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

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 in 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-V1 (Na-smectite, 46-49 wt%) was used and the experiments were carried out at dry densities of 0.8, 1.4, 1.6 and 1.8 Mg/m3 and temperatures of 22.5 ± 2.5 and 60 ± 0.1degC by in-diffusion method. The experiments in the systems with silica sand of 30 and 50 wt% were also carried out only at a bentonite dry density of 1.6 Mg/m3. Since Pb is much contained in the bentonite, 210Pb which is radioactive, was used as a tracer in all experiments and analysed by a liquid scintillation counter. All experiments were performed in a N2 atmospheric glove-box (O2 concentration < 1 ppm). Additionally, the background of 210Pb 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, 1.8 Mg/m3), saturation period (40-71 d) and bentonite slice thickness (0.2-2 mm). Furthermore, a HNO3 solution used for removal of 210Pb from bentonite slices, liquid scintillator and an empty polyethylene vial were also analyzed. Consequently, no significant difference in counts per minute (cpm) between bentonite dry density, saturation period and slice thickness was found in the background measurements and it was approximately constant between 2 and 4 cpm over the experimental conditions. The cpm values for HNO3, liquid scintillator and an empty polyethylene vial were also approximately the same degree as those for bentonite. This indicates that obtained cpm is neither originated from bentonite nor HNO3. The diffusion of 210Pb is quite slow and the distance penetrated in the diffusing period (∼ 210 d) was several mm at the maximum. The obtained Da values, in a range of 10-17 to 10-15 m2/s order at 22.5degC, decreased with increasing bentonite dry density and showed a tendency to increase with increasing silica sand content in bentonite and temperature. Furthermore, Da values were well correlative with smectite partial density which was defined by the density of only smectite part in bentonite. This indicates that Pb diffusion is predominantly controlled by the properties in part of smectite. The conservativity of Kd for Pb used for the reference case in the second progress report was confirmed from comparison between Kd calculated from obtained Da and that used in the second progress report. (author)