Geological discrete-fracture network model (version 1) for the Olkiluoto site, Finland

This report describes the methods, analyses, and conclusions of the modelling team in the production of a discrete-fracture network (DFN) model for the Olkiluoto Site in Finland. The geological DFN is a statistical model for stochastically simulating rock fractures and minor faults at a scale ranging from approximately 0.05 m to approximately 500 m; an upper scale limit is not expressly defined, but the DFN model explicitly excludes structures at deformation-zone scales (∼ 500 m) and larger. The DFN model is presented as a series of tables summarizing probability distributions for several parameters necessary for fracture modelling: fracture orientation, fracture size, fracture intensity, and associated spatial constraints. The geological DFN is built from data collected during site characterization (SC) activities at Olkiluoto, which is currently planned to function as a final deep geological repository for spent fuel and nuclear waste from the Finnish nuclear power program. Data used in the DFN analyses include fracture maps from surface outcrops and trenches (as of July 2007), geological and structural data from cored boreholes (as of July 2007), and fracture information collected during the construction of the main tunnels and shafts at the ONKALO laboratory (January 2008). The modelling results suggest that the rock volume at Olkiluoto surrounding the ONKALO tunnel can be separated into three distinct volumes (fracture domains): an upper block, an intermediate block, and a lower block. The three fracture domains are bounded horizontally and vertically by large deformation zones. Fracture properties, such as fracture orientation and relative orientation set intensity, vary between fracture domains. The rock volume at Olkiluoto is dominated by three distinct fracture sets: subhorizontally-dipping fractures striking north-northeast and dipping to the east, a subvertically-dipping fracture set striking roughly north-south, and a subverticallydipping fracture set striking approximately east-west. The subhorizontally-dipping fractures account for most (55% - 60%, depending on fracture domain) of the total observed fracture intensity at Olkiluoto. Fracture intensity shows a correlation with depth below the ground surface, with exponential functions used to describe the change in volumetric fracture intensity (P32) with depth. Fracture intensity is greatest within the intermediate fracture domain and least in the lower fracture domain. Three alternative DFN models are presented, based on different weightings of the importance of individual datasets. In Case 1, the fracture orientation and intensity models are based solely on borehole data; this case is focused on correctly modelling intensity in the upper fracture domain. Case 2 is focused on producing a better match between simulated and observed fracture trace maps at surface outcrops. Finally in Case 3, the size model is modified so as to give greater weight to fracture traces observed in the ONKALO tunnel. Together, these three models provide an initial estimate of variability in geological DFN parameters. (orig.)