THM Coupling Process In Unsaturated Bentonite Buffer Material With Construction Joints And Self-healing Effects

The development of nuclear energy is accompanied by the production of a large amount of radioactive nuclear waste.At present,the internationally accepted treatment method is geological disposal by establishing a repository,and separating nuclear waste from the geological body away from the human life circle through the engineering barrier.The buffer material is an important part of the engineering barrier.It is generally backfilled around the waste tank in the form of high-pressure bentonite block splicing.The gap between the tank body,the block and the surrounding rock is filled by the joint material,and the buffer material will be filled.The process of undergoing a combination of thermal-hydraulic-mechanical-solute migration.In this paper,the influence of joints on the workability of buffer materials and the healing effect of joints are studied.The bentonite constitutive model model equations and calculation methods are improved to better analyze the influence relationship between various factors.The main research contents include:(1)According to the soil mechanical properties of the buffer material and the case of construction joints,the coupled theory of unsaturated soil is used to establish the thermal-hydraulic-mechanical coupling and solute transport model of unsaturated soil.The model can study the soil.Heat conduction,seepage,evaporation/condensation,solute transport,hygroscopic expansion,thermal expansion,compression deformation and other issues.The multi-field coupled finite element software is used to solve the problem.The China-Mock-up test is taken as an example to compare the numerical calculation results with the monitoring data to verify the feasibility of the model.(2)By calculating the complete three-dimensional block-seam model,the multi-physics evolution law of the block and joint during the working process of the buffer material is analyzed.The results show that the block will produce a large expansion force during the saturation process.The joint material is compressed and improve the performance of the buffer material,this process is called self-healing.Study result of the joint design parameters,such as the type,number,width,splicing form and dry density of the joint,shows that the final average permeability,thermal conductivity,diffusion coefficient and swelling pressure of the material mainly depends on the average dry density,while the distribution of the joints and the splicing of the block are less affected.Provide a reference for the design of buffer material.(3)In the calculation of multi-field coupling of bentonite block,in order to solve some problems of inconvenient calculation of finite element software,the application of smooth particle method(SPH)in heat-water-steam migration and other conduction problems can be used in numerical calculation as a the auxiliary mean.A new kernel function form is proposed to solve the parabolic mathematical equation,which has higher precision and more precision than the traditional Gaussian kernel function.Fast solution speed.The nuclear function was used to study the effect of evaporation on the seepage and the effect of seepage on the heat transfer during the thermal-hydraulic-steam migration.The results show that the evaporation effect enhances the influence of temperature gradient on water migration,and the effect of seepage on heat conduction.Soil permeability is related to boundary conditions,and the effect of seepage in low permeability clay on heat conduction is negligible.The saturation process of the bentonite block was calculated and found to have a positive correlation between the saturation rate of the bentonite and the external water pressure and heat source temperature.(4)Aiming at the problem of dry-wet cycle of unsaturated soil,a new SWCC hysteresis model was proposed,and the SPH algorithm program for the calculation of the heat-water coupling process under the dry-wet cycle of unsaturated soil was prepared.The corresponding soil column test was designed to study the dynamic response of temperature field and water content distribution under the action of temperature rise and fall of unsaturated clay.It was found that the water migration of soil has obvious hysteresis,and the new hysteresis model is capable to describe the hysteresis effect.