Study On The Swelling Properties Of Gaomiaozi Bentonite Under Coupled Water And Heat Conditions

The amount of high-level radioactive waste is increasing rapidly with the expansion of nuclear power in the past decades.In order to ensure the sustainable development of nuclear power,it's particularly important to dispose of high-level radioactive waste safely.In China,multiple barrier design is adopted in the high-level radioactive waste repositor-y,which is composed of the natural surrounding rock,buffer material and canister.Buff-er material is one of the most important components of high-level radioactive waste repository.The long-term stability of the repository largely depends on the swelling capability of buffer material.As the base material of buffer material,it is very important to predict and evaluate the swelling property of bentonite accurately.The environment in the disposal repository is extremely complex due to concrete interacting with groundwater to form high alkaline pore water,and the radiant heat is continuously emitted by the high-level radioactive waste in the canister.In this work,swelling tests were conducted to obtain the different swelling properties between GMZ07 bentonite and GMZ24-200 bentonite.NaOH solution of different concentrations was used to represent the high alkalinity pore water,whilst the oven was employed to represent the radiant heat.The swelling properties of GMZ bentonite in different environment were tested.The main conclusions are obtained as follows:(1)Exponential relationship between the dry density and maximum swelling pressure is obtained in swelling tests of GMZ07 bentonite and GMZ24-200 bentonite.The relationship between the effective content of Montmorillonite and the maximum swelling pressure is analyzed.It is found that the dry density is the main factor affecting the swelling pressure of bentonite.Double peak structure is observed in swelling pressure curve of GMZ07 bentonite with low dry density,because of the collapse of soil skeleton resulting in the decrease of swelling pressure.At high dry density(1.89g/cm~3 and 1.90g/cm~3),the double peak structure is not obvious.However,double peak structure is not appeared in swelling pressure curve of GMZ24-200bentonite.The maximum swelling pressure of GMZ07 bentonite is higher than that of GMZ24-200 bentonite,because the content of montmorillonite in GMZ07 bentonite is much higher than that in GMZ24-200 bentonite.With increasing time,the swelling capacity of GM24-200 bentonite increases gradually and reaches the maximum value at a certain time.There is a good linear relationship between the maximum swelling rate and dry density.(2)Swelling tests were conducted on GMZ24-200 bentonite with four different concentration solution.When the pH of solution is 12,the mineral composition of GMZ24-200 bentonite did not change.The maximum swelling rate of bentonite is the same as that of deionized water.When the solution is Beishan water,the swelling rate of bentonite is the smallest due to no chemical reaction between Beishan and decrease of the interlayer space and swelling rate.With the solution pH 13,,the swelling rate is the largest under four different concentration solution since the mineral composition has changed.The reason is that a large number of Na~+infiltrates,resulting in a large swelling of crystal layer.(3)The evolution of GMZ24-200 bentonite swelling pressure under different temperature and pH value is obtained.The pH value and electro-conductivity is recorded when bentonite is immersed in alkali solution.At normal atmospheric temperature,the maximum swelling pressure of GMZ24-200 bentonite after 190 days of direct infiltration of three alkaline solutions is approximately the same as that of GMZ24-200 bentonite powder after 190 days of mixing and soaking.The difference of swelling pressure may be due to the influence of temperature in the former experiment,or the difference of Na~+content in the latter More,the larger the ion hydration capacity,the greater the swelling pressure.