Full-scale 3D modelling of a nuclear waste repository in the Callovo-Oxfordian clay. Part 2: thermo-hydraulic two-phase transport of water, hydrogen, 14 C and 129 I

Abstract

AbstractThe development of deep geological repositories for nuclear waste requires a sound system understanding, to which performance analyses by means of numerical flow and transport simulations form an important contribution. In this context, we present modelling studies of the thermo-hydraulic two-phase transport of water and hydrogen in the planned French repository in part 1. This paper, part 2, uses the same modelling concept extended to include radionuclide transport. The numerical TOUGH2-MP-model encompasses the host rock from cap to bed rock including the repository in full 3D. We performed a series of single-phase and two-phase transport simulations of volatile radionuclides (14C) and highly soluble ones (129I). The sensitivity analyses concerned different conditions and scenarios, for example, early canister failure, varied hydrogen generation regimes and transport parameters. The results allow determination of the dominant radionuclide transport paths and transfer processes. They demonstrate that hydrogen generation inside the repository has a large impact on the transfer of 14C, favouring its advective transfer in the gas phase along the backfilled tunnels, whereas it has only minor influence on the transfer of 129I which is mainly transported by diffusion in the formation water of the host rock. The study demonstrates the feasibility of full-scale radionuclide transport simulations.