MEASUREMENTS OF EFFECTIVE DIFFUSIVITIES OF TRITIATED WATER IN SAND-MIXED BENTONITE

摘要

Effective diffusivities of tritiated water in the mixtures of sand and bentonite were determined by measuring the effective diffusivities of tritium labeled water (H{sup}3HO). The bentonite used was Kunigel V1. Two kind of the sands, natural silica sand and spherical glass beads, were used to consider effects of shapes and grain sizes on the diffusivity in the mixture. The diffusivities were measured using a tracer transmission measurement method. The diffusion experiments were carried out at room temperature under an ambient atmospheric condition. In the experiments using natural silica sand, the bentonite content was varied from 10 to 100 wt.% under a condition that the dry density of the mixture was 1.6 × 10{sup}3 kg m{sup}(-3). In the experiments using glass beads, amounts of the sand in the mixture were varied under a condition of a constant bentonite gel density in order to determine structural factor introduced by the sand. Additionally, particle sizes of glass beads were varied in the range of 0.05 to 5.0 mm to evaluate size effect, under a condition that the dry density of mixture and bentonite content are 1.6 × 10{sup}3 kg m{sup}(-3) and 20 wt.%, respectively. The effective diffusivity of tritiated water in the sand-mixed bentonite decreased with increasing the bentonite content in the mixture under a condition that the dry density of the mixture is constant. Structural factors produced by the sands, natural sand and glass beads in the mixture were empirically evaluated as a function of a volume fraction of the sand in the mixture. The shape of the sands slightly affected structural factor in the mixture. The effective diffusivity (structural factor) is not dependent on the grain size of the sand in a range of 0.05 - 5.0 mm. In the ranges of dry density of 0.5 - 2.0 kg m{sup}(-3) and bentonite content of 10 - 100 wt.%., the effective diffusivities of tritiated water in the mixture were calculated from the bentonite volume fraction in the mixture, structural factor experimentally obtained in this study, and empirical relation between the effective diffusivity of water in bentonite and bentonite dry density.