Tritium transport in non-saturated concrete under temperature fluctuations.

This work studies the effect of the environmental conditions on tritium transport in concrete cells storing radioactive waste. Water with high concentration of tritium was leaking from drains situated in these concrete cells, which indicates flow of water and tritium transport in these cells. 2D numerical models together with temperature and humidity measurements suggested that this leak of tritiated water is caused by a combination of thermo-hydraulic processes occurring in the unsaturated concrete, that comprise capillary rise from the groundwater, evaporation and condensation due to temperature gradients caused by seasonal temperature fluctuations outside. Following this conceptual model, numerical models have been developed to study the tritium transport in the concrete. Model results show that tritium concentration varies according to evaporation and condensation processes inside the concrete cell. An analysis of tritium fluxes revealed that differences in transport mechanisms in liquid and gas can cause concentration gradients of tritium in zones of high water content very different or even opposite to those in zones with low water content. From our numerical calculations, we conclude that there is no release of tritiated water to the groundwater.