Abstract
This study deals with the diffusion and sorption mechanisms of 137Cs,90Sr in clay,which can be considered as a kind of better buffer material in radwaste disposal. Preventing the groundwater intrusion and possessing excellent sorption capability of nuclides are the two important criteria for an ideal buffer material. In this study, Jishin soil and Wyoming bentonite (MX-80) were selected as candidate buffer materials. The sorption parameters of radionuclide were determined by batch experiment. The diffusivities of Cs+, Sr2+ were performed by steady state method in the clay thin-layer diffusion column, with a compacted dry density of 1.8 g cm-3. According to the results of zeta potential measurement, sorption data in batch method were compared with in order to understand the variable electrostatic forces between charged surfaces and nuclide species. The adsorption coefficients of strontium are more dependent on pH values than that of cesium in the same condition. The adsorption of strontium on bentonite increases in the same pH region and in a similar manner as the zeta potential indicating an electrostatic binding of solvated Sr2+ ions which are placed in the diffusive double layer to negtive surface groups. The mechanism for nuclide diffusion is discussed in both pore water diffusion and surface diffusion. In this experiment, it was found that surface diffusion is an important factor in MX-80 bentonite than Jishin soil. The dependence of the diffusion of Cs+ and Sr2+ on the sorption intensity is accommodated by a model encompassing diffusion of the sorbed cations within the electrical double layer next to the mineral surface in addition to diffusion in the pore water. Because of the sorption character of Sr2+, one can find greater surface diffusion in MX-80 bentonite. It is believed that to find the different sorption and diffusion behaviors and their parameters for 137Cs and 90Sr nuclides in MX-80 bentonite and Jishin is useful for disposal assessment.