Abstract: The hybrid buffer material added with the auxiliary aggregate not only retains the material’s sealing and anti-seepage ability, but also overcomes the defect of low thermal conductivity and inferior construction performance of pure bentonite blocks. As an alternative for the buffer material of high-level radioactive waste repository, it is becoming a new research hotspot. Based on the previous research data and the permeation theory of unsaturated porous media, a 3D calculation model containing one tunnel and a single wellbore is established using COMSOL Multiphysics, which considering the wetting expansion of bentonite and the real-time change of material physical parameters (density, saturation, thermal conductivity, etc.). By simulating the THM coupling process of the hybrid buffer material (bentonite-sand mixture) in the barrier system for 100 years, the time evolution and spatial distribution of each physical quantity are analyzed, and the influences of wetting expansion and sand mixing rate on the evolution process of the barrier system are discussed. The temperature and saturation of the material in the system are related to the distance from the vitrified HLW and the rock wall. The stress in the tunnel and wellbore is mainly compressive stress, and the deformation tends to compress first and then expand. The wetting expansion of bentonite-based materials has little effect on temperature evolution, but it will slightly accelerate the saturation process and cause significant time evolution and regional distribution differences in the stress and strain of the material. The stress in the area close to the rock wall in the wellbore and roadway rises quickly, and significant vertical displacement occurs at the junction of the roadway floor and the wellbore. Increasing the mixing rate can reduce the surface temperature of the tank effectively, enhance the heat transfer capacity of the barrier system, reduce the maximum historical stress of the buffer material, and control the vertical displacement on the axis of the borehole. On the other side, it will also weaken the anti- seepage capability of the system.

Key words: high-level radioactive waste, barrier system, wetting expansion, hybrid buffer material, numerical simulation, multiphysics coupling