Migration Of Eu(Ⅲ) In Compacted Bentonite-Sand Mixtures As Buffer/backfill Material

Deep geological disposal has been regarded as a feasible way to dispose of the high-level radioactive waste (HLW). Radionuclides will diffuse to the outside in case that the overpack has been destroyed in the HLW disposal repository. Therefore, buffer/backfill material serves as a significant part of engineering barrier, preventing convective water due to the low permeability of bentonite, and retarding the migration of radionuclides through buffer by sorption to the matrix. Hence, the hydraulic conductivity and diffusion coefficient of radionuclides on compacted bentonite-sand mixtures play a very important role in assessing the performance of buffer/backfill material and in designing the deep geological disposal.Bentonite has been considered as the ideal buffer/backfill material in HLW repositories due to its high absorption, high swelling property and low permeability. The GMZ bentonite is selected as the major ingredient and quartz sand is taken as the additive of the buffer/backfill material in this paper. And then, quartz sand is uniformly added into the bentonite with Rs=0,10,20,30,40and50%, respectively, followed by compacting the mixtures to specimens with objective water contents and objective dry densities. The hydraulic conductivity and migration of Eu(III) are examined using a flexible wall permeameter for a solute concentration of2.0×10"5mol/L to assess the performance of buffer/backfill material.The research shows that:hydraulic conductivities of compacted bentonite-sand mixtures with sand ratio varying from0%to50%are K=2.07×10-10～5.23×10-10cm/s, regarding2.0×10-5mol/L Eu(III) solution as influent. Moreover, there is not a significant change in hydraulic conductivity with sand ratio ranging from0%to50%. This indicates that even though a certain content of sand is added into bentonite, the bentonite-sand mixtures can still maintain the permeability as low as that of pure bentonite. This verifies again that the bentonite-sand mixtures as buffer/backfill materials can meet the requirement of low permeability. The apparent diffusion coefficients of Eu(III) in compacted bentonite-sand mixtures are Da=1.44×10-14～9.41×10-14m2/s. Generally, apparent diffusion coefficient decreases as effective dry density increases. The reduce in Da with increasing effective dry density is mainly attributed to two factors:1) The sorption of Eu(III) on mixtures is dominated by bentonite, thus an increase in clay content in per unit-volume of the mixtures results in an increase in the sorption ability of bentonite-sand mixtures, therefore, decline the diffusion coefficient.2) Increasing effective dry density will lead to an increase in soil specific surface area and a decrease in the tortuosity, thus, the diffusion coefficient will decrease.