Permeability Of Bentonite-Sand Buffer Block For HLW Disposal

High-level radioactive waste is a by-product produced after using nuclear energy.If they are not disposed of in a timely and proper manner,this type of waste will seriously threaten the survival of mankind.At the current stage,deep geological disposal method is the most mature and reliable method to dispose of this kind of waste.The method is based on a multiple barrier system composed of waste tanks,buffer backfill materials and underground surrounding rock masses.The buffer backfill material is generally composed of high-pressure bentonite-sand（or pure bentonite）blocks,which is the core part of the multi-barrier system.Buffer blocks use bentonite as the main material,mainly because bentonite has a strong cation exchange capacity and can absorb possible leaking nuclides.At the same time,bentonite has extremely low permeability,it can further prevent the migration of nuclides and also prevent the groundwater from getting close to the waste tank and causing corrosion.Therefore,the hydraulic conductivity is an important index to evaluate the quality of the block and its preparation process.This study takes the B-type mixed buffer block independently designed by Lanzhou University as the research object.Three kinds of blocks with sand content of30%,water content of 18%and dry density of 1.6,1.7 and 1.8 g/cm³ were pressed.After the blocks were cut,a ring cutter was used to sample from different parts and directions of the blocks.Finally,18 different round pie-shaped samples were obtained.They have a height of 20 mm and a diameter of 102 mm.The rigid wall permeability tests were carried out on the prefabricated dry density samples of 1.6,1.7,1.8 g/cm³.The hydraulic conductivity of different parts and directions of the block was tested,and the influence of the dry density on the permeability of the block was discussed.In the meantime,the P-wave velocity,thermal conductivity and CT number of the samples were tested.The test results were compared and discussed by statistical methods,and the effect of the penetration process on the blocks was discussed.At the same time,a permeability test under the expansion condition was carried out on the samples with dry density of 1.8 g/cm³.The purpose of this test is simulating the different working conditions of the block.The permeability test results of the rigid wall show that as the dry density increases,the permeability and hydraulic conductivity of the sample decrease significantly.The hydraulic conductivity of the samples with the preset dry density of 1.6,1.7 and 1.8g/cm³ are respectively 7.25×10^-10,6.18×10^-10,and 2.16×10^-10 cm/s.In the flexible wall contrast test of the samples,it was found that although a confining pressure of 400 k Pa was applied to the samples,but the volumes of the samples still expanded by 10-20%after absorbing water.After water absorption and expansion,the density of the sample decreased,and the hydraulic conductivity of the sample increased significantly compared with the rigid wall samples.In the permeability test of the flexible wall,the average hydraulic conductivity of the sample with a dry density of 1.8 g/cm³ is 3.75×10^-10 cm/s,which increased by 73.61%compared with the parallel samples in the rigid wall permeability test,but does not exceed one order of magnitude.Under the two permeability conditions,the hydraulic conductivity of all samples is in the order of 1×10^-10 cm/s.It is proved that the water conductivity performance of the mixed buffer block with a sand mixing rate of 30%meets the requirement of low permeability of buffer material.The P-wave velocity,thermal conductivity,and CT number of the samples after penetration are significantly lower than those before penetration.In addition,the standard deviations of the P-wave velocity,thermal conductivity,and CT number of the same dry density block were significantly lower than those before penetration.These test results indicate that the overall uniformity of the blocks become better after penetration.The t test and normal test results of the samples show that the t value of the samples after infiltration increases,indicating that the distribution of CT number of the samples after infiltration are more uniform.The P value of the normal test also increases obviously,indicating that the distribution range of the block CT number is more concentrated.At the same time,the CT images of the same samples show that the cracks of the samples had active healing during the infiltration process.It shows that the long-term saturated permeation process makes the density distribution of the sample more uniform,and is also beneficial to the healing of the cracks inside the sample.