Analysis of the Long-Term-Coupled Thermo-Hydro-Mechanical Performances and the Retardation Capacities of Composite Bentonite Buffers Using a Numerical Simulation Method

A bentonite buffer is an important component of the engineering barrier of a high-level radioactive waste (HLW) repository, which can delay the infiltration of groundwater and retard the migration of radionuclides. The long-term-coupled thermo-hydro-mechanical (THM) performances and the retardation capacities of composite bentonite buffers are studied in this work. A theoretical coupled THM model is presented, and a finite element code, namely LAGAMINE, is used for the numerical simulation under coupled THM conditions. Six kinds of composite bentonite buffer computational models are considered. The boundary conditions of these models and computational parameters of bentonite are presented. The repository time for HLW is considered to be 10,000 years. The evolution processes of temperature, saturation degree and swelling pressure are presented according to the computational results. The combination forms of blocks and joints have an obvious influence on the swelling pressure in the bentonite buffers. The circumferential and radial joints can significantly reduce the maximum value of the swelling pressure. The influence coefficient for the retardation capacity ψ is proposed for evaluating the retardation capacity of composite bentonite buffers. According to the value of ψ, the six kinds of computational models are ranked in terms of retardation capacity, and three kinds of models are suggested for usage in design. The results of this investigation are helpful for optimising the design of bentonite buffers for HLW repositories.