This paper presents a study on the transmissivity change of pre-existing fractures due to fracture shearing in a deep geological repository of spent nuclear fuel. The heat generation from the spent nuclear fuel, glacial loading and earthquake are three scenarios considered in this study based on the geological settings at Forsmark, which is the candidate site for a repository of spent nuclear fuel in Sweden. Two-dimensional near-field models are used based on discrete fracture network (DFN) of the size 40 m × 40 m in which one to five deposition holes are modeled by means of vertical and horizontal sections. The Discrete Element Method code UDEC is applied for the numerical modeling of thermo-mechanical and dynamic behavior of sparsely fractured rock mass. This study demonstrates that fracture transmissivity change due to the fracture shearing during the lifespan of a geological repository is irreversible and important for the long-term integrity of the repository.
Permanent change of fracture transmissivity in the repository can affect the long-term barrier functions of the repository by affecting buffer and backfill resaturation time and radionuclide transport. This paper presents a study on the transmissivity change of pre-existing fractures due to fracture shearing in a deep geological repository of spent nuclear fuel. The heat generation from the spent nuclear fuel, glacial loading and earthquake are three scenarios considered in this study (Figure 1) based on the geological settings at Forsmark, Central Sweden, which is the candidate site for a repository of spent nuclear fuel. The Discrete Element Method (DEM) code UDEC (Itasca 2014) is used for the numerical modeling of thermo-mechanical and dynamic behavior of sparsely fractured rock mass. The current study explicitly considers numerous fractures represented by Discrete Fracture Network (DFN) for coupled thermo-mechanical analysis, which can give a realistic estimation of thermal shearing of rock fractures. This study focuses on key mechanisms such as transmissivity increase due to shearing and irreversible nature of shearing displacements due to the elasto-plastic behavior of rock fractures. For fractures under high normal stress, a residual increase of transmissivity due to dilation is anticipated contrary to the usual expectation of complete transmissivity recovery after unloading.
