Experimental studies for sorption behavior of tin on bentonite and rocks, and diffusion behavior of tin in compacted bentonite

In the safety assessment for geological disposal of high-level radioactive wastes (HLW), distribution coefficients (Kd) and diffusion coefficients of radionuclides are used to estimate the migration of radionuclides in a near-field of repository. 126Sn is one of the important nuclides for the safety assessment in Japan and its behavior under repository conditions has not beep understood. This report provides the experimental informations for the sorption of Sn on bentonite, tuff and granodiorite, and the diffusion of Sn in a compacted bentonite. The Kd values of Sn on bentonite, tuff and granodiorite were determined by the batch-type sorption experiments as 103-106 [ml/g], 104-105 [ml/g] and 103-105 [ml/g], respectively. The sequential extraction experiments for adsorbed Sn on bentonite were also performed to investigate its desorption behavior. These experimental results indicated that the mechanisms of sorption onto bentonite were dominated by the sorption reactions on smectite and pyrite and consisted of reversible and irreversible sorption on solid and stable fixation in solid. On the other hands, the apparent diffusion coefficients (Da) in compacted bentonite were measured by the diffusion experiments as 10-13 [m2/sec] and 10-14 [m2/sec] for dry densities of 0.4 [g/cm3] and 1.0 [g/cm3], respectively. Moreover, the Kd values in compacted bentonite were calculated according to the relationship with the measured Da values, and the solubilities in the porewaters of compacted bentonite were calculated by use of the calculated Kd and the obtained diffusion profiles. It is found that the derived solubilities almost agreed with the solubilities of amorphous SnO2 reported by Amaya et al. (1997), however, the derived Kd values were lower than that measured from the batch-type sorption experiments. (author)