Study On The Characteristics Of Damage Healing And Reconsolidation Of Salt Rock

Under the background of carbon peak and carbon neutrality,nuclear power,as the clean and low-carbon energy that can replace fossil energy on a large scale,has ushered in a new development opportunity.At present,the treatment of high-level nuclear waste is a relatively weak link in China’s nuclear power industry and one of the important factors restricting the development of China’s nuclear power.In this paper,according to the construction of underground salt rock high level radioactive waste disposal repository in China as the background,based on the three research status quo of the insufficient research on damage healing characteristics of salt rock,unclear understanding of deformation characteristics of granular salt reconsolidation,and unclear permeability and thermal conductivity characteristics of reconsolidated salt,a systematic study on damage healing and reconsolidation characteristics of salt rock was carried out by means of experimental research,theoretical analysis and numerical simulation.The main research contents and conclusions are as follows:（1）The salt rock macroscopic fracture strength and permeability recovery experiments,and the salt rock damage microcracks healing experiments at the mesoscopic scale were carried out.The salt rock damage healing law under the influence of different factors was studied.The results have shown that water is an important condition for salt rock crack healing.Under the condition of water participation,crack healing in salt rock can take place at room temperature and low pressure.Salt rock damage healing is manifested in microcrack filling and crack segmentation caused by grain growth.The increase in temperature,humidity and normal stress all promote the healing of salt rock damage.The degree of salt rock healing increases with the increase of healing time,and the relationship between healing variables and healing time can be expressed by an exponential decay function.（2）Based on the analysis of the microscopic morphology of salt rock damage after healing,combined with the theory of crystal growth,the physical mechanism of salt rock damage healing is revealed:with the participation of water,the chemical potential difference within the crack drives the dissolution and precipitation of substances and the recrystallization with Na Cl solute;microscopic behaviors such as grain growth,volume diffusion,grain boundary diffusion,dislocation annihilation and recrystallization related to crystal properties under high temperature or high pressure conditions.Based on the high temperature and high pressure environment of the underground salt rock high-level nuclear waste repository,the impact of damage healing in salt rock on the mechanical properties and permeability evolution of the surrounding rock of the repository will be more obvious.（3）The reconsolidation experiments of granular salt rock under constant strain rate loading and creep loading and the orthogonal experiments of granular salt rock reconsolidation under different geological environment factors were carried out to study the deformation characteristics and influencing factors of granular salt rock reconsolidation process.The results have shown that consolidation stress,consolidation temperature and water content of granular salt rock have an important influence on the deformation characteristics of granular salt rock during reconsolidation.The final porosity of the reconsolidated granular salt rock depends to a large extent on the consolidation stress.The greater the consolidation stress,the smaller the final porosity,and the effect of the consolidation stress on the porosity of the reconsolidated salt is mainly due to instantaneous loading.It has little effect on the deformation rate of the granular salt rock in the creep consolidation stage.The increase of temperature can accelerate the deformation rate of granular salt rock during the reconsolidation process,which is one of the main factors affecting the deformation rate of granular salt rock in the creep consolidation stage.The addition of a certain amount of water can greatly promote the reconsolidation of the granular salt rock.Under the condition of drainage consolidation,when the initial water content of the granular salt rock exceeds 6%,the continuous increase of water content has no obvious promoting effect on the consolidation effect of granular salt rock.（4）The porosity and permeability evolution experiments during the reconsolidation of granular salt rock,and the experiments of the influence of different inlet pressures and confining pressures on the permeability of the reconsolidated salt were carried out to study the permeability characteristics of the reconsolidated salt.The results have shown that the decrease in porosity during the reconsolidation of granular salt rock is not only a decrease in the number of pores but also the conversion of large pores to small pores and micropores and the decrease in connectivity between pores.The relationship between the permeability and porosity of the reconsolidated salt can be expressed by a power function.Compared with salt rock and mudstone,the influence of Klinkenberg effect in the gas seepage of reconsolidated salt is more obvious.The absolute permeability of the reconsolidated salt sample with a porosity of 5%can be as low as the order of 10^-19 m².It can fully meet the shielding requirements of groundwater from the buffer materials in the salt rock high-level nuclear waste repository.（5）The thermal characteristic parameter tests of the reconsolidated salt under different porosity conditions in the temperature range of 20-250℃were carried out,and the numerical simulation of the evolution of the temperature field during the operation of the underground salt rock repository was carried out.The results have shown that the thermal conductivity of the reconsolidated salt decreases with the increase of temperature and increases with the decrease of porosity.Based on the experimental data,a model of the effective thermal conductivity of the reconsolidated salt was established.The temperature of the waste body of nuclear waste began to drop after reaching a peak in about 10 years.After 1000 years,the influence of the decay heat of nuclear waste on the temperature of the repository is no longer obvious.The increase in the porosity of the buffer material and the density of the disposal chamber will increase the temperature of the repository.Under the same disposal conditions,the peak temperature on the surface of the waste tank of the underground salt rock HLW storage can be reduced by nearly half compared with that of the granite storage,which indicates that the underground salt rock HLW storage has great advantages in heat conduction and temperature control of the storage.