The transmissivity of the crystalline rock, which is a major rock type for geological repository of nuclear waste, is dominated by the presence of fracture networks due to the low permeability of the host rock. Reliable characterization and prediction of fluid flow through the fracture network is challenging due to the stress dependent deformation of fractures and uncertainty in the geometrical properties of fracture network among others. The current study constructed the three dimensional fractured rock model based on in situ test carried out at Äspö HRL(Hard Rock Laboratory), Sweden. A threedimensional DEM(Discrete Element Method) is applied capturing the stress-dependent mechanical behavior of fractures. Aperture evolution induced by the stress re-distribution due to the tunnel opening compared well with the transmissivity distribution monitored from the in situ injection tests. Numerical modeling of three interference tests was partially successful with reasonably good match to two of three in situ interference tests. The current model can be a basis for the performance assessment of the geological repository for spent nuclear fuel.
The performance of a geological repository for spent nuclear fuel should be carefully evaluated in various aspects. Transmissivity of the bedrock is one of the most critical parameters which can directly affect the performance of the geological repository. For crystalline rock which is the main candidate rock type of many countries, the existence of fracture networks tends to dominate the transmissivity of the whole rock mass due to the low permeability of the host rock. Since the permeability of the fractures is easily affected by deformations of fractures induced by the normal closure/opening and the shear dilation (Rutqvist and Stephansson, 2003), the hydro-mechanical characteristics of the fractures should be determined to evaluate the stability of the natural barrier.
The ultimate goal of this research is to predict the transmissivity evolution for the performance of the geological repository for spent fuel during its lifetime. This research is a part of Task G in DECOVALEX(DEvelopment of COupled models and their VALidation against Experiments) project, which is the international research and model comparison collaboration for understanding coupled thermo-hydro-mechanical-chemical (THMC) processes in geological systems.