Driving force for diffusion of cesium cations through compacted sand-bentonite mixtures

Document available in extended abstract form only. After emplacement of high-level radioactive waste in a deep geological isolation system, long-lived 135Cs may be leached from the waste packages and may subsequently be transported through surrounding buffer materials to the geosphere. Compacted sand-bentonite mixture is a candidate for the buffer material. The permeability of water through the sand-bentonite mixtures is so low that the only possible mechanism of 135Cs transport is diffusion. The Fick's first law of diffusion is applied to the diffusion of ions in sand-bentonite mixtures. When the concentration gradient is defined based on amount of diffusing species in unit volume of porous material, the proportional constant between the flux and the concentration gradient is called apparent diffusivity (Da). When based on the concentration of diffusing species in pore water, on the other hand, the proportional constant is called effective diffusivity (De). In this study through-diffusion experiments were performed through compacted sand-bentonite mixtures for Cs under several water compositions. Da and De were calculated and examined which was the fixed constant, namely, proportional constant between diffusive flux and concentration gradient driving the diffusion. Through-diffusion experiments were carried out at various experimental solution compositions, which were 0.01 mol dm-3 NaCl solution, 0.5 mol dm-3 NaCl solution, 0.5 mol dm-3 NaOH solution (series-1-1, 1-2, 1-3, respectively) and 0.3/0.03 mol dm-3 (NH4)2CO3/Na2S2O4 solution (series-2). In the experiments, mixtures of bentonite and silica sand, whose ratio was 7:3 in dry weight, were compacted in acrylic diffusion column to obtain dry density of 1600 kg m-3. Each side of sand-bentonite mixture was covered with sintered stainless steel filters with porosity of 40% and 1 mm thickness. The central acrylic diffusion column holding the sand-bentonite mixture was assembled with two reservoirs. The assembled diffusion cell was soaked in each experimental solution under vacuum to evacuate all air from the pores in the specimens. The series-1 and series-2 diffusion experiments were started by adding CsCl and 137CsCl, respectively, to the solution in one of the reservoir. The experiments were performed at room temperature under Ar. The concentrations of Cs and 137Cs in the both reservoirs were determined by ICP-MS and γ-spectrometry, respectively, during the experiments. The exact solution of the diffusion equation for the case decreasing inlet concentration-increasing outlet concentration was applied to the diffusion of highly adsorbable Cs, for which steady-state diffusion was not reached. Distribution coefficient Kd was calculated. The effective diffusivity of Cs was determined by fitting the concentration of Cs at position x and time t to the concentrations of Cs in the reservoirs. The apparent diffusivity was, on the other hand, estimated from the De and Kd. Further this analysis method was applied to the diffusion data of Cs obtained in similar diffusion experiment using 0.3/0.03 mol dm-3 NaHCO3/Na2S2O4 solution by Yamaguchi et al. [2], De and Da were re-estimated. Effective diffusivity (De) values of 5.2x10-10 - 5.9x10-9 m2 s-1 were obtained. The variation was somewhat large in the De values. Apparent diffusivity (Da) values, on the other hand, were 2.0x10-12 - 6.2x10-12 m2 s-1, which shows small variation. The results indicate that, in applying Fick's first law of diffusion, diffusive flux is proportional to the apparent concentration gradient of Cs in the sand-bentonite mixture rather than the gradient of Cs concentration in pore water. Since the apparent concentration gradient in sand-bentonite mixtures is nearly equal to the gradient of adsorbed Cs, diffusion of Cs under adsorbed state would be the main mechanism of diffusion of Cs in sand-bentonite mixtures