Review of groundwater numerical simulation for deep geological disposal of high-level radioactive waste

Groundwater numerical simulation is an important part of the safety assessment for high-level radioactive waste (HLW) disposal sites. The complexity of deep geological disposal media and the relative lack of deep bedrock data lead to uncertainties in the simulation results. How to characterize the deep groundwater dynamic field and evaluate the possible risks has become a key issue in the safety assessment of HLW disposal. Based on a lot of literature investigation, this paper reviews the application cases of groundwater numerical simulation and uncertainty analysis of HLW deep geological disposal sites in typical countries, and summarizes the research experience in this field. The results show that (1) the structure and fracture development and distribution of deep geologic repositories determine the groundwater circulation conditions, and exploration of the new hydrogeological test method suitable for fractured bedrock areas is the basis of improving the accuracy of groundwater numerical simulation. (2) The integration of different scale models is an effective technical method to solve the groundwater simulation for deep geological disposal. The equivalent continuum method is mostly used at a regional scale, the coupling model of equivalent continuous porous medium and discrete fracture network is used at a site scale, and the discrete fracture network method is used at a repository scale. Furthermore, it is necessary to pay more attention to the migration and transformation of radionuclides in geological formations, to simulate and predict the potential impact of the long-term groundwater environment evolution on radionuclide migration in the site area. (3) Considering different host rocks in the disposal layers and the thermal-hydrogeological-mechanical-chemical process occurring in multiple media, the commonly used software for groundwater numerical simulation in HLW deep geological disposal includes Porflow, Modflow, GMS and MT3DMS, which are used to deduct pores or fractures into equivalent continuum, and groundwater flow and nuclides migration in fractures of crystalline rock and granite can be generalized by using Connectflow, Feflow and Fracman, whereas TOUGH series is mainly used to simulate flow, solute and heat transport in dual media. (4) Targeted uncertainty analysis of the models and parameters should be carried out to reasonably reduce workload and improve the model accuracy. Moreover, the prediction simulation of repository long-term evolution, waste canister failure, extreme rainfall and other scenarios can provide basic data support for the safety assessment and design of the repository. (5) In view of the current researches on groundwater numerical simulation of HLW deep geological disposal in China, it is necessary to strengthen the investigation and monitoring of geology, hydrogeology, fissure measurement and field test in the next step, and the multi-medium coupling model, multi-field coupling simulation and uncertainty analysis will be the focus of future researches.