| With the vigorous development of the nuclear industry,a certain amount of highlevel radioactive waste(referred to as high-level radioactive waste)has been accumulated.The safe disposal of high-level radioactive waste is of great significance for ecological environment safety and sustainable development of nuclear energy.At present,deep geological disposal method is widely used internationally.This method is implemented by placing high-level radioactive waste in a waste tank.After that,the waste tank will be wrapped with bentonite as a buffer material.Finally,the bentonitewrapped waste irrigation will be buried in the surrounding rock at a depth of 500-1000 meters.Thus,the permanent isolation between high-level radioactive waste and the biosphere is realized.With the continuous infiltration of groundwater,the bentonite gradually tends to be saturated.A large amount of gas will be generated in the repository due to the corrosion of the waste tank and the decomposition of microorganisms.The high pressures created by gas accumulation will have a significant impact on the safety and stability of the entire repository unit.Therefore,it is urgent to carry out research on the migration law of gas in saturated bentonite.At the same time,the understanding of the fluid-solid coupling mechanism of bentonite is the key to realize the anti-seepage stability evaluation of the repository under the influence of the interface effect and the rigid-flexible boundary of the surrounding rock.In this thesis,laboratory test,theoretical modeling and numerical simulation are used to study the migration law and breakthrough mechanism of gas in saturated bentonite.The following innovative results have been achieved:(1)The microscopic pore structure characteristics of homogeneous bentonite and heterogeneous bentonite after saturation were analyzed by SEM scanning,CT scanning and mercury intrusion test,and the interface between bentonite blocks healed after water saturation was found.The change rule of swelling force for homogeneous bentonite and heterogeneous bentonite was obtained by water injection test,and the swelling force of heterogeneous bentonite is lower than that of homogeneous bentonite when it tends to be saturated.The water-holding characteristics of homogeneous bentonite and heterogeneous bentonite were analyzed through water-holding test under two boundary conditions of lateral confinement and free expansion.The critical values of capillary pressure under different boundaries for the two samples were calculated based on the van Genuchten model and the soil-water characteristic curve.(2)According to the characteristics of low permeability and water swelling of Gaomiaozi bentonite,a multi-field coupling permeation system was developed.The system can record the changes of physical quantities such as tiny pore pressure,relative content of argon and water in the overflow gas composition in real time during the test process.A computational model for permeability in ultra-low permeability medium is proposed based on continuum theory and micropore pressure changes.The gas permeation test in saturated Gaomiaozi bentonite was carried out by this system.The influence of boundary conditions,interface conditions and gas injection times on permeability,growth rate of outlet pressure,argon and water content and gas breakthrough pressure of saturated bentonite was studied through gas permeability test.the gas breakthrough mechanism is revealed from the aspects of capillary rupture,dilatancy effect and interface effect.(3)Considering the influence of deformation and damage on permeability,the deformation equation of bentonite with pore pressure introduced is established based on the elastic theory of porous media,At the same time,the coefficient of damage effect is added to the evolution equation of permeability based on the Coulomb-Mohr criterion,the criterion of maximum tensile stress and the theory of damage evolution.A fluidmechanical interaction model considering bentonite damage is established.Numerical simulation of bentonite stress field and seepage field is realized by using MATLAB and COMSOL code.The applicability of the model is verified by comparing the breakthrough pressure and permeability with the measured values.The influence of boundary conditions,confining pressure,dry density and heterogeneity coefficient on gas breakthrough pressure,permeability and the expansion process of pores in bentonite was analyzed and simulated.(4)For the bentonite samples before and after the test,the 3D pore structure models at low resolution and high resolution were obtained by CT physical scanning technology and improved simulated annealing algorithm,respectively.The 3D pore models at different scales are superimposed by using an improved superposition algorithm,and the 3D pore model of bentonite is obtained at multiple scales.Through the quantitative characterization of 3D pore model,the influence laws of boundary conditions and interface conditions on effective porosity,connectivity,seepage path length,tortuosity,pore coordination number and pore throat ratio are obtained.The evolution law of pore structure characteristics and seepage path of bentonite samples before and after breakthrough was explored,which revealed the gas migration law and breakthrough mechanism in bentonite from a microscopic perspective.There are 121 figures,36 tables,and 172 references in this thesis. |
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