Chemical evolution of a HLW cell in Callovo-Oxfordian Claystone: taking into account the oxic transient period
Résumé
Numerous calculations have been performed to represent the long-term chemical evolution of the
autochthonous (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. The
oxic transient related to the operating period (due to the access drift ventilation) in High-Level and
long-lived Waste (HLW) repository facilities is often neglected in the simulation of their long-term
chemical evolution. However the initial oxidation of the reduced environment prevailing in the
claystone and the oxic corrosion of the carbon steel in the HLW cell head combined with the anoxic
corrosion of the carbon steel and H2 production in the using part of the cell may lead to a complex
oxidizing/reducing front (De Windt et al., 2014). The present study intends to simulate the transition
phase 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 are
altered by atmospheric oxygen and carbon dioxide during the operating period;
• and how these alterations affect the long term chemical evolution of the system after closure
of disposal cell.
The modelling strategy relies on a two steps procedure. Two phases flow simulations were carried out
with Comsol Multiphysics, in order to obtain the temperature and water saturation profiles as a
function of the different operating steps. The chemical evolution of the HLW cell was then simulated
with the reactive transport code CrunchFlow (Steefel et al., 2014) with fixed water saturation and
temperature profiles derived from the thermal-hydrology simulations. The code flexibility enabled the
simultaneous consideration of the irreversible reaction describing the pyrite oxidation by O2, and
subsequent sulphates release, and the reversible reaction of pyrite dissolution/precipitation under
anoxic conditions. The simulation of the oxic period led to pyrite oxidative dissolution together with
iron oxi(hydr)oxides and gypsum precipitations (Fig. 1). As a result of pyrite dissolution, the pH value
in 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 the
establishment 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 clayminerals
under reducing conditions was characterised primarily by a transformation of pyrite into
pyrrhotite. In addition, formation of greenalite was simulated at the interface between the claystone
and the metallic material. Those two predictions are in agreement with results obtained on short term
experiments (Truche et al., 2009; Bourdelle et al., 2014). Simulation results indicated also a
destabilization of the illite–smectite and quartz minerals of the claystone (Fig. 2).