Study On The Thermal Conductivity Performance Of HLW Buffer Barrier Masonry Structure

Deep geological disposal of high-level radioactive waste（HLW）is recognized as the most effective disposal method at present.In the deep geological disposal repository of HLW,the buffer barrier composed of buffer blocks plays a vital role in the safety of nuclear waste disposal.Under the influence of radiation heat generated by nuclear waste decay and groundwater seepage,the buffer barrier is situated in a thermal-hydraulic-mechanical coupling environment.The joints between the spliced blocks play as both the weak interfaces of the buffer barrier and the preferential seepage channels of groundwater,and hinder the decay heat transfer to the surrounding rock.The influence of joints on the thermal conductivity of buffer barrier is of importance to the buffer barrier sealing evaluation.In this study,taking the spliced block with joints as the research object,laboratory tests and numerical simulation were conducted to study the change of thermal conductivity of buffer barrier masonry structure with groundwater saturation process.Based on previous research results,test samples were compacted by bentonite-quartz sand mixture with sand content of 30%,and the joint sealing material selection were prepared by bentonite particle and bentonite powder mixture with particle rate of 70%,and a particle diameter of 0.25～0.50 mm.Firstly,thermal conductivity tests were conducted on simulated blocks and joint sealing materials to determine the thermal conductivity coefficients of simulated blocks and joints was various dry densities and saturation conditions.Followed by the establishment of normalized prediction model of thermal conductivity coefficient of simulated blocks and joint sealing materials with saturation degree and dry density.Then,through rigid wall permeability test,taking simulated block test samples and simulated block test samples with joints as research objects,to simulate the healing process of the blocks and joints under the action of groundwater during the period of operation of the repository,and measurement of the thermal conductivity coefficient of the test samples after permeability saturation by thermal conductivity testing.Saturation permeability coefficient and thermal conductivity coefficient were used to quantitatively evaluate the degree of joint healing.Finally,using COMSOL Multiphysics field coupling finite element software,by establishing a spliced block model with joints,a numerical simulation of the thermal-hydraulic-mechanical coupling effect experienced by the blocks and joints during the period of operation of the disposal repository was conducted to predict the change in thermal conductivity performance of the buffer barrier masonry structures.For simulated block test samples and bentonite particle-power mixture compacted test samples,tests of thermal conductivity tests were conducted to study the relationship between the thermal conductivity coefficient and the dry density and saturation degree of the sample.The test results show that the thermal conductivity coefficient of the simulated block test samples gradually increases with increasing degree of saturation and dry density;The influence of saturation degree on sample thermal conductivity coefficient becomes more significant with the increase of sample dry density.From the fit results,there is an approximate linear growth between the thermal conductivity coefficient and the dry density and saturation degree of the simulated block test samples.The thermal conductivity coefficient of bentonite particle-power mixture compacted test samples gradually increases with increasing degree of saturation;If the degree of saturation is less than 60%,there is a significant difference in the coefficient of thermal conductivity between different test samples of dry density.From the fit results,there is an approximate linear growth between the thermal conductivity coefficient and the dry density and saturation degree of the bentonite particle-power mixture compacted test samples.By comparing the test results of simulated block test samples and bentonite particle-power mixture compacted test samples,it is found that under the same condition of dry density and the degree of saturation,the thermal conductivity coefficient of the simulated block test samples is larger than that of the bentonite particle-power mixture compacted test samples;As the degree of saturation increases,the difference between their coefficients of thermal conductivity gradually decreases.For simulated block test samples and simulated block test samples filled with bentonite particle-power mixture containing joints,rigid wall permeability tests were conducted.Following permeability tests,characteristic points were selected for thermal conductivity tests.The permeability test results show that for simulated block test samples and simulated block test samples containing joints with initial dry density of1.6 g/cm³,their saturated permeability coefficients are 7.27×10^-12 m/s and 9.97×10^-12m/s respectively,which have little difference and are in the same order of magnitude,indicating that after permeability saturation,joint sealing effect is good.Thermal conductivity tests show that,before and after saturation of the permeability,at the joint filled with bentonite particle-power mixture,the thermal conductivity coefficient increases from 0.75 W/m·K to 1.33 W/m·K,while the one at the block increases from1.29 W/m·K to 1.39 W/m·K,with a difference of 4.32%.As the thermal conductivity coefficient of the simulated complete block compaction test sample is increased from1.27 W/m·K to 1.42 W/m·K,the difference in thermal conductivity coefficient at block is small compared to the filler sample of bentonite particle-power mixture,indicating that after filling the joint with bentonite particle-power mixture,the degree of joint healing is good,and the influence on the overall thermal conductivity performance of the simulated block is small.Simulation results of thermal-hydraulic-mechanical coupling of blocks spliced with joints show that under the constant pressure infiltration condition,joints become preferential infiltration pathways.Under the action of expansion force,the dry density of joints increases significantly,from the initial value of 1.1 g/cm³ to 1.27 g/cm³,while the dry density of blocks only decreases slightly.After full saturation there is still some difference in permeability between the blocks and the joints,and the degree of joint healing has not achieved the ideal condition.The change in dry density and degree of saturation significantly improves the thermal conductivity coefficient of the joint,between the the initial value of 0.49 W/m·K and approximately 1.59 W/m·K,while the thermal conductivity coefficient of the block increases from the initial value of 1.30W/m·K to 1.97 W/m·K.As the blocks and joints become saturated with water absorption,the difference between their coefficients of thermal conductivity decreases significantly,a good degree of joint healing is achieved,and the overall thermal conductivity performance of the buffer barrier masonry structure tends to be uniform.