An evaluation model of the impact of hydrogen ‘piston effect’ on water displacement in a deep geological disposal of radioactive waste

Abstract

One of the major issues in the safety of a deep geological repository (DGR) for radioactive waste is the migration of hydrogen (H2) resulting mainly from anaerobic metal corrosion and from water and waste radiolysis. The migration of hydrogen may have adverse effects on the mechanics of the host rock and of the engineered barrier systems within a DGR. Some studies pointed out involvement of hydrogen “piston effect” in the transport of radionuclides. In this mechanism, the gas phase displaces the contaminated water, along backfilled but still relatively permeable drifts, towards the main shafts. The precise quantification of such a displacement is still a challenging task because it involves simulations at the scale of a DGR and only few studies were dedicated to this subject. In this work an attempt has been made to verify, through a simplified DGR model, if the usual parametrizations could give rise to a substantial and continuous water movement during the first 100,000 years after the closure of a DGR, when a significant H2-flux reaches its main drift. The assessment of the piston effect importance, related to the H2 production and release, is evaluated in terms of cumulative liquid-phase travel distance (CLTD) within the main drift and its persistence in time. Scenarios simulated by our model show that, in the investigated conditions, piston effect is not negligible (i.e., CLTD-values are greater than 100 m) and that water would be displaced towards the main shafts. However, this work is focused on the mechnisms involved, and the results obtained cannot therefore be genralized to any disposal concept. Furthermore, additional studies are necessary for improving this model by analyzing uncertainty propagation in its parameters, and by considering, e.g., gas-entry pressure and hysteresis phenomena usually neglected in the simulation models.