Simulation of hydraulic disturbances caused by the underground rock characterization facility in Olkiluoto, Finland

A final repository for spent nuclear fuel is sought in Finland by Posiva (the company responsible for the construction and option of the final nuclear-waste disposal facility in Finland, http://www.posiva.fi). A part of this project involves an underground rock characterization facility (ONKALO), which will consist of an 8,500 m long and 520 m deep system of tunnels, to be potentially extended with the drifts of the repository. The maximum hydraulic disturbance (water table drawdown, deep saline-water upconing and tunnel inflow) that might result from the construction and operation of the ONKALO facility were assessed by site-scale finite-element simulations. The drawdown was calculated by employing the free-surface approach. Tunnel inflows were obtained from the state of equilibrium, while the evolution of the salinity distribution was simulated using a time-dependent and coupled flow and salt transport model. The simulations showed that without engineering measures to limit inflow of groundwater into the ONKALO facility, the hydraulic disturbances would be significantly greater than with these measures implemented. Most of the inflow (330-1,100 L/min) would come from the conductive subhorizontal fracture zones intersected by the drifts. The water table could sink to a depth of about 200 m, and the depressed area could extend over the Olkiluoto Island. Groundwater salinity (expressed as total dissolved solids [TDS]) could locally rise from 22 g/L to over 50 g/L at the tunnel depth of 520 m. With tight grouting, the simulations showed that the depression in the water table can be confined to the immediate vicinity of the ONKALO, the maximum drawdown of the water table remained around 10 m, and the total inflow to the tunnels was about 20 L/min. Moreover, upconing of the saline water remained moderate.