Thermal Conduction Experiments on Compacted GMZ Bentonite Blocks as the Potential Buffer Materials for HLW Disposal Purpose

Compacted Na-bentonite blocks, with the original water content of 9% and compacting density of 1800kg/m³, from the Gaomiaozi (GMZ) deposit in Inner Mongolia Autonomous Region have been experienced about two months experiment in order to reveal the transmission patterns of temperature and the heat induced moisture movement in GMZ Na-bentonite blocks under high-level radioactive waste repository-like conditions. Based on the design, temperature and humidity composed micro-sensors have been used and allocated in different positions in the bentonite blocks as the check points of the experiment. There are two thermal conduction stages in GMZ bentonite blocks in a closed cylinder heat conduction system. In the first stage, the temperature at the check points in the bentonite blocks near the heater in the centre of the cylinder increased to 55°C in 20 hours, and the temperature at the check points distributed near the edge of the cylinder reached about 48°C in the same period. The temperature was maintained at about 60°C for about 35 days in the first stage. The heat induced moisture movement in bentonite blocks in the cylinder is different from changes of the temperature. The relative humidity at the check points near the heater increased quickly at the beginning of the heating, and then slowly decreased with the temperature maintained at 60°C. The average radial temperature gradient (G_T) and radial relative humidity gradient (G_HR) in bentonite blocks are 0.85°C/cm and 1.32%/cm respectively in the first stage. At the end of the first thermal conduction stage, the temperature of heater was slowly increased into 85°C and maintained this temperature for about 25 days as the second thermal conduction stage. The transmission patterns of temperature and the heat induced moisture movement in bentonite blocks are similar to the first stage. However, the radial temperature and humidity gradients are higher than that in the first stage. The average radial temperature gradient (G_T) and radial relative humidity gradient (G_HR) in bentonite blocks are 1.57°C/cm and 1.89%/cm respectively in the second stage.