Workreport 2009-78

Modelling of Groundwater Flow and Solute Transport in Olkiluoto - Update 2008

Löfman, J., Mészáros, F., Keto, V., Pitkänen, P. & Ahokas, H.

English

274

Posiva Oy is preparing for the final disposal of spent nuclear fuel in the crystalline bedrock in Finland. Olkiluoto in Eurajoki has been selected as the primary site for the repository, subject to further detailed characterisation which is currently focused on the construction of an underground rock characterisation and research facility (the ONKALO). An essential part of the site investigation programme is analysis of the deep groundwater flow by means of numerical flow modelling. This study is the latest update concerning the site-scale flow modelling and is based on all the hydrogeological data gathered from field investigations by the end of 2007. The work is divided into two separate modelling tasks: 1) characterization of the baseline groundwater flow conditions before excavation of the ONKALO, and 2) a prediction/outcome (P/O) study of the potential hydrogeological disturbances due to the ONKALO.

The flow model was calibrated by using all the available data that was appropriate for the applied, deterministic, equivalent porous medium (EPM) / dual-porosity (DP) approach. In the baseline modelling, calibration of the flow model focused on improving the agreement between the calculated results and the undisturbed observations. The calibration resulted in a satisfactory agreement with the measured pumping test responses, a very good overall agreement with the observed pressures in the deep drill holes and a fairly good agreement with the observed salinity. Some discrepancies still remained in a few single drill hole sections, because the fresh water infiltration in the model tends to dilute the groundwater too much at shallow depths.

In the P/O calculations the flow model was further calibrated by using the monitoring data on the ONKALO disturbances. Having significantly more information on the inflows to the tunnel (compared with the previous study) allowed better calibration of the model, which allowed it to capture very well the observed inflow, the groundwater level and the hydraulic heads in the deep drill holes. However, due to a lack of representative salinity samples in the monitoring system and, in particular, a lack of a time series for the observations, it was difficult to judge the validity of the salinity model when tunnels were present.

The current study also took the first steps to incorporate more heterogeneity (with respect to the transmissivity of the zones and the conductivity of sparsely fractured rock) in the flow model. In addition, a preliminary analysis was conducted to evaluate uncertainties and sensitivities by means of the Ensemble Kalman filter.

groundwater flow, salt transport, numerical modelling, spent nuclear fuel, Olkiluoto

Modelling of Groundwater Flow and Solute Transport in Olkiluoto - Update 2008 (pdf) (26.6 MB)