Gas-driven radionuclide transport in undisturbed and disturbed Boom Clay

In deep geological repositories for the disposal of radioactive waste, gas can be generated by different mechanisms including anaerobic corrosion, radiolysis and microbial degradation. If the gas generation rate is larger than the capacity for the diffusive transport of the dissolved gas, a free gas phase will be formed, eventually leading to gas-breakthrough events. Depending on the timing of gas breakthrough, dissolved radionuclides and contaminants could be driven out of the clay faster than the normally expected diffusive transport. A column experiment was designed in which a water-saturated clay core is placed directly on top of a thin BoomClay core that has been previously saturated with a tracer solution (0.01mol l21NaI). A helium gas pressure is applied and stepwise increased. Upon gas breakthrough, the water on top of the column is expelled and analysed for its iodide content. The measured concentration iodide is linked to the amount of NaI-saturated pore water that was displaced. It can be concluded that the transport of radionuclides and contaminants because of a gas-breakthrough event is possible but appears to be very limited. The volume of water displaced is very low (three orders of magnitude) compared to the volume of gas transported upon breakthrough.