Numerical Prediction of Thermo-Mechanical Spalling Around a Deep Nuclear Waste Repository in Crystalline Rock

ABSTRACT: The focus of this study is on stress-driven rock damage potentially developing around the tunnels and deposition boreholes of a planned underground nuclear waste repository at the Forsmark site in Sweden. This type of damage is commonly referred to as "spalling" or "brittle failure". In hard, crystalline rocks, such as those at the Forsmark site, spalling typically manifests itself in the form of extensional (Mode I) cracking parallel to the excavation boundaries followed by buckling of thin rock flakes. During the construction phase of the repository, mechanical spalling may be induced by overstressing (under unconfined conditions) of the excavation surface periphery. During the operational phase, thermal spalling may develop due to additional thermo-elastic stresses forming in response to the increasing rock mass temperature field induced by the heat-emitting spent nuclear fuel. Since rock failure may negatively affect the long-term isolation properties of the host rock within the multi-barrier disposal concept to be adopted at the Forsmark site, prediction of its occurrence, location, and extent is critical for an effective repository design and long-term safety. In this study, the response of underground structures was studied using Geomechanica's Irazu software, which is novel 3D coupled thermo-mechanical simulator based on the finite-discrete element method (FDEM). It is the first numerical study to date that explicitly captures both mechanical and thermal fracturing processes while using the latest repository design and site-specific geomechanical input data. A sensitivity study is performed to investigate different combinations of rock mechanical properties, in-situ stresses, and deposition tunnel geometry on the host rock behaviour. Rock mass deconfinement is shown to promote the development of tensile damage in the tunnel sidewalls and floor with fracture surfaces growing parallel to the excavation boundaries. The negative effects deriving from the adoption of a relatively narrower tunnel cross-section and from an increase of horizontal in-situ stresses are highlighted. Thermo-mechanical analyses capture the rock mass behaviour following an increase of borehole surface temperature to 100°C. Numerical results indicate that the temperature evolution is affected by the shape of the underground cavities and their distance from the heated boreholes. The coupled thermal expansion of the rock induces additional stresses which, in turn, promotes further damage. Despite this increase, however, the total amount of induced rock damage at final conditions remains relatively low.