Microscopic Homogeneity Of Bentonite-Sand Buffer Block For HLW Disposal

Deep geological disposal is the most effective method for HLW(high-level radioactive waste)disposal at present,which is based on continuous exploration.Lanzhou University chooses compacted bentonite-sand buffer block to build buffer barrier,referring to the research results at home and abroad.Homogeneity study plays an important role of characterization of bentonite-sand buffer blocks,and it is also the main basis of optimization and overall evaluation of block compaction technology,which has great academic value and application value.The study of homogeneity at micro-scale can provide a concrete explanation for the anisotropy of macro-engineering properties of blocks,and make the study of buffer block more comprehensive.Gaomiaozi bentonite and Yongdeng quartz sand are used as experimental materials in this dissertation,and semi-automatic compaction equipment is used to compact bentonite-sand buffer blocks in different physical states.CT scanning(Computed Tomography)and micro-structure research are carried out.In the study of CT scanning of buffer block,the density of bentonite-sand buffer block is approximated by CT number,t-test method is used in radius direction,and the method of take CT number of a small area is used in angle and height direction.The spatial distribution and homogeneity of density of bentonite-sand buffer block are discussed.By observing the morphology of CT images,the development of cracks and the division of density plane are studied.Based on the results of CT scanning test of buffer block,the direction of height is determined as the key research direction.SEM(Scanning Electronic Microscopy)test is designed to observe the micro-morphology and study the pore characteristics.The hydraulic jack is used to compact simulated samples in different physical states,and the shear resistance and CT scanning are studied.In shear test,the relationship between cohesion,internal friction angle and initial moisture content and dry density are discussed.In the study of compacted simulated sample CT scanning,the homogeneity test is also carried out by t-test,and the trend of CT numbers along the height direction of small samples is analyzed.The homogeneity of bentonite-sand buffer block is preliminarily explored from the micro-scale by the test results of buffer block and compacted simulated sample.CT scanning tests of buffer block show that CT number(density)of bentonite-sand buffer block in the radius and angle directions are uniformly distributed,and the differences between them are small.The distribution of CT number(density)in the height direction is slightly different.In the height direction,from top(compacted layer)to bottom of the block,CT number(density)of the buffer block sample shows a trend of decreasing first and then increasing.Bentonite-sand buffer block with low moisture content or low dry density exhibit certain non-uniformity in density distribution.SEM tests show that there are three kinds of micro-morphological and structural characteristics of bentonite-sand buffer blocks,namely,block structure,flocculent structure and laminated structure.Laminated structure and flocculent structure mostly exist at top and bottom of the block.Block structure is most widely distributed in the middle layer of buffer block.The apparent porosity of bentonite-sand buffer block samples falls in the range of 30.0-40.0%,which is affected by the height,initial moisture content and dry density of the block.The fractal dimension of the plane pore distribution of bentonite-sand buffer block samples ranges from 1.35 to 1.50,and there are local differences.Study on the relation between buffer block and compacted simulated sample shows that with the increase of water content,the cohesion,internal friction angle and shear strength of the samples increase first and then decrease.With the increase of dry density,the cohesion and shear strength of the samples also increase,the angle of internal friction rises slightly in a steady state,and the overall fluctuation is not large.The size of the sample has an important influence on homogeneity of bentonite-sand buffer block.The larger size of the buffer block,the greater difference and less homogeneity it is.With the increase of size,the density distribution law of block in the direction of height will change slightly.The larger the initial moisture content,the more obvious the size effect;the larger the dry density,the less obvious the size effect.