On the fate of oxygen in a spent fuel emplacement drift in Opalinus Clay

Persistence of aerobic conditions after backfilling of a spent fuel emplacement drift is of high relevance for the selection of canister material and canister design, as well as for the prediction of the onset of H2 production. This paper presents the results of the O2 monitoring during the backfilling and early heating phases of the Full-scale Emplacement Experiment at the Mont Terri underground laboratory (Switzerland), as well as the first results of laboratory experiments and 3D reactive transport modelling which aim at identifying the processes controlling O2 concentrations. The monitoring shows the disappearance of gaseous O2 and onset of anaerobic corrosion in sections not affected by O2 inflow from the access tunnel within weeks after backfilling, and even before closure of the drift for the deepest parts of the experiment. The laboratory experiments show that O2 can significantly adsorb on granular bentonite exposed to high temperatures and might in principle also adsorb at ambient temperature and relative humidity as low as 55%. Thus, gas sorption should be considered in the balance of O2 in the experiment. These results are consistent with the numerical model indicating that the excavation damaged zone but also the bentonite backfill are the main sinks for O2, whereas corrosion of metallic components seems to play a negligible role. Previous studies estimated the duration of aerobic conditions in an emplacement drift between a few years and several decades; our results show that anaerobic conditions may be reached within a few weeks up to maximum a few months after closure of an emplacement drift for spent fuel.