Understanding oxidizing transient conditions in clayey rocks

Abstract

Large amounts of carbon steels are foreseen in most deep geological disposals of high-level radioactive waste. The disposal cells will be initially aerated and the ventilation will renew oxygen at their front edge for years to decades. Excavation damaged zones (Edz) along underground works (tunnels, drifts) may promote oxygen migration. The diffusion and reactivity of oxygen within such zones is still poorly known despite that steel corrosion is significantly enhanced in the presence of oxygen. With this respect, four tests were implemented in the Tournemire Underground Research Laboratory, France. In each test, the oxygen partial pressure and the total pressure within a chamber isolated from the atmosphere by a packer were recorded over 90–150 days. Oxygen was never completely depleted in the chambers, likely due to oxygen diffusion through the Edz, whereas pyrite oxidation within the Edz was the main oxygen sink. The four in situ tests are different in terms of geometries (volume and surface of chambers) and drilling modes (compressed air flushing vs. pressurized argon flushing). A sequential global optimization of the relevant parameters (diffusion and reaction) has been developed, aiming at reproducing as closely as possible the measured four in situ tests data by modelling. The key controlling parameters are shown to be the Edz thickness and the ratio Dekre (effective diffusion coefficient over oxygen consumption first-order effective rate), whose optimum was found between 23 m2 and 45 m2. The Edz extent active for oxygen diffusion ranges between 0.6 and 1.0 borehole radius, with mean water saturation of 0.7, a mean De of 6 × 10−7 m2·s−1 and a mean kre of 2 × 10−8 s−1 (or kr = 6 × 10−7 s−1 per equivalent pore volume). With respect to the safety assessment of radioactive waste disposals, any plug system that would not intercept the Edz could not ultimately prevent oxygen entry into the structure, which could have an impact on dimensioning carbon steel components of disposal concepts. The proposed optimization methodology could be used to quantify diffusion and reaction kinetics in other contexts where pyrite reactivity and/or oxygen diffusion in water unsaturated media play a key role, such as the leaching of mining ores and acid mine drainage.