Swellinging And Permeability Of Graphite-bentonite Mixture

Regarding the disposal method of high-level radioactive waste(referred to as "highlevel radioactive waste"),deep geological disposal is currently recognized as the safest and most reliable method in the world,that is,the high-level radioactive waste is sealed in a suitable rock body and has a certain distance from the ground surface.Set up mult ip le artificial barriers to suppress the leakage and migration of nuclides,and ultimately achieve the goal of safe disposal of high-level waste.Buffer backfill material is one of the significant components in the high-le ve l radioactive waste repository system.Studying the performance of the buffer material is the key to building a high-level radioactive waste repository.The buffer layer of the high-le ve l depot requires not only good isolation performance,but also excel ent thermal conductivit y.To this end,the thesis is based on sodium-based bentonite,blended with high thermal conductivity natural graphite to form a uniform graphite-bentonite mixture,equipped with a buffer material with both isolation and thermal conductivity,and tested the partial buffering performance of the graphite-bentonite mixture the study.The fundamental goal of the research on hybrid buffer materials is to determine the proportion of the materials incorporated,and maximize the graphite-bentonite mixture to meet the swelling and permeability requirements.The thesis starts with the main influenc ing factors such as graphite particle size,graphite incorporation rate,porosity ratio,water content,etc.,and carries out water-force characteristic tests such as free swel ing rate,constant volume swel ing force and permeability of graphite-bentonite mixture.Swelling and permeability of the mixture were investigated.With the help of test instruments such as particle size analyzer,scanning electron microscope,mercury intrusion instrument,etc.,the change process of the mixture under the condition of hydration and swel ing and the change law of internal pore diameter were studied.The paper obtained the following test results:1)Overall,when graphite with a particle size of 200 mesh is incorporated,the selfhealing ability and impermeability of the graphite-bentonite mixture reach the optimal state,which is closely related to the contact mode between the particles in the mixture.2)The swel ing force of the graphite-bentonite mixture is greatly affected by the init ia l dry density and the graphite incorporation rate,and the initial dry density is roughly exponentially related to the maximum swel ing force.In comparison,the swel ing force of the graphite-bentonite mixture is less affected by the initial moisture content.3)The permeability of graphite-bentonite mixture is similar to that of swelling property,both increase linearly with the increase of initial dry density,and decrease with the increase of graphite incorporation rate.In addition,the permeability is also affected by the size of the pressure and the size of the pressure gradient.4)When the graphite incorporation rate exceeds a certain ratio,the swel ing and permeability of the graphite-bentonite mixture will deteriorate sharply.Taken together,when the graphite incorporation rate is 20% to 30%,the e swelling and permeability of the graphite-bentonite mixture have the best cost performance.5)Through the two methods of "effective bentonite density" and "diffused double electron layer",the swel ing pressure model is established to predict,and both have achieved good fitting results.