Research On Thermal And Mechanical Damage Of Granite For Geological Disposal Of High Level Radioactive Waste

The safety of high-level radioactive waste?HLW?disposal is the key to the sustainable development for nuclear industry.Nowadays the deep geological disposal of HLW has been widely accepted worldwide.The deep geological disposal method separates HLW from biosphere by burying it 500 m¹000 m underground.Firstly the excavation of repository will inevitably cause damage on surrounding rock,inducing Excavation Damage Zone?EDZ?.Secondly,the decay heat of HLW will increase the temperature of rock,which will induce thermal-stress in surrounding rock.Overall,excavation and the decay heat will cause damage on surrounding rock,which would induce cracks in rock,to provide potential immigration pathway for nuclide.Thus to deepen the understanding of EDZ and thermal damage on granite is of great importance for HLW disposal engineering.Beishan granite is the most potential host rock for HLW repository for China.To deepen the understanding of the influence of temperature,stress and excavation method on the physical and mechanical properties on granite,many research works have been conducted as follows:?1?Experimental study of temperature effect on physical properties of granite.Firstly,a laboratory thermal cracking experiment was conducted to study thermal cracking process under temperature gradient field in granite samples.Secondly,thermo-gravimetric and differential thermal analysis test and acoustic emission monitoring?AE?were conducted to study physicochemical reaction and the development of thermal induced micro-cracks of granite from room temperature to800?;Thirdly,the influence of temperature on physical property of granite was studied.?2?The study on meso-and micro-characteristics of thermally damaged granite.Firstly,the development of micro-cracks induced by thermal damage was studied using SEM and microscope;Pore size distribution?PSD?of thermal damaged granite was analyzed based on MIT and NMR results.Then the fractal characteristic of PSD was analyzed using NMR test results?T₂?.The evolution of fractal dimension of PSD for thermally damaged granite shows good agreement with the evolution of permeability:porosity,fractal dimension and permeability decrease with temperature in 100?³00?,characterized as?pore healing?,and increase with temperature in 400?⁷00?.The fractal dimension of PSD was used in a proposed empirical formula to calculate the permeability of rock.In addition,a code was devised to analyze percolation law and fractal dimension for NMR images of thermally damaged granite.In addition,a code was devised to analyze percolation law and fractal dimension for NMR images of thermally damaged granite.?3?Experimental study on excavation damage zone of granite.Systematic EDZ study was conducted in Beishan Exploration Tunnel?BET?using AE monitoring system,multipoint displacement meter,ground penetrating radar,ultrasonic velocity measurement device and borehole TV.The development and characteristic of EDZ were studied.And the suitability and effectiveness of used techniques were evaluated.In situ EDZ monitoring techniques system was proposed for EDZ study in HLW disposal engineering.The damage process was identified as blast damage stage and progressive damage stage based on the evolution of AE hits.Significant damage was observed in 10cm²0 cm from excavation surface,and rock within 40 cm⁵0 cm inner the excavation surface was slightly influenced.?4?Study on mechanical properties of Beishan granite under and after high temperature treatment.Cycle loading and unloading compression test,axial,tri-axial compression test and Brazil test were conducted to study the influence of temperature on mechanical properties of granite.And the influence mechanism of temperature on mechanical properties of granite is analyzed using micro-characteristics and fracture mechanics.Result shows that mechanical properties of thermal treated granite were slightly strengthened in 100?³00?,then slightly deteriorated in 300?⁵00?,and significantly deteriorated once temperature higher than 500?.In addition,cycle loading and unloading compression test for granite under 20?to 90?was conducted to study mechanical properties under high temperature.The strength of granite under 90?is lowered 15%compared to unheated granite.The strength parameters?c and?value?were synchronously weakened under high temperature.The evolution of strength parameters during compression test for granite under different temperature shares similar characteristic,which can be summarized as CWFS_W model.According to the evolution of deformation parameters for the whole compression process,a method to determine strength parameters for strain softening model was proposed using conventional axial and tri-axial compression test data.?5?Numerical simulation of thermal-mechanical problem for HLW disposal.A strain-softening model coupled simplified elastic-plastic CWFS_W model was proposed,and the model was verified using FLAC3D.Result shows the model is effective in characterizing strength and deformation of granite.At last,a numerical model,taken the decay power,EDZ and thermal damage into account,was used to study the evolution of temperature,stress and plastic strain for HLW disposal engineering.The influence of distance between disposal pits and tunnels on the maximum temperature in HLW repository was discussed.And the influence of the existence of EDZ and thermal damage on the safety of repository was preliminarily studied.