Simulations of hydrogeological evolution at Olkiluoto

On assignment by its owners, Fortum and TVO, Posiva will take care of the disposal of spent fuel from the nuclear power plants at Loviisa and Olkiluoto. The site (Olkiluoto) for the repository has been chosen on the basis of site investigation programme, which currently is focused on the construction of an underground rock characterisation and research facility (the ONKALO). The hydrogeological conditions at the site may constitute a significant factor with regard to the performance of the disposal facility. Of particular importance are the changes within the glacial cycles, during which major climatic changes are expected to occur. The glacial cycles consists of several periods ranging from temperate to glacial climate conditions, which affect the groundwater flow not only in the soil but also deep in the bedrock. This study comprises of the numerical modelling of the hydrogeological evolution at Olkiluoto over the next glacial cycle. Because of the high uncertainties relating to the prediction of the long-term climate changes (e.g., the future evolution of the atmospheric CO2 concentration is highly uncertain) and the associated climate-driven boundary conditions, the modelling was not striving for a detailed continuous simulation of the whole glacial cycle, but it was focused on assessing the impact of a range of the expected most relevant climatic conditions (temperate, permafrost, ice sheet retreat). The hydrogeological evolution for the three time windows were modelled using climate-related boundary conditions on the top surface, which for the temperate and permafrost time windows were derived from the separately conducted surface hydrological model, while the glacial simulations were based on an ice sheet model. Both the hydrogeological and the surface hydrological models used permafrost data from a separate permafrost evolution model. The Olkiluoto surface hydrological model was used to compute water fluxes in the overburden soils and in the shallow bedrock to provide proper treatment of the hydraulic connection in the geosphere-biosphere interface zone. In this study the main aim of the Olkiluoto surface hydrological model was to compute the hydraulic head boundary condition at level z = 0 m for deep groundwater flow model both in temperate and in permafrost conditions. All the spatial input data needed for the hydrological modeling were provided by the GIS-based UNTAMO toolbox. The future hydrological evolution of the Olkiluoto site from the present day condition to the year 52,000 AD is primarily driven by postglacial crustal uplift and the uncertainty in the crustal uplift model influences the accuracy of the results. Other uncertainties are related to overburden and bedrock hydraulic properties, the network of small streams, depth of small streams, and the climate scenarios used in the computations. During the operational period the open tunnels draw groundwater from all directions in the bedrock (e.g. fresh water from the surface and saline water below the tunnel system). After closure of the repository the hydrological disturbances cease and the flow conditions start to recover gradually back towards the natural state. The recovery lasts hundreds to thousands of years. During the next 50,000 years the continuous fresh water infiltration dilutes groundwater, resulting in a slowly decreasing trend of the average and minimum salinity in the repository rock volume. During the permafrost period the surface hydraulic conductivity decreases several orders of magnitude, resulting in a decrease in the hydraulic gradients and the fresh water infiltration at all depths. The flow rates in the repository rock volume follow the permafrost evolution and reduce significantly compared to the temperate period. Appearance of permafrost interrupts the groundwater dilution started during the temperate period and results in a slow, weakly increasing trend of the salinity values in the repository rock volume. However, essentially the salinity levels during the permafrost period will remain at the same level than those prevailing before the onset of the permafrost. During the ice sheet retreat period the flow conditions evolve with time as the icemargin moves across the site. The presence of the ice sheet and the location of the icemargin affect significantly the flow directions and flow rates in the repository rock volume. The flow rates were approximately three to five-fold compared to the corresponding values at the end of the temperate period and they remain high until the ice-margin approaches and passes the repository area. With a constant retreating rate the ice-margin stays over the site such a short time, that there is no major change in the salinity distribution at the repository rock volume. However, if the ice-margin stays immobile at the site the salinity in the repository rock volume starts to rise slowly after the margin stops. Upconing is strongest when the ice-margin is located above the repository area and the weakest when it has passed the area. The average salinity is at most approximately 15–20 % higher than in the initial state by the end of simulation period of 1000 years.