Abstract: The impermeability of compacted buffer block is a key parameter to ensure the sealing of high-level radioactive waste (HLW) repository. It is importance of testing and evaluating the uniformity and anisotropy of the permeability of the buffer material blocks to the design of buffer barrier because the blocks are compressed under uniaxial loading with a large size. In this work, full-sized blocks of bentonite-sand mixture were compacted into different dry densities, cut with electric saw to get smaller pieces at various positions and directions, and tested by both rigid-wall and flexible wall permeameter to evaluate the overall hydraulic conductivity of the blocks. Test results show that the hydraulic conductivity decreases with the increase in block density, and all blocks fall into the order of 10?10 cm/s along the directions of z, ? and r of the blocks. This proves that the compacted blocks possess satisfactory homogeneity and marginal anisotropy in permeability, and meet the low requirements of the buffer barrier for HLW repository. With respect to both permeameters, specimens in flexible wall permeameter expanded 10%?20% than those in rigid-wall permeameter, leading to a slight increase in the hydraulic conductivity than the rigid one. Before and after permeation, CT test was introduced to investigate the microstructure evolution of the blocks. After permeation, the density distribution of the specimen as indicated by CT value transformed uniformly than before, and the micro crack was healed to become unobservable.

Key words: high-level waste(HLW), hybrid buffer material block, permeability, anisotropy