Some Methodological Modifications of Determination of Diffusion Coefficients in Compacted Bentonite
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Some Methodological Modifications of Determination of Diffusion Coefficients in Compacted Bentonite Dušan Vopálka, Helena Filipská1, Antonín Vokál2 Department of Nuclear Chemistry, Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Břehová 7, 115 19 Prague 1, Czech Republic 1 Centre for Radiochemistry and Radiation Chemistry, Czech Technical University in Prague, Břehová 7, 115 19 Prague 1, Czech Republic 2 Nuclear Research Institute Řež plc, Husinec-Řež 130, 250 68, Czech Republic ABSTRACT The results of 3H, 36Cl and 137Cs diffusion experiments through compacted bentonite using a new design of diffusion cell and a new methodology of diffusion coefficients evaluation are presented. The diffusion cell was made from the stainless steel and enables to connect it directly to the input and/or output reservoirs without any tubing. The evaluation of diffusion coefficients utilizes a compartmental model developed in the environment of the GoldSim transport code. It enables to determine diffusion coefficients for various types of boundary conditions, including also input and output filters. The influence of the diffusion through filters on the determined values of both effective (De) and apparent (Da) diffusion coefficients was numerically demonstrated for the through diffusion method. This effect is most important for Da, the value of which would be underestimated using standard ways of evaluation for neutral and positively charged species, mainly in the case of high effective porosity. The comparison of standard and the newly developed method of evaluation of diffusion coefficients showed a significant influence of diffusion in filters for HTO. Contrary to the standard method of evaluation, the evaluation taking into account filters showed here no difference between total and effective porosity. The effect of filter resistance was negligible for Cl-, especially at high dry density of compacted bentonite, due to the anion exclusion effect. The numerical model developed enabled to determine Da values of Cs+ from the concentration change in the inlet reservoir. INTRODUCTION The sorption and diffusion coefficients of critical radionuclides play a very important role in the safety analyses of high-level radioactive waste repositories. Despite extensive research in the area of radionuclide diffusion through compacted bentonite, the question of real ranges of diffusion coefficients needed for performance assessment studies is, however, still open. This is confirmed, for example, by a literature survey of Yu and Neretnieks [1], in which tabulated values of diffusion coefficients differ not only due to differences in specified conditions. In addition, there seems to be a lack of a standard methodology. The usual ways of determination of diffusion coefficients of radionuclides in the plug of compacted bentonite are based on analytical solutions of the diffusion equation for specific boundary conditions. E.g., the time-lag method used for the evaluation of through diffusion experiments require
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