MODELLING OF RESATURATION, GAS MIGRATION AND THERMAL EFFECTS IN A SF/ILW REPOSITORY IN LOW-PERMEABILITY OVER-CONSOLIDATED CLAY-SHALE

Radioactive wastes conditioned for final disposal in repositories in deep, low permeability formations will produce a significant amount of gas due to corrosion and microbiological degradation processes. This paper addresses the process of gas release from disposal tunnels for spent fuel (SF) and long-lived intermediate level wastes (ILW) in a clay-shale formation. For the SF the impact of additional heat transfer is considered. The issues under consideration are the interference of the repository resaturation process by the build-up of gas pressures in the disposal tunnels and the impact of waste-generated thermal loads on fluid and gas flow. Numerical simulations were conducted, representing the backfilled disposal tunnels (SF and ILW, respectively) and the surrounding host rock formation by a two-dimensional cross-section. The simulations were carried out with the TOUGH2 EOS5 module. The paper gives results of a sensitivity study, aimed at investigating the impact of repository-induced processes (heat, gas generation) and parameter uncertainties on the evolution of porewater pressure in the backfilled disposal tunnels. The numerical simulations are of an indicative nature and did not make use of the final reference values for NAGRA's demonstration project of SF/HLW/ILW disposal feasibility in the Opalinus Clay formation.