Analysis of the long-term hydraulic-gas transient in the central zone of a deep clay repository

Abstract

In the Andra repository concept, a large amount of hydrogen gas will be generated through radiolysis of water and corrosion of metal parts from the infrastructure and the packages containing radioactive waste. The fate of the gas could have a significant impact on the rate of resaturation of the porous media, and needs to be accounted for in order to make accurate predictions of the long-term evolution of the repository. The central zone of the repository consists of a 4.5-kmlong network of galleries which connect the storage zones with the surface through three shafts. These shafts, which will be backfilled and sealed after closure, will have a key role in gas and water exchange between the repository and the overlying aquifer. The aim of this work was to describe the evolution in time of both the water and gas flows in the central zone up to the end of the hydraulic-gas transient. The issue was addressed by means of numerical simulations. The simulations produced local predictions of flow rates and pressures, water saturation and amounts of dissolved gas. The results were analysed to determine how water and gas flows combine in the central zone, when the saturation of the seal will be complete, when the free gas will reach the overlying aquifer, and when the saturation of the central zone will be complete.