Study On Thermal Conductivity Of Bentonite-Sand Mixed Buffer Layer

Deep geological disposal is a widely used method of high-level radioactive waste engineering disposal in the world.During the operation of the disposal repository,high-level radioactive waste will decay and release heat for a long time,which will affect the performance of buffer materials and thus affect the safety and stability of the disposal repository.Buffer material is an important part of the multi-barrier system in the disposal repository,its thermal conductivity parameters are very important for the design of the disposal repository system.The increase of buffer layer temperature not only affects the barrier encapsulation performance of buffer material,but also produces thermal stress and strain inside the buffer layer,and has a significant impact on its physical and mechanical properties.In this study,the proportionally reduced bentonite-sand mixture samples were used as the buffer layer after splicing and healing,simulate the temperature field change of buffer layer when high-level waste decays and exhales heat.Heat conduction simulation tests were carried out on buffer layer samples with different parameters at the same heat source temperature and buffer layer samples with optimal parameters at different heat source temperature in radial and axial directions through the use of self-designed test equipment,record the temperature value of each measuring point at different time.The thermal-mechanical coupling processes of temperature field,stress field and deformation field in the buffer layer were simulated and analyzed,the thermodynamic characteristics of the near field of the disposal repository are obtained.It is found that increasing the dry density,water content and sand content of samples can improve their thermal conductivity;The radial direction of buffer layer samples is directly affected by heat transfer over a large area of heat source,the temperature of each measuring point is high,and the stress and strain change greatly,the results of samples with different water content changed significantly;The axial direction of the buffer layer sample was affected by the heat transfer path,the temperature of each measuring point changed gently,compared with the radial direction,the stress and strain changed little,and the stress reached equilibrium earlier under the influence of temperature.The temperature,stress and strain of the buffer layer near the heat source are the largest,and gradually decrease along the radial and axial directions to the outer boundary,the values change significantly at the initial moment.Under the same test conditions and sample parameters,compared with coarse aggregate samples,the samples with medium-grained quartz sand as auxiliary materials have higher coefficient of thermal conductivity and enhanced thermal conductivity.Moreover,coefficient of thermal conductivity increases with the increase of dry density and sand content,the increase was significant under high moisture content.The higher the heat source,the greater the increase in buffer layer temperature,on the contrary,the change is gentle.According to the symmetric buffer layer model,the middle temperature is higher than the top and bottom temperature,so the temperature distribution presents the shape of ">".Under the influence of temperature,when the buffer layer is in a high temperature heat source,its stress and strain change greatly,and its stability decreases to some extent.Therefore,the disposal repository is more stable when the heat source temperature is lower.