A Study on Diffusion and migration of lead in compacted bentonite; The Effects of dry density, silica sand content and temperature on diffusion and migration of Pb-210 in sodium bentonite

Sato, Haruo; Miyamoto, Shinya

We have studied performance as a diffusion barrier of bentonite which is one of the candidate buffer materials for geological disposal of high-level radioactive waste. Various functions are expected for bentonite and a retardation function in diffusion process of radionuclides released from vitrified waste is also one of them. In this study, diffusion and migration of Pb i bentonite, particularly for the effects of bentonite dry density, silica sand content and temperature on apparent diffusion coefficients (Da) were experimentally studied from the viewpoints of (1) database development and expansion for important nuclides in dose evaluation, (2)confirmation of the validity or conservativity of distribution coefficient (Kd) used in the second progress report, and (3)understanding the mechanism of diffusion and migration behaviour in bentonite. In diffusion experiments, a Na-bentonite, Kunigel-VI (R) (Na-smectite, 46-49wt%) was used and the experiments were carried out at dry densities of 0.8, 1.4, 1.6 and 1.8Mg/m and temperatures of 22.52.5 and 600.1C by in-diffusion method. The experiments i the systems with silica sand of 30 and 50wt% were also carried out only at a bentonite dry density of 1.6Mg/m. Since Pb is much contained in the bentonite, Pb which is radioactive, was used as a tracer in all experiments and analysed by a liquid scintillation counter. All experiments were performed in a N atmospheric glove-box (O concentration 1ppm). Additionally, the background of Pb in the bentonite was measured to obtain reliable data. The measurements were carried out as a function of bentonite dry density (0.8, 1.6, 1.8Mg/m), saturation period (40-71d) and bentonite slice thickness (0.2-2mm). Furthermore, a HNO solution used for removal of Pb from bentonite slices, liquid scintillator and an empty polyethylene vial were also analyzed. Consequently, no significant difference in ...