Advances in groundwater numerical simulation in deep geological disposal of high-level radioactive waste

Groundwater numerical model is not only an effective tool for understanding the formation and evolution mechanism of deep groundwater dynamic field, but also the basis for establishing numerical model of nuclide migration. Therefore, it is an important technical means in the site selection and safety assessment of high-level radioactive waste (HLW) disposal repository. There are many numerical simulation methods for groundwater flow in deep geological disposal of HLW, and how to choose the suitable method is also a problem worth paying attention to. This article focuses on the research of groundwater numerical simulation technology for deep geological disposal of HLW. Through reviewing a lot of relevant papers, systematically expounds the research progress, applicable conditions and practical applications of four kinds of commonly used groundwater numerical simulation methods. In addition, summarizes the model uncertainty analysis methods and research results commonly used in deep geological disposal, and lists the numerical simulation software of groundwater flow suitable for geological disposal of HLW and its application in waste disposal selection and safety assessment. The results show that the equivalent continuum model is suitable for large, long sequence and high fracture development or uniform areas, with the advantages of mature method and easy to obtain the required data and parameters, but the flow characteristics of groundwater in fractured media cannot be described accurately. The discrete fracture network model is suitable for solving the groundwater flow problems that need to be finely described, such as disposal site and repository canister. However, due to the need for a large number of fracture characteristics, connectivity and related parameters data, this method has the disadvantages of heavy workload and time-consuming. The dual medium model is mainly used to solve the problem of regional-scale fractured groundwater flow, but it cannot show the characteristics of anisotropy and discontinuity of fractured media, so the scope of application has certain limitations. The equivalent discrete coupling model can adopt the equivalent continuum model for the area with high fracture density and the discrete fracture network model for the area with low fracture density through the domain decomposition method, which is more in line with the characteristics of fracture seepage under general geological conditions, but there is also the problems that the exchange capacity is difficult to determine and the coupling technology of two models. Sensitivity analysis sorts the influence degree of different sensitive factors on the model sensitive indexes, so as to improve the model accuracy and reduce the workload of parameter uncertainty analysis. Monte Carlo method is a commonly used method for model uncertainty analysis, which is simple in principle and beneficial to implementation. Finally, the author points out that numerical model simulation ability, uncertainty analysis, prediction simulation and multi-medium coupling model research should be strengthen in the future.