Modelling of Salt Water Upconing in Olkiluoto

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). During the repository operation the open tunnels and shafts of the ONKALO, and the subsequent repository, are likely to create a hydraulic disturbance to the site's groundwater system for hundreds of years (the greatest impact concerns only the time of the operational period, i.e., c. 100 years). In particular, during the operational phase of the repository upward flow below the tunnels may give rise to the upconing of deep highly saline groundwater up to the planned repository rock volume, which is a concern with regard to the performance of the tunnel backfill material after the closure of the tunnels.

This study concerns the supporting groundwater flow analysis for the Rock Suitability Criteria (RSC) programme, which has been set up to develop host rock requirements for the repository design and layout adaptation. The objective is to assess the potential upconing of deep highly saline groundwater into the planned repository rock volume. The work is divided into three separate sub-tasks: characterization of the expected upconing on the basis of 1) observations in the drillholes, 2) numerical groundwater flow modelling, and 3) the analytic groundwater flow modelling.

Monitoring data of groundwater sampling and electrical conductivity (EC) of the fracture specific groundwater and the drill hole water do not indicate the upconing of saline groundwater at the monitoring sections in the deep drill holes or in the ONKALO, which is congruent with the modelling results.

The numerical modelling shows that locally the salinity may rise to rather high levels, especially below and around the access tunnel and the shafts, which are hold open for the whole operational phase. The saline groundwater rises also below the repository panels, but to a lesser extent, because each of them is hold open only for a short period of time. The upconing in the repository rock volume lasts until the closure of the access tunnel and the shafts. The duration of the recovery lasts hundreds of years. The maximum salinity (occurring at the bottom depth of 470 m) in the site-scale reference volume surrounding the repository ranges between 41−80 g/L, depending on the input parameters applied in the various calculation cases. The average salinity in the reference volume, however, remains practically unchanged or lower than the initial value of 12 g/L. With the decreasing flow and diffusion porosity the upconing increases also below the actual repository panels (during the short period of time that they are open) and becomes increasingly dependent on their location. In the calculation case, in which the input parameters are modified simultaneously (so that the effect of the open tunnels would be more pronounced than in the base case), the maximum and the highest average in the southern panels 6 and 7 range between 60−75 g/L and 25−45 g/L, respectively.

Simplified analytical models have been applied to build confidence on the results of the numerical modelling and to support understanding of the processes that affects the salt water upconing. Similarly to the numerical modelling the analytic calculations, which are based on the pressure balance, indicate that the leakage to the ONKALO/central tunnel panel is likely to result in the upconing of deep more saline water. Leakage to the disposal panels raises the median salinity of water into the repository tunnels, but not the brine that is in much greater depth below the repository.