Supercritical Temperature Effect On The Hrdro-mechanical Performance Of Graphite-Bentonite Mixtures

Currently,with the exhaustion of fossil energy,nuclear energy is favored by more and more countries in the world.However,while nuclear fuel brings many benefits,it also produces a large amount of nuclear waste.Bentonite is used as the preferred backfill/buffer material for nuclear waste disposal repositories due to its high swelling pressure,low hydraulic conductivity and high adsorption performance.After the metal tank containing nuclear waste is buried in the underground disposal warehouse,the decay heat will be continuously released.If the heat is not diffused in time,the heat will accumulate.As a result,the surface temperature of the metal tank increases,and the adjacent bentonite ACTS at supercritical temperature(>100?)for a long time.When the temperature exceeds the operating standard of the disposal library(100?),the bentonite used for the buffer layer may undergo mineralogical changes(such as silication),thereby affecting the barrier performance of the buffer layer.Through the existing research foundation and literature,the thesis uses self-developed and designed instruments to simulate the swelling pressure and hydraulic conductivity of bentonite specimens at different temperatures(25,60,105?)and different temperature action sequences,and found that under 105?,the traditional method cannot be used to test the hydraulic conductivity of the specimen,and a new method for testing the hydraulic conductivity of compacted bentonite under supercritical conditions is proposed,and the hydraulic conductivity of the specimen under the action of supercritical temperature is measured.The above experimental results show that temperature can weaken the hydro-pressure performance of the compacted bentonite specimen.To this result,graphite with superior thermal conductivity is added to accelerate the heat transfer of the buffer layer,and the thermal conductivity and water of the compacted bentonite-graphite mixture with different forms of graphite are studied.The influence of mechanical properties provides a scientific reference for the preparation of bentonite-graphite mixture buffer materials.The main conclusions drawn are as follows:1)The self-made high-pressure bentonite thermo-hydro-pressure simulation tester has good performance,which can simulate the decay thermal action of nuclear waste within the range of-20?to 200?,as well as the process of bentonite wetting and swelling,and realize semi-automatic data collection.The test results of the swelling pressure and hydraulic conductivity of the compacted bentonite specimens with dry density of 1.65g/cm³ show that the rise in temperature can increases both the swelling pressure and the hydraulic conductivity of compacted bentonite.Compared with the initial value at 25?,when the temperature rises to 60?,the swelling pressure and hydraulic conductivity increase by 1.7times and 2.0 times respectively.After being heated at 105?for 15 days,the hydro-pressure property of bentonite weaked.2)Under the action of decay heat and groundwater,the buffer materials in the deep geological disposal repository of high-level radioactive waste will produce complex thermo-hydro-pressure coupling phenomenon.As water boils and vaporizes at 100?,it is difficult to measure the hydraulic conductivity at the critical temperature of the buffer layer.In order to accurately measure the hydraulic conductivity of buffer layer at supercritical temperature,a new method for testing the hydraulic conductivity of compacted bentonite under supercritical condition was proposed.In this method,the boiling point of water is increased by rising the water pressure,and different water pressure is applied to the upper and lower parts of the specimen at the same time,so that the water on the upper and lower sides of the specimen does not boil at the supercritical temperature.The hydraulic conductivity of the specimen is calculated by the principle of axial translation.Taking the compacted pure bentonite specimen with dry density of 1.65g/cm³ as an example,the hydraulic conductivity of the compacted pure bentonite specimen at 105?was measured to be 1.0×10^-13m/s.3)With the increase of the incorporation ratio of graphite(flake graphite and spherical graphite),the thermal conductivity of the graphia-bentonite mixture is higher than that of pure bentonite.Under the same pore ratio,with the increase of graphite incorporation ratio,the swelling pressure of the two(spherical graphite-bentonite mixture and flake graphite-bentonite mixture)remained close.However,the saturation hydraulic conductivity of the spherical graphia-bentonite mixture was smaller than that of the flake graphia-bentonite mixture,and the difference increased obviously after the incorporation ratio of 20%.On the whole,the amount of graphite,the overlap mode between graphite and graphite,the overlap mode between graphite and bentonite and the evolution of the pore volume of the mixture after wet swelling are the key factors affecting its thermal conductivity,hydraulic conductivity and free swelling ratio.