Investigation of nuclide migration in complex fractures with filling properties

Abstract

Natural fractures in rock masses commonly feature filling properties, which greatly impact the process of fluid flow and mass transport in the geological disposal of high-level radioactive waste. This study develops a unified pipe-network method to numerically investigate the mechanism of nuclide migration in rough-walled fractures with filling properties based on mobile–immobile models (MIMs). Benchmark tests are conducted against analytical results with respect to the concentration distribution along a single filled fracture to demonstrate the numerical reliability. A case study is performed at the Xinchang (Beishan) selected site as the geological disposal repository for high-level radioactive waste. The migration process of U-238 in a fracture-network system close to a water-conducting zone is investigated with varying fracture-filling properties, fracture-filling proportions in mobile and immobile domains, and hydrodynamic properties. A safety classifier is developed using a support vector machine (SVM) based on 500 sets of numerical results for the concentration evolution of nuclides in complex fractures. This is to effectively evaluate the avoidance distance between the repository and the water-conducting zone with respect to factors including the fracture delay coefficient, the porosity of the fracture mobile domain, the hydraulic gradient, and the equivalent hydraulic aperture.