Numerical Simulation Of¹²⁹I&¹³⁵cs Nuclide Migration In EDZ Of The High-Level Radioactive Waste Repository

Deep geological disposal of high-level radioactive waste is currently recognized as the most realistic and feasible method.Because there are fractures in the excavation disturbance zone?EDZ?of high-level radioactive waste repository,these fractures are important channels for radionuclides to migrate from near field to far field.Therefore,numerical simulation of nuclide migration in EDZ is of great significance.This article based on the conceptual model of high-level radioactive waste repository;EDZ of Tarim clay rock was used as a research object;the nuclide migration of ¹²⁹I and ¹³⁵Cs is simulated by TOUGH2 simulation platform.The main contents of my thesis are as follows:?1?Under isothermal conditions,EDZ model is established by using equivalent continuity?EC?model and dual media?DP?model simplified fracture;the effects of different model on nuclide migration of ¹²⁹I and ¹³⁵Cs in EDZ are studied.The experiment results show that the nuclide migration velocities of ¹²⁹I and ¹³⁵Cs in the DP model are faster than that in EC model,which indicates that the different model will affect the nuclide migration regular pattern of ¹²⁹I and ¹³⁵Cs in EDZ and the blocking effect of EC model is better than that of DP model.?2?The sensitivity analysis of diffusion coefficient and partition coefficient of ¹²⁹I.The experiment results show that ¹²⁹I is more sensitive to diffusion coefficient than to partition coefficient.?3?Ignoring EDZ,a far-field model is established based on the fact that the simulated areas are all far-field clay rocks.The experiment results show that the migration velocities of ¹²⁹I and ¹³⁵Cs in EDZ model are higher than those in far-field model,which indicates that EDZ cannot be ignored in the design of repositories and safety evaluation stages.?4?Based on the exothermic of radioactivity radionuclide decay,the EDZ model under variable temperature is established.The experiment showed that EDZ reached the highest temperature of 73.3? in the 10 th year and returned to the initial temperature in 5000 years.The change of temperature will cause the change of seepage.Under the action of convection,the horizontal temperature gradient is gradually larger than the vertical temperature gradient.?5?Under Temperature effect,the nuclide migration velocities of ¹²⁹I and ¹³⁵Cs in EDZ model and far field model will increase,and the increase of ¹²⁹I is larger than ¹³⁵Cs.?6?The far-field model under the action of temperature is established.The experiment showed that with the change of temperature,the difference of migration velocity between ¹²⁹I and ¹³⁵Cs in EDZ model and far field model is larger than that under isothermal condition.Therefore,the existence of EDZ cannot be neglected in the design and construction of high-level radioactive waste repositories under the actual situation that the radioactivity of radionuclide decay will generate a lot of heat.The results show that the presence of EDZ has a negative impact on the safety of high-level radioactive waste disposal,and the increase of temperature will increase the impact.Therefore,the influence of EDZ and temperature should not be neglected in the safety evaluation of high-level radioactive waste repositories.