Investigation of Equilibrium Characteristics of Granite-groundwater System at Different Temperatures in Beishan Underground Laboratory Ramp Area

The Beishan granite area in Gansu Province is the primary candidate site for the geological disposal of high-level radioactive waste (HLRW) in China. Understanding the evolution of the host rock surrounding the disposal repository is an essential part of the safety evaluation of the repository. Targeting China’s Beishan Underground Laboratory (which is under construction), compositional analysis of the granite and groundwater in the ramp area was conducted, and the equilibrium characteristics of granite with groundwater under different temperatures were explored in this study. The granite sample (R-1130) was taken 1130 m from the surface adit in the ramp area. XRD analysis reveals that the main mineral components include quartz, feldspar, annite, and chlorite. XRF analysis shows that the iron content of the R-1130 sample, expressed as Fe₂O₃, is 0.91% (mass fraction). Additionally, the groundwater sample (W-1359), collected 1359 m from the surface adit in the ramp area, was analyzed. Based on the mineral and chemical composition of the R-1130 sample, the interactions between the R-1130 sample and two types of fluid (anoxic ultrapure water and anoxic groundwater) at 60, 90, and 120 ℃ were simulated using the geochemical modeling software PHREEQC. The simulations focused on the resulting aqueous geochemistry, including the concentrations of major ions, pH, and the formation of secondary minerals to reveal the equilibrium characteristics of granite-groundwater systems at different temperatures in the ramp area. The results indicate that while temperature has a relatively minor influence on the types and concentrations of secondary minerals and dissolved ions, it does affect reaction progress and system pH. In anoxic ultrapure water, the secondary minerals formed from Beishan granite are illite, kaolinite, and goethite. In anoxic Beishan groundwater, illite, kaolinite, goethite, Ca-montmorillonite, and calcite are the main secondary minerals. As the reaction progresses, the solution pH gradually increases but slightly decreases with higher temperatures, remaining within the alkaline range. The secondary clay minerals produced by granite alteration may seal granite fractures and enhance the adsorption of radionuclides, thereby slowing down radionuclide migration. The high temperature equilibrium characteristics of granite-groundwater system in Beishan underground laboratory ramp area not only confirm the retardation ability of granite to HLRW, but also provide a new example for the geochemical evolution of deep crystalline rocks under thermal disturbance, which provides an important scientific basis for the design and safety evaluation of HLRW disposal repository.