Analysis of Uranium Diffusion Coefficients in Compacted FEBEX Bentonite
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$QDO\VLVRI8UDQLXP'LIIXVLRQ&RHIILFLHQWVLQ&RPSDFWHG)(%(;%HQWRQLWH M. García-Gutiérrez, J. L. Cormenzana1, T. Missana, M. Mingarro, U. Alonso. CIEMAT, Caracterización Hidrogeoquímica de Emplazamientos, Av. Complutense 22, 28040 Madrid 1 Empresarios Agrupados $%675$&7 In the Spanish concept of a high level radioactive waste (HLRW) repository, the canister with the spent fuel would be surrounded by a layer of FEBEX bentonite (Ca-Mg smectite clay) compacted to a dry density of 1.65 g/cm3. Since the permeability of the compacted bentonite is very low, the main mechanism for radionuclide transport in the clay barrier is expected to be diffusion, retarded by the sorption on the solid. Since the mayor component of the radioactive waste is uranium it is very important to study its diffusion behaviour in the compacted clay. Generally, effective (De) and apparent (Da) diffusion coefficients are obtained from "through-diffusion" and "in-diffusion" experiments, respectively. The obtention of both parameters in the same sample is a very interesting issue because the former includes the information on the porous medium structure (connectivity, constrictivity, tortuosity) and the latter takes into account the sorption on the solid phase. In this work, through-diffusion experiments were carried out for studying uranium diffusion and both effective and apparent diffusion coefficients were estimated, from the same experiment, by using different theoretical approaches. ,1752'8&7,21 The parameters needed for describing the diffusive transport of a radionuclide in a porous media are the effective and apparent diffusion coefficients. Effective diffusion coefficients (De) are usually estimated for conservative or slightly sorbing elements by means of "throughdiffusion" (TD) experiments. Since TD experiments are very time-consuming for medium/high sorbing elements, "in diffusion" (ID) experiments are usually carried out with these elements. From these experiments apparent diffusion coefficients (Da) can be obtained. Consequently, in most cases, only the apparent diffusion coefficient is experimentally determined, whereas the effective diffusion coefficient is calculated taking into account two others independent parameters obtained in different experiments: the distribution coefficient between the radionuclide and the solid (Kd), generally taken from batch experiments, and the theoretical porosity of the medium. This approach might not be correct since the accessible porosity for a radionuclide is strongly dependent on its size and charge and therefore can be significantly different from the theoretical porosity. Uranium for example, in presence of carbonates, occurs mostly as an anionic carbonate complex and may suffer anionic exclusion. In addition, the Kd values obtained in batch experiments often differ from those estimated from diffusion experiments, this increasing the uncertainty of the calculation. For this reason, it is useful to develop strategies to obtain both diffusion coefficients from the same experiment. The aim of
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