2011:26 The influence of temperature and fluid pressure on the fracture network evolution around deposition holes of a KBS-3V concept at Forsmark, Sweden

In preparation for the review of SKB’s license application for disposal of spent nuclear fuel, SSM is conducting studies to evaluate the performance of the multi-barrier principle on which the KBS-3 concept is based. Copper canisters containing the spent nuclear fuel are placed into granitic bedrock at about 500 m depth and embedded in clay. Thus, the rock, the clay and the copper canister are acting as barriers in order to retard the possibility of spent fuel to escape the repository and reach the biosphere. During a very long time period (e.g. millions of years), the rock will be subjected to thermal, mechanical and hydraulic changes that can induce failure and propagation of the existing fractures providing new pathways for the spent fuel to escape the repository.

For SSM, the goal of this study is to improve the scientific basis for the evaluation of the performance of the bedrock as a barrier in the KBS-3 repository concept and enhance the knowledge about the thermal-mechanical-hydraulic processes affecting the bedrock. These simulation series focus on the natural and/or induced fractures in the bedrock surrounding the canister and their possibility to initiate or propagate due to the changes of stress and water pressure in the bedrock. These changes will be caused by the construction of the repository for spent nuclear fuel, its thermal phase and, in particular, by a glacial period affecting the bedrock at the repository site.

According to the modeling results, during construction and thermal phase of the repository, no major alterations of the fracture network are expected. However, a large increase of fluid pressure due to a glaciation period has a pronounced impact on the fracture network. In a high magnitude stress field, fractures tend to close and mobilize their frictional resistance. However, in a low magnitude stress field, existing fractures tend to propagate with consequent major changes the fracture network. In conclusion, the understanding of the magnitude of the in-situ stress field at Forsmark appears to be critical for the evolution of the fracture network.