Migration Experiments in Compacted Ca-Bentonite
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Migration experiments were carried out in compacted Ca-bentonite as a complement of the FEBEX project large scale tests. Through diffusion and permeation experiments with tritiated water were used to perform a deep characterisation of the clay diffusion parameters (diffusion coefficient, effective porosity and permeability). In through diffusion experiments, a variation of tracer concentration in both reservoirs was allowed. Since in these experimental conditions an analytical solution of the diffusion problem does not exist, an analytical approximation was used and the results compared with a numerical model which takes into account the presence of the sintered steel filters confining the clay plug. The migration behaviour of Se in this compacted bentonite has been studied and the retardatioci factor obtained. The calculated Kd is at least one order of magnitude lower than the one obtained by means of batch sorption experiments due to a lower availability of sorption sites when increasing the compaction degree. INTRODUCTION
According to the Spanish concept for the disposal of high level radioactive waste, a clay engineered barrier is located around the canisters, due to its low permeability, good swelling properties and high sorption capability. In this medium the migration of radionuclides is expected to occur mainly by diffusion. Different tracers [1] simulating the radioactive waste were introduced inside the Ca-bentonite used in the two large-scale tests of the FEBEX (Full-Scale Engineered Barriers Experiment in Crystalline Host Rock) project [2], in order to study sorption and diffusion capability of the clay barrier. The properties of Nabentonites have been extensively investigated, but only few diffusion data are available on Ca-bentonites. Therefore, the interpretation and modelling of these large-scale tests need laboratory experiments to obtain retention and sorption parameters. The migration behaviour of a conservative tracer, tritiated water (HTO), was studied, using the clay compacted at different densities, by means of throughdiffusion (TD) and permeation experiments. The behaviour of a non conservative tracer, 75-selenium isotope, was studied by means of permeation experiments. Most of the work found in the literature concerning TD experiments uses the classical analytical solution of the Fick's first law considering, as a boundary condition, the tracer concentration at the inlet as a constant, or the concentration at the outlet zero. In this work, the effective diffusion coefficients were determined according to an evaluation based on the Fick's second law, allowing concentration 695 Mat. Res. Soc. Symp. Proc. Vol. 556 ©1999 Materials Research Society
variations in both reservoirs and finally comparing the results obtained by means of a numerical approach. EXPERIMENTAL All the experiments have been carried out using FEBEX Ca-bentonite saturated during 4 weeks under oxic conditions. The detailed characteristics of this clay are described elsewhere [3]. The bulk density of the clay was 1.18, 1.35
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