Chemical evolution of a HLW cell in Callovo-Oxfordian Claystone: taking into account the oxic transient period

Abstract

International audienceNumerous calculations have been performed to represent the long-term chemical evolution of theautochthonous (e.g. argillaceous rock) and allochthonous (e.g. cement based materials, carbon steel)materials that may interact within the French underground radioactive waste repository concept. Theoxic transient related to the operating period (due to the access drift ventilation) in High-Level andlong-lived Waste (HLW) repository facilities is often neglected in the simulation of their long-termchemical evolution. However the initial oxidation of the reduced environment prevailing in theclaystone and the oxic corrosion of the carbon steel in the HLW cell head combined with the anoxiccorrosion of the carbon steel and H2 production in the using part of the cell may lead to a complexoxidizing/reducing front (De Windt et al., 2014). The present study intends to simulate the transitionphase between the periods with oxidative and reductive conditions in order to determine:• how the chemical compositions of the metallic materials, clay and cement in the HLW cell arealtered by atmospheric oxygen and carbon dioxide during the operating period;• and how these alterations affect the long term chemical evolution of the system after closureof disposal cell.The modelling strategy relies on a two steps procedure. Two phases flow simulations were carried outwith Comsol Multiphysics, in order to obtain the temperature and water saturation profiles as afunction of the different operating steps. The chemical evolution of the HLW cell was then simulatedwith the reactive transport code CrunchFlow (Steefel et al., 2014) with fixed water saturation andtemperature profiles derived from the thermal-hydrology simulations. The code flexibility enabled thesimultaneous consideration of the irreversible reaction describing the pyrite oxidation by O2, andsubsequent sulphates release, and the reversible reaction of pyrite dissolution/precipitation underanoxic conditions. The simulation of the oxic period led to pyrite oxidative dissolution together withiron oxi(hydr)oxides and gypsum precipitations (Fig. 1). As a result of pyrite dissolution, the pH valuein the pore water decreased, but is rapidly buffered by carbonate dissolution at the wall of the drift.After the sealing of the disposal cell by addition of 3 meters of bentonite and 4 meters of concrete(2009 version of Andra’s concept), iron canister corrosion consumes the O2, which leads to theestablishment of reducing conditions. Once O2 is depleted, the canister corrosion then produces H2.Magnetite and siderite were simulated as being the main corrosion products. The alteration of the claymineralsunder reducing conditions was characterised primarily by a transformation of pyrite intopyrrhotite. In addition, formation of greenalite was simulated at the interface between the claystoneand the metallic material. Those two predictions are in agreement with results obtained on short termexperiments (Truche et al., 2009; Bourdelle et al., 2014). Simulation results indicated also adestabilization of the illite–smectite and quartz minerals of the claystone (Fig. 2)