Bentonite-Sand Mixtures As Buffer/Backfilling Materials

High-level radioactive waste(HLW) can make great harm to natural environment and development of human society because of its strong radioactive, toxicity, heat and long half-life nuclides. It is absolutely necessary to reliably isolate HLW with the living environment of human in the long-term. Attentions have been devoted in recent years to the challenge of how to safely sotre HLW, which is the most important subject in the field of nuclear waste disposal. The deep geological disposal is currently believed to be the best feasible disposition way, which is based on the conceptual model of a multiple barrier system. The model combines an isolating geological environment with an engineering barrier system including the vitrified HLW, canister, and buffer/backfilling materials to seal HLW up safely at a depth of500-1000m below the surface, in which the buffer/backfilling materials are the most inportant barrier to prevent the migration of radionuclides by groundwater. The buffer/backfilling materials play an absolutly important role in slowing groundwater seepage, preventing nuclide migration, keeping the stability of disposal canister and transferring nuclide decay heat.As buffer/backfilling materials of the geological HLW repository in China, GMZ bentonite is the preferred choice because of its high absorption and good swelling sealing and low permeability and considerable mechanical strength. However, the pure GMZ bentonite as buffer/backfilling materials has two limitation which are low thermal conductivity properities and poor constructability. As a modern trend in the develpoment of buffer/backfilling materials, bentonite is optimized by addition of certain content of quart sand to improve the thermal conductivity properities and constructability without obvious lowering of permeability and swelling properties.The prime aim to the compacted bentonite-sand mixtures is to investigate the optimum sand addition, under which the mixtures will provide a best adsorption to radionuclides, impermeability, thermal conductivity, swelling sealing and mechanical strength for meeting the requirments of HLW disposal at the same time. As a specific research, objectives of this study is to reveal the compression properties of bentonite-sand mixtures with different dry densities and sand ratios performed by WG oedometer, providing a basal data for the final optimum ratio of sand content of bentonite-sand mixtures. It is found that the compression coefficient a1-2decreases quadratically with the increase of dry density, but it increases nonlinearly with the increase of sand ratio. Based on the concept of effective clay density and effective water content, we tried to explain the changes of the compression coefficient a1-2with dry density and sand ratio. The regression analysis model of bentonite-sand mixtures with various dry densities and sand ratios are fitted based on test data, and it hopes that this result can be used to provide a related basis of the optimization of buffer/backfilling materials.