| Under the goal of"carbon peaking and carbon neutral",the development of clean and low-carbon transition is being accelerated in China,and the efficient development of nuclear power on the basis of ensuring safety is an important policy for China’s energy construction.With the rapid development of China’s nuclear power industry,the safe and proper disposal of high-level nuclear waste is a huge challenge for the nuclear technology and defense science industries.At present,the generally accepted solution is adopting the combined engineering and natural barriers to achieve the safe storage of nuclear waste by means of deep geological disposal.Granite has been selected as an ideal natural barrier for the deep geological disposal of high-level nuclear waste in China because of its large storage capacity,low permeability and high mechanical strength.Due to the large burial depth of the nuclear waste repository,the macro-mechanical properties,permeability and deformation characteristics of granite will be highly affected under the long-term complex coupling of mechanical deformation,fluid flowing through the repository and thermal loading from the decaying waste.Therefore,it is important to evaluate the thermo-hydro-mechanical（THM）treatments on the macro physical and mechanical properties of granite ensuring the safety of the disposal systems.However,for granite,which is a complex discontinuity formed by microdefects such as pores and microcracks as well as the aggregation of multiple minerals,the mechanical behavior and damage characteristics are directly determined by the internal meso-structure.Especially when the granite is subjected to THM treatment,the thermal expansion of minerals and the seepage of water in the pores will induce stress concentration on the mineral grain interfaces,which will lead to the obvious evolution of meso-structure and finally lead to the change of macro-mechanical properties of granite.Therefore,the meso-mechanical behavior of each component inside the granite after THM treatment should be fully studied,which can provide an important basis for the reasonable prediction of the varied macro mechanical properties of natural surrounding rocks in the deep geological disposal.However,limits still exist in the current research focusing on the multi-scale damage mechanisms of rocks.Besides,the interaction between multiple physical fields inside the granite leads to the complex damage mechanism of granite,and the quantitative relationship between the macro-mechanical response and meso-mechanical mechanism of granite under THM treatment is still confused.Therefore,in order to predict the changes in the mechanical properties of granite during the operation process of nuclear waste disposal,based on the laboratory tests,theoretical analysis and numerical simulations,the meso-macro damage evolution mechanism of granite under the THM treatment was investigated from the perspective of damage characteristics in meso scale,investigated,which is of great significance to provide key references for the design and safe operation of the deep nuclear waste repository.The main research work and conclusions are illustrated as the following aspects:（1）Evolution of meso-mechanical properties of mineral grain boundaries under THM treatmentFirst,the three-point bending tests of the mineral grain interface were carried out by using the self-developed single mineral grain interface mechanical test system with nanometer-level loading control accuracy under different THM conditions.Based on the Dugdale-Barenblatt（D-B）model,the tensile strengths of quartz,feldspar and quartz-feldspar grain interfaces were derived,and the mechanical strength degradation characteristics of different mineral grain interfaces in granite under THM treatment were directly obtained and distinguished for the first time.The experimental results showed that the tensile strength of the mineral grain interface was more sensitive to the pore pressure than the temperature,and the degradation effect of THM treatment on the mineral grain interface was more obvious in the quartz-feldspar interface,followed by the feldspar mineral grain interface and then the quartz mineral grain interface with the highest average tensile strength.Finally,based on the tensile strength distribution characteristics of different mineral grain interfaces under THM treatment,the meso-mechanical strength degaradation models of mineral grain interfaces related to THM treatments were proposed.（2）Evolution of pore characteristics of granite under THM treatmentFirst,the THM coupling test of granite was carried out adopting the temperature-confining pressure-pore pressure-axial pressure coupled controlling MTS815mechanical test system,and the effects of THM coupling on the evolution of granite permeability were first investigated.It is shown that the effect of high pore pressure（P>8 MPa）on the permeability of granite is greater than the effect of mild temperature（T<125 oC）,while the effect of low pore pressure（P<4 MPa）is the weakest.Subsequently,based on the low-field NMR testing and digital image processing technique,the pore characteristics,i.e.pore size distribution and pore volume of granite under THM treatment were quantitatively characterized.Finally,based on the correlation analysis of the macro and meso structure characterization of granite,the mechanism of the THM effect on the meso-structure evolution of granite was revealed,mainly including the strengthening,weakening and double effects of confining pressure,pore pressure and temperature on the pore size and large pore volume.（3）Simulation algorithm of meso-macro damage evolution of granite under THM treatmentFirst,based on the particle flow discrete element algorithm PFC2D,the weakening factor of the pore was introduced to characterize the correlation between the thermodynamic properties and the aperture of the pore.Subsuquently,the pore characteristics evolution results obtained in research（2）were substituted into the aperture dependent relationship to clarify the constitutive equations between weakening factors of pore,temperature and pore pressure in granite,and the meso-mechanical strength degradation model of mineral grain interface proposed in the research content（1）was embedded into the simulation algorithm to characterize the strength degradation properties of mineral matrix in the granite related to the THM treatments.In this regard,a meso-structure baased model which can truly describe the internal pore characteristics and mineral component properties of granite was reconstructed,and the simulation algorithm which can fully consider the mechanical degradation effects of mineral grain interfaces and pores under THM treatment was developed.Finally,the numerical results of TM,HM and THM were compared with the experimental results,and it was verified that the simulation algorithm can accurately predict the low-temperature thermal load strengthening and high pore pressure weakening effects on the macro mechanical properties of granite under THM treatment.（4）Mechanism of meso-macro damage evolution in granite under THM treatmentFirst,based on the simulation algorithm of damage evolution proposed in the research content（3）,the quantitative relationship between the internal mineral components of granite and its macroscopic mechanical properties was systematically analyzed under the THM conditions satisfying the engineering design environment of the nuclear waste repository.Subsequently,the THM processes in the granite were accurately replicated with the evolutions of pore pressure,THM-induced microcracks and meso structure compactness during the THM treatiment analyzed,and the evolution mechanism from"grain boundary tensile damage→cohesion deterioration→shear slip"meso-damage to"crack initiation→propagation→penetration"macro-failure of granite under THM treatment was revealed.Finally,the differences in the influences of each mineral component on the macro mechanical response of granite under THM treatment were quantified,and an evaluating system was established to quantify the relationship between the heterogeneous characteristics of granite meso-structure,coupled effects of temperature and pore pressure,and the meso-macro mechanical properties of granite. |
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