Earthquake-Induced Shear Failure in 3D Fracture Systems with an Application for Long-Term Safety Assessment of Nuclear Waste Repositories

The safety of nuclear waste repositories in crystalline rock has been a crucial issue for many countries which need to geologically isolate radioactive waste in the subsurface. The repositories are usually buried in crystalline rock at ~500 m depth, which inevitably contains complex and intersected fracture networks. Over the lifespan of the repositories, large earthquakes may occur in the vicinity of the repository site and trigger coseismic shear displacements of secondary fractures, endangering the safety of the repository system. A shear displacement of over 50 mm may be able to affect the integrity of waste canisters and thus lead to the leakage of radionuclides into groundwater. Therefore, the impact of potential earthquake-induced shear failure of fractures on the repository’s safety should be cautiously assessed. Based on the detailed site data of a proposed repository in Forsmark, Sweden, we use a distinct element method-based commercial software (3DEC) to build a three-dimensional faulted and fractured rock mass model, in which a primary fault zone together with a secondary fracture network obeying a power-law length scaling are generated. An earthquake is simulated as a rapid rupture along the primary fault plane spreading outwards in a circular pattern from a predetermined hypocenter at a specified rupture speed, with the rupture processes driven by a time-dependent strength weakening law. We explore various plausible scenarios of fracture networks and analyze their responses to a potential post-glacial earthquake with a moment magnitude of Mw = 5.68. We observe that the earthquake-induced shear displacements of fractures show a significant dependence on the depth and diameter of fractures as well as their outcropping condition. Considerable slippages can be accommodated by fractures that outcrop at the ground surface and the slip magnitude may surpass the safety threshold of 5 cm.