SSM perspective
Background and objective
High level radioactive waste, such as spent nuclear fuel, will continue to generate heat after the final disposal in an underground repository. The decay heat results in thermal expansion of the repository rock mass which can induce deformation of the fracture system. Near-field effects may involve thermally induced slip of a fracture crossing a deposition hole. Far-field effects could be fault reactivation which can generate a dynamic disturbance in the fracture system by seismic loading. The thermally induced deformation can also affect the permeability of the repository rock mass. Therefore, it’s important to understand how the decay heat influences a fractured rock mass hydraulic and mechanic behaviour. To further explore the potential thermal impact of the rockmass, a research project was performed that was divided into two main parts: laboratory experiments of thermally induced fracture slip and benchmark numerical modelling.
Results and conclusions
Three types of fracture surfaces were considered in the experiment: a smooth, sawcut fracture surface, a semi-rough fracture surface where one side has been deformed by laser-marking, and a rough fracture surface generated by tensile splitting. The experiment indicates that fracture roughness has a substantial impact on the amount of shear displacement and fracture dilation that occurs due to thermal stress increase. Compared to the sawcut and laser-marked fracture surfaces, the tensile-split fracture surfaces experienced the largest amount of normal and shear displacements. The progressive increase in both shear and normal displacement for the tensile-split fractures under thermal loading underscores the critical need to account for thermally induced expansion from radioactive decay. In the context of a deep geological repository, these displacements along rough fracture surfaces can potentially alter aperture distribution and fracture connectivity, thereby influencing the long-term integrity of the rock mass and its hydrogeological behavior.
Numerical modeling was conducted through the Particle Flow Code 3D (PFC3D), where the crystalline rock mass from the thermoshearing experiment was represented as a particle assembly. Two scenarios were evaluated for modeling the rough fracture surface: a mated case, characterized by a high degree of initial surface interlocking where contact is represented by smooth joint segments, and an unmated case. In the latter, a deliberate offset of 2–3 mm was introduced along the fracture plane to reduce interlocking and simulate the effects of prior shear displacement. For the unmated case, parallel bonds between smaller-sized particles along the rough fracture surface were used as a replacement for the smooth joint contacts.
The PFC3D model very effectively captures temporal evolution of temperature throughout the particle assembly during simulated thermal loading. The shear and normal displacement curves also align well with experimental results, validating the PFC3D simulation approach. While cumulative amounts of simulated shear and normal displacement tend to fall below those observed in the thermoshearing experiment, particularly for the unmated fracture case, the initial rapid increase followed by a slower, steady increase in shear displacement is consistent with initial rapid heating during the thermal loading phase. Simulated thermal stress accumulation is accompanied by shear displacement jumps, indicative of high slip velocities. Unlike planar fractures, the rough fracture surface exhibits significantly different behaviour, in that spatial distribution of shear displacement and stress distribution is highly heterogeneous. Particularly in the case of high fracture interlocking, represented by the mated fracture model, there is the potential for large amounts of compressive stress to accumulate before being released in sudden shear slip events. These results emphasize the importance of considering surface roughness when simulating thermo-mechanical processes in a fractured rock medium.
Need for further research
One of the primary safety concerns in a nuclear waste repository is the risk of groundwater infiltration into deposition holes, which can accelerate the degradation of engineered barriers and increase the potential for radionuclide transport. Studies have shown that when fracture dilation occurs due to shear slip, the fracture aperture increases heterogeneously, which can potentially create preferential pathways for fluid migration. This is especially of interest in fractured crystalline bedrock, where permeability is highly dependent on fracture connectivity. This emphasizes the need for more detailed modeling that explicitly incorporates fracture roughness and its effects on permeability evolution.