A novel DFN-DEM approach to simulate long-term behavior of crystalline rock under effects of glacial climate conditions

The safety assessment of nuclear waste repositories is a critical yet challenging task. Several processes acting on different time scales and multiphysical couplings must be taken into account. The complexity of such assessments often requires integrating different software into a single numerical workflow. This study focuses on the long-term behavior of fractured rock masses and the alteration of the hydrogeological system under changing global climate conditions. The investigated impacts include the response of the rock mass due to heating of a generic repository, development of permafrost, and advance and retreat of an ice-shield. To represent THM processes on a timescale of ∼100,000 years in crystalline rock, a novel DFN-DEM modelling approach has been developed. The modelling approach combines two numerical software tools. The thermomechanical calculations are carried out using the distinct element method (DEM) based tool 3DEC [1]. The flow field and the hydraulic heads in the fracture network are calculated with the DFN-tool DFN.Lab [2]. A proprietary script allows to couple and update parameters and variables that are needed to perform THM calculations linking the two different tools. The long-term THM simulations are tested on a generic model of crystalline rock that includes deterministic fault zones as well as a stochastic DFN as the key feature of the hydrogeological system. The modelling approach allows to analyze the behavior of the fracture network including fracture growth and the development of flow pathways within the fracture network as a response to the external THM impacts. The results of the simulations show that the superposition of the different THM processes favors the development of preferential flow pathways along large-scale faults during cold climate conditions.