Abstract: The hybrid buffer block as a buffer barrier requires good thermal conductivity for geological storage of high-level waste (HLW). In this study, Gaomiaozi (GMZ) bentonite was mixed with standard quartz sand, and the sand mixing rate Rs was controlled at 30% according to the orthogonal test principle. The moisture contents were set at 7.0%, 11.0%, and 15.0%, and dry densities were set at 1.60, 1.70 and 1.80 g/cm3, respectively. A large buffer block was pressed with a semi-automatic hydraulic press. The heat transfer test was conducted on the cut and decomposed block. The relationship among the moisture content, dry density and thermal conductivity was established, and the spatial distribution characteristics of thermal conductivity were analyzed. Test results show that the thermal conductivity of buffer blocks increase linearly with the increases in water contents and dry densities. The mean value of thermal conductivity more than 0.8 W/m ? K satisfies the requirement recommended by International Atomic Energy Agency(IAEA). The thermal diffusion coefficient slightly decreases first and then increases, and the overall trend also increases. The spatial distribution and uniformity of the thermal conductivity of the hybrid buffer block were also analyzed. It is found that the thermal conductivity of the block edge fluctuates larger than that of the middle part, but the overall difference and non-uniformity are not obvious. The t-test method was used to test the uniformity of thermal conductivity of the hybrid buffer block. The test results show the block satisfies the uniformity test condition and the sample is uniform. Data analysis shows that the thermal conductivity of the hybrid buffer block can be predicted from the physical index using the Johansen model with a deviation of ±10%.

Key words: high-level waste(HLW), geological disposal, buffer block, bentonite-sand mixtures, thermal conductivity