Study On Permeability Thermodynamics Of Bentonite Sand Graphite Buffer Material Based On Nuclear Waste Disposal

Buffer material is the last engineering barrier of geological repository of nuclear waste and other high-level radioactive waste（HLRW）.From the construction to operation of nuclear waste repository,buffer material will work under complex thermo-hydro-mechano-chemical（T-H-M-C）multi field coupling conditions,and each field has close and complex interaction with each other.In order to predict and evaluate the actual performance of buffer materials scientifically and reasonably,it is necessary to have a deep understanding of the main engineering characteristics and multi-field coupling behavior of unsaturated buffer materials under the combined action of temperature field,seepage field and chemical field.Therefore,the in-depth and systematic study of the T-H-M-C coupling characteristics and engineering application of the buffer materials for the engineering barrier of the HLRW repository in China,which is of frontier,good application prospects and social benefits.In this paper,BSG mixture is investigated to reveal its hydro-mechanical properties,swelling properties,heat transfer characteristics and microscopic mechanism.The characteristics of BSG buffer material are systematically studied by means of laboratory tests,mechanism analysis and model research.The main research contents and results are as follows:（1）Based on the self-developed swelling pressure and permeability test device,the variation of swelling pressure and hydraulic conductivity of BSG mixture at different temperatures,salt solution concentrations and p H values of Na OH solution were systematically studied.Also,the effects of different factors on the swelling pressure and hydraulic conductivity of BSG mixture were analyzed in detail.The results show that with the increase in grap Hite content,the maximum swelling pressure of BSG mixture decreased,while the hydraulic conductivity increased;at the same grap Hite content,the hydraulic conductivity increased with the increase in the maximum grap Hite mesh number.The addition of grap Hite changed the soil structure and contact mode between the particles of the BSG mixture;and affected the internal pore distribution in the mixture,which resulted in the variations of the thermal conductivity,swelling pressure and hydraulic conductivity in the BSG mixture.Sand content can effectively restrain the maximum swelling pressure,but with an increase in sand content,the inhibition effect is weakened;while the hydraulic conductivity of BSG mixture increases with an increase in sand content,and the greater the sand content,the greater the hydraulic conductivity.For a given cation type,with an increase in salt solution concentration the swelling pressure decreased,while the hydraulic conductivity increased;the BSG specimens wetted with Na Cl solution developed lower swelling pressure than those with Ca Cl₂ solution at room temperature,whereas the opposite tendency appeared at 60-80°C.The effects of temperature on the swelling pressure are explained by the role of the lattice contraction.The degeneration of the interlamellar adsorbed water at high temperature weakens the crystalline swelling.For a given temperature,the decreasing surface potential and increasing osmotic suction with solution concentration both weaken the clay swelling.With an increase in p H and temperature,the swelling pressure of BSG mixture decreased,while the hydraulic conductivity increased;the optimum grap Hite content should not exceed 20%,the maximum grap Hite mesh number should be between 100-200,and the sand content should be less than 20%.（2）Based on the self-developed swelling deformation test device,the swelling deformation characteristics of BSG mixture at different temperatures,salt solution concentrations and p H values of Na OH solution were systematically studied.The results show that the swelling strain of BSG mixture increased with an increase in initial dry density and grap Hite content,respectively;while shows a decreasing trend with an increase in sand content.The optimum ratio of BSG buffer material is determined,that is,the mass fraction of bentonite,sand and grap Hite is 85:10:5;the maximum grap Hite mesh number can be 100 or 200.In the semi logarithmic coordinate system,the evolution curve of swelling strain with time can be roughly divided into three stages:initial expansion,main expansion and secondary expansion.At low temperature,the evolution curve shows a smooth"S"shape;at higher temperature,the evolution curve of BSG mixture was fluctuating;the increase in temperature accelerates the expansion process of BSG mixture and increases the expansion deformation in the initial expansion stage and main expansion stage.The concentration of salt solution significantly inhibited the expansion deformation of the sample,but the decrease rate of the maximum expansion rate decreased with the increase in temperature,indicating that the temperature can offset the inhibition effect of some chemical solutions on the expansion to a certain extent.However,alkali solution will destroy the internal structure of bentonite,change its mineral composition and content,and ultimately reduce the expansion property of bentonite.The coupling effect of temperature and high concentration of salt solution may change the structure of montmorillonite in bentonite,while the coupling effect of temperature and high concentration of alkali solution may change the types of minerals in bentonite.In addition,based on the expansion mechanism of bentonite,according to calculation method of Komine,the concept of expansion volume strain of montmorillonite was proposed.Combined with the diffusion electric double layer（DDL）theory,the calculation model of diffusion electric double layer theory is modified based on fractal model,and fitted through experimental data.A prediction model of expansion deformation of BSG mixture under the action of salt solution with high accuracy is obtained.（3）Based on the unsteady state thermal needle technique,the thermal properties of BSG mixtures were systematically studied.The results show that the thermal conductivity increased with an increase in volumetric water content,dry density,and degree of saturation.The effect of pore distribution and pore water arrangement on thermal conductivity is different under different water retention conditions.The influence of the size of sand on the thermal conductivity of soil is mainly due to the change of the number of contact points between particles in the unit volume of soil.In addition,the relationship between thermal conductivity and volumetric water content has inflection point,and the volumetric water content corresponding to the resistivity inflection point can be used as the inflection point of thermal conductivity.Based on the inflection point of thermal conductivity,an improved serial-parallel thermal conductivity model is proposed.The prediction results of the model are in good agreement with the laboratory test results,and the predicted value of the model is within the Wiener boundary.（4）The microstructure evolution of BSG buffer material under thermo-chemical action was revealed by scanning electron microscope（SEM）,X-ray diffraction（XRD）and infrared spectroscopy（FTIR）.The results show that the diffraction peaks of montmorillonite and quartz in unheated sodium bentonite are significant,and with the extension of heating time and the increase in temperature,the diffraction peaks of montmorillonite gradually decrease,some diffraction peaks almost disappear,and silicon oxide appears.The diffraction peak intensity of montmorillonite decreases gradually with an increase in p H value;after the bentonite is eroded by Na Cl or Ca Cl₂ solution,the particles aggregate and more macropores appear.Thermo-chemical coupling causes the dissolution and p Hase transformation of the main mineral（montmorillonite）in bentonite,the filling of secondary chemical precipitation into the pores of bentonite,and the chemical cementation of bentonite caused by amorp Hous colloid.The formation of non-expansive minerals by p Hase transformation of clay minerals will not well fill the cracks caused by the dissolution of bentonite,and the secondary precipitation accumulation will reduce the expansion performance of bentonite buffer barrier.