Distribution Coefficients and Apparent Diffusion Coefficients of Cesium in Compacted Bentonites
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Compacted bentonite has been considered as a candidate buffer material for high-level waste disposal in Japan. Since compacted bentonite has low permeability, diffusion is the principal mechanism of transport of most species. Diffusion coefficients are, therefore, important parameters for safety assessment and for understanding the diffusion mechanism in highly compacted bentonites. It has been reported that some cations diffuse through bentonite at greater rates than those predicted from pore diffusion and adsorption on bentonite.' 2 This phenomenon was explained by a surface diffusion mechanism1 or migration within the electrical double layer next to the mineral surface. Oscarson et al.45 , however, have asserted that there is no need to invoke a surface diffusion mechanism at any conditions of underground repositories. In this study apparent diffusion coefficients and distribution coefficients of cesium in some compacted bentonites were determined by the penetration profile method combined with the in-diffusion method. The diffusion mechanisms of cesium in compacted bentonites are discussed from these parameters. EXPERIMENTAL PROCEDURE Sodium bentonite used in this study was Kunipia F®which consists of more than 95 wt% 1091 Mat. Res. Soc. Symp. Proc. Vol. 556 " 1999 Materials Research Society
montmorillonite. The chemical composition of Kunipia F® is shown in Table I. The chemical formula of Kunipia-F is estimated from the chemical composition as follows: (Na0 4 Ca0 .03 KO.0 1 )(All 6 MgO. 3Feo I)Si 4 01 0 (OH)2 . The molecular weight is approximately 372. Bentonite powder was compacted into cylinders of 10 mm in diameter and 10 mm in height with the dry density of 0.8 to 1.6 Mg/in 3 . Each compacted bentonite was inserted in an acrylic resin column as shown in Figure 1 and saturated with deionized water for a month. After the compacted bentonites were saturated with water, the deionized water in the upper part of the acrylic column was replaced with a tracer solution of cesium. After a diffusion period of two weeks at room temperature, the columns were disassembled. Then the compacted bentonite was pushed out from the column and was sliced in steps of 1 mm. The weight of each slice was measured before and after drying to determine the water content in the slice and the thickness of the slice. The experiments were carried out by using non-radioactive solutions (cold tests) and radioactive solutions including 34Cs (hot tests) as shown in Table II. The pHs of the solutions were measured after the diffusion periods. Cesium in each slice in cold tests was extracted by 50 mL of IN-NH 4 0H for 72 hours, then concentrations of cesium in the extractant and tracer solutions were measured with an Inductively Coupled Plasma Mass Spectrometer. The activity of "'Cs in each slice in the hot tests was directly measured with a Ge(Li) detector. Table I. Chemical composition of Kunipia F® used in this study. Constituents contents (wt%) SiO2 58.36 A1203 20.36 Fe203 1.34 FeO 0.51 TiO2 0.13 MnO O) = O. Cp(O< xt=O) = O. For this ex
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