Radionuclide transport during glacial cycles: Comparison of two approaches for representing flow transients

► We model the effect of ice sheet movement and permafrost on geosphere radionuclide transport. ► A novel Monte Carlo time-domain particle tracking scheme is used for the transport simulations. ► We compare transport in steady-state flow with transport in pseudo-transient flow. ► The steady-state method with varying velocity magnitude captures main breakthrough features. ► The pseudo-transient method is needed to capture discharge locations during a glacial cycle. The effect of future, transient ice sheet movement and permafrost development on transport of radionuclides from a proposed repository site is investigated using numerical groundwater flow and radionuclide transport modelling. Two different transport approaches are compared, both utilizing groundwater flow simulations of future climate conditions. The first transport approach uses steady-state particle trajectories representing temperate climate conditions, but modifies the transport velocity along the trajectories according to the changing climate. The second approach is pseudo-transient by performing particle tracking in each individual flow field representing a given time epoch. Two different climate sequences are analyzed. First, a simplified sequence is assessed in order to understand if the two different transport approaches yield significantly different breakthrough characteristics. Second, a sequence representing conditions relevant for real safety assessment applications is considered. Results indicate that the transport approach using fixed trajectories tends to significantly over predict breakthrough during permafrost conditions relative to the pseudo-transient approach. The major difference between the two approaches is related to discharge locations. The fixed trajectory approach yields discharge locations constant in time whereas the pseudo-transient approach is characterized by discharge centres moving in time according to the different climate conditions.