The Water Retention And Permeability Of Granular Bentonite Material For HLW Disposal

Bentonite particles and their powder mixtures(granular bentonite materials)are considered as excellent sealing materials for the joints of blocks in the high-level radioactive waste disposal repository due to strong expansibility,low permeability and the ability to fill irregular voids.During the operation of the repository,the granular bentonite materials will remain unsaturated for a long period of time due to groundwater erosion and radiant heat.Its retention capacity and permeability will affect the permeability coefficient and expansion performance of the buffer barrier,and have a great influence on the swell-seaming of joint.Usually,gradation and compactness are important factors affecting the retention capacity,permeability and other engineering characteristics.In this dissertation,the best composition of granular bentonite material was determined by natural bulk density and free expansion rate test.Then,both the pressure plate and vapor transfer technique were used to measure the soil-water characteristic curves(SWCC)of granular bentonite material,and discussed the effects of dry density and particle size of bentonite pellets on retention capacity in drying process.Finally,the saturated permeability coefficient of granular bentonite was measured by variable head permeability test,and the unsaturated permeability of granular bentonite was predicted by combining the results of soil-water characteristic curve test.From the results of natural bulk density and free expansion rate test,it is known that when the particle fraction is 70%,the swelling property of the granular bentonite is the best,the packing density is the largest,and the bulk particle size of bentonite is larger,the bulk density is also larger.However,the granular bentonite having a particle diameter of 1~0.5mm exhibits an expansion property superior to particle diameter of 0.5~0.25 mm.Soil-water characteristic curves show that at low suction section,dry density has a significant influence on the soil-water characteristic curve,as the dry density increases,the moisture content corresponding to the same suction decreases and at the high suction(when the moisture content is less than 20%),the dry density has no influence on the soil-water characteristic curve.The larger the size of bentonite pellet,the larger the internal pores of the sample,the faster the water loss rate and the lower the retemtion capacity.Gardner is the most suitable model for fitting the soil water characteristic curve of granular bentonite.Microscopic test results show that the granular bentonite material exhibits a pore size distribution curve consistent with the bentonite powder,which is a typical bi-modal formed by intra-grain and inter-grain.The dry density has no effect on the pores in the intra-grain,mainly affecting inter-grain.After the granular bentonite materials are saturated,the high-density bentonite pellets in the material are not in a homogeneous state with the surrounding bentonite powder,and the inner layer of the aggregate contained therein is not completely expanded.And as the size of the bentonite pellets increases,the unexpanded aggregate in the sample also increase.Permeability coefficient equations for granular bentonite materials predicted by soil water characteristic curve and saturated permeability coefficient show that with the increase of dry density,the unsaturated hydraulic conductivity of granular bentonite materials decreases,and with the increase of the size of bentonite pellets,the unsaturated permeability coefficient increases.