Regional-to-site scale groundwater flow in Kivetty

The work describing numerical groundwater flow modelling at the Kivetty site serves as a background report for the safety assessment TILA-99. The site scale can roughly be taken as the scale of detailed borehole investigations, which have probed the bedrock of Kivetty over about 3 km2 large and 1 km deep volume. The site model in this work covers an area of about 16 km2. The depth of the model is 2000 m. The site scale flow modelling produced characteristics of the deep groundwater flow both under the natural conditions and in the case of a spent fuel repository. The hydraulic gradient in the intact rock between the repository and the fracture zone nearest to it (about 50 m off) was assessed for the block scale model. The result quantities were the hydraulic head h (as the base quantity) and its gradient in selected cross sections and fracture zones, the flow rates around the repository, flow paths and discharge areas of the water from the repository. Two repository layouts were discussed. The numerical simulations were performed with the FEFTRA code based on the porous medium concept and the finite element method. The regional model with a no-flow boundary condition at the bottom and on the lateral edges was firstly used to confirm the hydraulic head boundary condition on the lateral edges of an interior site model (having a no-flow boundary condition at the bottom). The groundwater table was used as the hydraulic head boundary condition at the surface of each model. Both the conductivity of the bedrock (modeled with three-dimensional elements) and the transmissivities of the fracture zones (described with two-dimensional elements in the three-dimensional mesh) decreased as a function of the depth. All the results were derived from the site model. With the exception of the western part of Repository A the outlined repositories are located underneath Kumpuvuori, where the flow has a significant subvertical component. The horizontal component of the deep groundwater flow trends south or south-west in the repository area. The range of variation of the hydraulic gradient immediately outside the repository was studied in the direction of the flow routes from the repository. The gradient outside the eastern repository varies between 1.5 % and 4.0 % (average 2.5 %). The corresponding range for the western repository is between 0.7 % and 0.9 % (average 0.8 %). The nodal hydraulic gradient of the two- dimensional elements representing the eastern repository varies between 0.2 % and 2.0 % (average 1.2 %). In the western part the respective range is from 0.2 % to 0.6 % (average 0.4 %). (orig.)