Diffusion coefficients and accessible porosity for HTO and 36Cl in compacted FEBEX bentonite

Abstract The transport properties of a Spanish bentonite (FEBEX bentonite) were studied. The behaviour of the conservative anionic species 36Cl− is compared with HTO (neutral and conservative tracer, tritiated water). Apparent and effective diffusion coefficients, as well as the accessible porosity, were determined in a wide range of clay dry densities. Effective diffusion coefficients for HTO and 36Cl− were measured in through-diffusion experiments. Apparent diffusion coefficients were measured for HTO with in-diffusion (ID) experiments. A very important parameter for describing the transport of radionuclides is the accessible porosity, which is often difficult to determine, especially at high clay densities, for the anions. Therefore, it is very important to use different methods to compare the results obtained in order to understand the limitations of each technique and finally provide reliable results. Furthermore, the use of different techniques and the independent determination of each transport parameter can be used to validate the results. Apparent and effective diffusion coefficients for both HTO and Cl− showed an exponential decrease when the dry density increased, with the decrease being more pronounced in the case of Cl−. The accessible porosity for HTO is equal to the total porosity. The accessible porosity for chloride is only a small fraction of total porosity (2–3% at a dry density of 1.65 g/cm3, when the total porosity is about 40%).