Simulation of hydraulic disturbances caused by the decay heat of the repository in Olkiluoto

The hydraulic disturbances caused by the decay heat of the repository was studied by a transient simulation of groundwater flow, solute transport and heat transfer. The three phenomena were coupled due to the dependence of density of water on salinity and temperature, and the dependence of viscosity of water on temperature. The construction and operation of ONKALO and repository was not taken into account in the simulations, but the whole tunnel system to a depth of 520 metres was made hydraulically active at the beginning of the simulation and was assumed to be open for 100 years. The simulations continued for 2000 years after present.

The results show that the decay heat of spent nuclear fuel rises the temperature of the repository and the surrounding bedrock several tens of degrees for many centuries. During the first 100 years the open ONKALO tunnels dominate the flow conditions in the vicinity of the repository and draw groundwater from all directions in the bedrock. After the closure of the tunnels the temperature rise induces an increasing upward groundwater flow, which for hundreds of years delays the recovery of the flow conditions towards the natural state and rises the salinity of groundwater in the vicinity of the repository.