Coupled Thermo-hydro-mechanical Process And Solute Transport Of Composite Bentonite Blocks Considering Joint Healing Effect

With the continuous development and utilization of nuclear energy technology,a large amount of high-level radioactive waste is produced.Deep geological disposal of high-level radioactive waste is currently recognized as the most potential and promising disposal scheme in the world.In the design of high-level radioactive waste repository,the buffer layer around the waste tank is composed of densely compacted bentonite blocks.Construction joints are inevitable in the construction process,and the construction joints are the potential hydraulic defects and weak strength part of buffer /backfill materials.During the operation of the disposal repository,the buffer/backfill materials are affected by the coupling effects of thermo-hydro-mechanical.Therefore,the performance of the buffer/backfill materials with joints under thermo-hydromechanical coupling has become a key issue in the barrier design and construction of the high-level radioactive waste repository.In this paper,Gaomiaozi(GMZ)bentonite buffer material with construction joints is taken as the research object.Based on the hydrothermal transport mechanism,mechanical model and boundary conditions of unsaturated soil,a coupled thermo-hydro-mechanical model of bentonite considering joint healing effect is established.Combined with laboratory tests and numerical simulation,the multi-physical field coupling process and solute transport process at pore scale of GMZ bentonite buffer material with construction joints were simulated.The main contents include:(1)The heat transfer induced by liquid water flow and water vapor was embedded into the energy conservation equation.Based on the Barcelona basic model(BBM),the coupled thermo-hydro-mechanical model of unsaturated bentonite was established by analyzing the swelling process of bentonite block and the compression process of joint material.The finite element software Comsol Multiphysics is used to simulate the China-Mock-up test,the rationality of the proposed model is verified with the test results.On this basis,the effect of joint self-healing on dry density,thermal conductivity and permeability coefficient of buffer material was further analyzed.(2)A laboratory test device suitable for unsaturated bentonite combined samples was developed.The evolution law of temperature and volumetric water content of GMZ bentonite combined samples at different positions with time before and after influent was measured by this test device.According to the test results,the thermal conductivity and hydraulic conductivity after healing at the joint of bentonite combined samples are obtained.The coupled thermo-hydro-mechanical model of unsaturated bentonite was used to simulate the evolution of temperature and moisture at different positions of bentonite combined samples with time under the two conditions,and the calculation results were compared with the test results to verify and analyze.(3)A coupled thermal-water-vapor model for unsaturated soil was established by considering the possible phase transformation of ice-water and water-vapor in unsaturated soil,and considering the effects of heat transfer of liquid water flow and vapor migration and temperature potential on liquid water flow and vapor migration.The smoothed particle hydrodynamics(SPH)method is used to solve the model.In order to verify the rationality of the theoretical model,the distribution of unsteady temperature field,volumetric water content and water vapor flux in semi-infinite space medium with thermal boundary conditions of the first kind are simulated,and the calculation results are compared and analyzed with the analytical solutions without considering the coupling.Finally,the thermal-water-vapor coupling of the buffer material bentonite in the high-level radioactive waste repository is simulated,and the effects of underground water pressure and heat source temperature on the internal hydrothermal evolution are further analyzed.(4)SPH method is used to discretize the Navier-Stokes equation describing pore water movement and the linear homogeneous second-order differential equation describing solute diffusion process,and a two-dimensional model which can simulate fluid flow and solute diffusion in porous media is established.The accuracy of the simulation method is verified by solving the one-dimensional definite solution of the convection diffusion equation at low Peclet number.On this basis,the pore water flow and solute diffusion in bentonite blocks and combined bentonite blocks with different width joints under the condition of low Peclet number are simulated at pore scale.Through the simulation experiment,the distribution of flow velocity of fluid particles in the pore channel of bentonite block and the joints between blocks,the penetration curve of solute penetrating bentonite block and combined bentonite block with different width joints,and the motion path of fluid particles in bentonite block are obtained.On this basis,the calculation methods of tortuosity are further given.