| For the amount and current situation of domestic nuclear waste,the state plans to build a cavern-type repository in Yangjiang for the disposal of low-and medium-level nuclear waste,its safety life is required to be 300 to 500 years.Therefore,granite is selected as the surrounding rock,bentonite as the buffer/backfill material,cement as the wrapping material of the radioactive waste,and wrap radionuclides through multiple barriers to meet their safety.However,with the passage of time,groundwater may erode the repository and barrier materials,leading to the outward migration of radionuclides.The study of Radionuclides migration can effectively predict the safety of nuclear waste repository.The safety of the repository and its ability to block key radionuclides need to be verified by various experiments before construction.At present,many scholars at home and abroad have studied the radionuclide migration of nuclear waste repository through laboratory experiments,disposal site regional experiments,numerical simulation and other methods.Near-surface geological disposal is the safest and most effective disposal method for medium-and low-level nuclear waste at present,and groundwater is the most important factor contributing to the diffusion and migration of radionuclides.In this paper,the characteristic parameters of granite in a repository are obtained from static and dynamic experiments to simulate a three-dimensional geological model of the repository with the help of a GMS（Groundwater Modeling System）.Study the radionuclide migration distance,influence range and the change trend of nuclide concentration at different observation points in the ultra-long time scale.The physical and chemical properties of the granite samples is characterized by X-ray fluorescence（XRF）,X-ray diffraction（XRD）,Fourier transform infrared（FT-IR）spectroscopy and scanning electron microscopy（SEM）.Analysis of main chemical components of groundwater by inductively coupled plasma mass spectrometry（ICP-MS）.The main conclusions are as follows:（1）Through various characterization study that the main components of the granite in the disposal repository are Si O2,Al2O3 and K2O.The main chemical components of groundwater are Ca、Si、Na、HCO3-、Cl-.（2）Study the adsorption capacity of the granite surrounding rock of the disposal the disposal repository to three nuclides through static adsorption.The adsorption distribution coefficient of granite to Cs（I）reached the maximum value at the 7th day,58.60 m L/g,Co（II）reached the maximum value at the 9th day,88.99 m L/g and Ni（II）reached the maximum value at the10th day,102.15 m L/g.（3）The dynamic shower experiment studies the time and speed of different nuclides penetrating the granite column with groundwater,and calculates the longitudinal dispersion coefficient of different nuclides penetrating the granite column.The longitudinal dispersion coefficient DL of Cs（I）is 3.19×10-7 m2/s,Co（II）is 1.11×10-8 m2/s,and Ni（II）is 2.49×10-7 m2/s.Comparison shows that Cs（I）penetrates the granite column the fastest,followed by Ni（II）and Co（II）.（4）The diffusion coefficients of Cs（I）,Co（II）and Ni（II）penetrating granite slices are3.29×10-10 m2/s、6.27×10-12 m2/s and 4.80×10-10 m2/s.The dispersion coefficients are 1.47×10-8 m2/s、7.72×10-8 m2/s and 5.80×10-8 m2/s.Compared with the diffusion coefficient and dispersion coefficient of the same nuclide,the diffusion experiment increase a certain pressure,which strengthened the nuclide diffusion ability and increased the concentration of penetrating granite slices.（5）Establish three-dimensional geological structure with three different situation to simulate the migration of three different nuclides in the groundwater.The adsorption retardation effect of 60Co on granite in this repository is better than that of 137Cs.Under the barrier of granite,bentonite and concrete,137Cs have the longest migration distance within 500 years after the release of nuclides,maximum migration distance is 80 m and the maximum nuclide concentration is about 12 mg/L.The results show that the medium-and low-level radioactive waste repository can effectively block the migration of radionuclides,shield its high heat release and high radioactivity hazards within the safe time,and meet the requirements of medium-and low-level radioactive waste disposal. |
PDF Full Text Request