A novel equivalent model of radionuclide migration in three-dimensional rough shear fractures

Abstract

Accurate prediction of radionuclide distribution in fractured rock mass is a key problem in high-level radioactive waste disposal engineering. However, it is inappropriate to directly regard rough fractures as smooth fractures when predicting radionuclide migration in real fractured rock masses. In this study, the characteristics of radionuclide migration in three-dimensional rough shear fractures are numerically analyzed considering the influence of fracture roughness, aperture distribution, contact ratios, and adsorption. Different fracture contact ratios and aperture distributions are realized by shearing two rough fracture surfaces. Based on the study, we found that the influence of surface contact is greater than that of surface roughness and aperture distribution on radionuclide migration in rough fractures with the same initial fracture aperture under laminar flow. The phenomena of dominant migration channels and low-velocity dispersion zones are observed for radionuclide dispersion in rough fractures containing contact areas. The higher the contact ratio, the more obvious this phenomenon. The breakthrough and steady state times of radionuclide are also significantly affected by the contact ratio. The higher the contact ratio, the shorter the breakthrough time, and the longer the duration from the breakthrough time to the steady state time. Adsorption delays the overall process of the radionuclide migration and prolongs the duration between the breakthrough time and the steady state time. A novel equivalent method of the radionuclide migration in rough shear fractures is proposed based on the study of radionuclide transport in rough fractures. In the proposed equivalent method, an adjustment coefficient for the transport velocity and a longitudinal dispersion coefficient for the rough fracture are defined to connect rough fractures and the corresponding smooth fractures with the same fracture aperture. The influence regularity of fracture contact and adsorption on the two defined coefficients is obtained based on over 400 numerical simulations. The specific formulas of the two coefficients are given based on the form of the analytical solution for the radionuclide transport in a smooth fracture. The dispersion coefficient and transport velocity in a rough fracture are equivalent to the dispersion coefficient and transport velocity in a smooth fracture and the fluctuation term affected by fracture surface contact and adsorption, respectively. It was verified that the equivalent model has higher accuracy, which can help characterize radionuclide migration in 3D rough fractures efficiently and precisely.