Coupled mechanics, hydraulics and sorption properties of mixtures to evaluate buffer/backfill materials

The buffer materials that retarding the migration of nuclides and making the canisters stable in a geological deep repository of high-level radioactive waste play a very important role. This study coupled engineering and sorption properties to evaluate the buffer/backfill materials. The Rd values of Cs, Se and I (10−4 M) with respect to various composite ratios of bentonite/laterite/quartz sand mixtures were measured using batch sorption tests in groundwater (GW) and seawater (SW) which simulate possible conditions for a deep geological disposal in an island. Deionized water (DIW) was used as the liquid phase for Atterberg limits tests, triaxial shear tests and hydraulic conductivity tests in order to determine the engineering properties of the mixtures. From the results of the Atterberg limits tests, seven samples were classified as inorganic clays of high plasticity and one sample was classified as inorganic clays of medium plasticity. Under the test condition of samples with dry density of 1.75 ± 0.05 g/cm3, the samples with 30% quartz sand content demonstrated higher shear strength. Very low hydraulic conductivities of four selected samples were measured, ranging from 1.46 × 10−11 m/s to 2.94 × 10−11 m/s. The sorption behavior of Cs, Se and I on every individual solid material (laterite, bentonite and quartz sand) can be summed up as follows: the sorption of Se is significantly more affected by the solid phase (Rdlaterite>Rdbentonite) than by the liquid phase (GW or SW). Contrarily, the sorption of Cs is more affected by the nature of the liquid phase (RdGW>RdSW) rather than solid phase. On the mixtures used in this work, the sorption of Cs was higher in GW suspensions, when the plastic index (PI) = 40-50 and a 30% quartz sand content. Regarding the relationship between sorption of selenium and engineering characteristics, the mixtures with PI = 40-50 (30% quartz sand content) were the potentially more adequate composite materials. Slight sorption of I was observed on all mixtures in GW and SW. On the basis of the results presented in this work, the composition of 30% quartz sand content which PI = 40-50 might be the best choice for the buffer/backfill materials. Finally, a local soil, laterite, exhibited higher sorption capacity of Cs and Se in both GW and SW than bentonite.