Consistency of the strontium transport parameters in Boom Clay obtained from different types of migration experiments
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Consistency of the strontium transport parameters in Boom Clay obtained from different types of migration experiments Marc Aertsens, Norbert Maes, Marc Van Gompel Waste & Disposal Department, SCK•CEN, Boeretang 200, B-2400 Mol, Belgium ABSTRACT The consistency of migration parameters obtained by different methods is examined for strontium in Boom Clay. In addition to a previous in-situ percolation experiment and electromigration experiments (using an electrical field as driving force), a lab percolation test and two through-diffusion tests (on clay cores of different lengths) have been performed. All experiments lead to a robust value for the apparent diffusion coefficient of strontium between 5 ? 10 -12 m2/s and 1 ? 10 -11 m2/s. No reliable value is obtained for the product R of the diffusion accessible porosity and the retardation factor R. For one of the through-diffusion experiments, the concentration decrease in the inlet as a function of time was fitted simultaneously with the strontium profile in the clay, leading to a R value around 400. Estimating R at the inlet/clay interface as the ratio of the bulk concentration and the pore water concentration leads to a similar value. The values obtained from the strontium profiles in the percolation and electromigration experiments are in line as well. However, fitting for both through-diffusion tests the outlet concentrations versus time leads to very low and unrealistic values of R (0.15 and 0.5). Modeling shows that the transport of strontium in Boom Clay cannot be satisfactorily described with the current assumptions (like e.g. instantaneous linear sorption equilibrium and boundary conditions), meaning that there is a problem to obtain a reliable value for the product R for strontium in Boom Clay. INTRODUCTION Performance assessment calculations need trustworthy values for the transport parameters of radionuclides in Boom Clay. Consistency between the values of the transport parameters, determined from different types of diffusion experiments, has previously been proven for tritiated water [1], dissolved silica [1] and cesium [2]. This study focuses on strontium. A previous in-situ percolation experiment [3] as well as electromigration experiments (using an electrical field as driving force) [4] already provide a reliable estimate for the apparent diffusion coefficient, but did not lead to trustworthy values for the product R of the diffusion accessible porosity and the retardation factor R. Therefore, an additional percolation test and two through-diffusion tests (on clay cores of different lengths) have been performed. EXPERIMENTAL The set-up of the experiments is described in [1]. In a lab percolation experiment [1, 2], an external pressure gradient is used to percolate clay water through the clay core. In an in-situ percolation experiment [3], the pressure gradient is due to the difference between the water pressure at the depth of the Underground Research Facility (URF), where the experiment is
performed, and the atmospheric pressure (being the pr
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