuclide (RN) transport through the geological barrier of a high-level waste repository (HLWR) [1], since colloids are particles with diameters lower than 1 µm, that can absorb radionuclides on their surface [2]. In order to assess the relevance of colloids on the RN migration, the determination of their nature and amount in a given environment [3] is critical. In particular, it is necessary to establish the conditions in which the compacted bentonite barrier generates colloids, to quantify the bentonite colloid “source term” and to evaluate the extent of colloids stability. There are several mechanisms that can lead to colloid generation [3-4]. The bentonite colloid generation will be primarily related to the hydration of the clay. Due to the entrance of water, effects of physical gradients (e.g. erosion) can promote bentonite colloid generation [5]. Kinetics of colloid generation will be related to the particle/surface interactions, as well as to the medium hydrodynamic conditions. In presence of a water flow, bigger particles have thinner bonding layers, so that just dragging or shearing forces may be sufficient for particle detachment [6]. If a local hydraulic gradient does not exist, the driving force for their mobilisation would be the diffusion, to overpass the bond energy barrier [7]. Once the particle is released, further diffusion towards the stationary water layer occurs. The static of the release processes is related
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