Study On Thermo-hydro-mechanical Coupling Process Of Composite Buffer Blocks For High-level Radioactive Waste Disposal Repository

As part of the engineering barrier of the high-level waste disposal repository(HLW),the bentonite blocks of different combinations are generally placed between the nuclear waste canister and the country rock to retard the migration of radionuclides.Because of the complex couplings between thermo-hydro-mechanical(THM)in bentonite,the coupling effects should be studied to evaluate the retarding ability of the bentonite blocks effectively.Based on the multi-field couplings theory of porous media,the simulation and analyzation of the THM couplings in the bentonite blocks composed of single ring and double rings are conducted with the finite element numerical simulation method.The main research programs and work done in this paper are as follows:(1)Considering the multi-physical phenomena such as phase transformation,dissolution,temperature driving,fluid transmission and wetting expansion,some works are done to sort the mathematical models of THM couplings in porous media based on the mixture theory,continuous medium theory and the principle of universal conservation,and the governing equations of the solution of air in liquid is re-derived.The equations above can be used to analyze the THM couplings in buffer materials.(2)On the redevelopment of the PDE mathematical module in the multi-physical field finite element software COMSOL and the inputs and definitions of the THM couplings governing equations and relative parameters,a numerical method for the analysis of the THM couplings in buffer materials is established.(3)By use of the physical,mechanical,thermal,hydraulic parameters of the Gaomiaozi(GMZ)bentonite,a numerical simulation of the test for the THM couplings in buffer material in China(China-Mock-Up)for 600 days is carried out to understand the changes of the temperature,seepage and stress fields.On the comparison with the experimental results such as the temperature,relative humidity and swelling force,the feasibility of the numerical simulation method is verified.(4)On the conceptual design of China's HLW repository and reference to relative foreign data and the existing manufacturing level in China,two buffer material models of single ring with 3 blocks and double rings with 6 blocks are suggested.Then considering the existence of the radial and circumferential gaps in the bentonite,a numerical simulation of the THM couplings for 10 years is perfonned.The results show the distribution and evolution of the temperature,seepage and displacement fields and reveal the sealing laws of the radial and circumferential gaps.The results of this study not only help to understand the THM couplings in buffer materials consist of different block combinations and provide a numerical simulation method for the study of the long-term performance of the buffer materials,but also guide the design of the buffer materials for the HLW repositories in China to some extent.