Étude expérimentale des interactions matériau cimento-bentonitique / argilite / fer et acier (MREA/COx/Fe ou acier) à 90°C en conditions de stockage géologique profond des déchets radioactifs (CIGEO)

In the current concept of radioactive waste deep geological disposal planned by Andra (CIGEO project), waste containers are stored into disposal cells drilled into the Callovo-Oxfordian claystone (COx) and encircled by a carbon steel liner. COx and in particular its pyrites, can be oxidized during the drilling of cells. Thanks to an original experimental set-up, involving several pyrite/mineral mixtures and a reactor coupled to a micro gas chromatograph, it is demonstrated, based on the measured O2 consumption that at 100°C in presence of calcite, less than 50% of the pyrite is oxidized. This oxidation induces a transient acidic plume and consequently the corrosion of the steel liner. Andra has developed and patented a low-pH (9<pH<12) cementitious bentonitic grout called MREA to neutralize this transient acidic disturbance and limit the steel corrosion. This material will be injected into the empty space between the steel liner and the COx. To evaluate the neutralization capacity of MREA, and its impact on the COx mineralogy and the reactivity of steel, interaction experiments were done between MREA, COx (oxidized or not), iron or carbon steel, at 90°C in presence of solutions at equilibrium with the initial materials. In all experiments with COx and MREA in equal proportions, with or without metal, interstratified clay minerals are destabilized and formation of Mg-Ca smectite is observed. Fe-hydroxides and Fe-rich silicates are formed but in small amounts. In an acidic medium (pH=5.8 at 25°C), simulated by 1 bar of CO2 added into the experimental system and in presence of the oxidized COx, metal is almost totally corroded and siderite crystallizes. Whereas in a neutral medium (pH=6.6) and in presence of the non-oxidized COx, only 30% of metal is corroded. Impact of a higher alkaline reserve (MREA/COx=8) was also tested by an experiment with carbon steel. The absence of measurable H2 produced into this experiment correspond to a drastic diminution of carbon steel corrosion rate. This study has demonstrated that MREA is competent to neutralize the transient acidic plume generated by the claystone during the exploitation of the disposal, provided that an adequate alkaline reserve, i.e. an enough amount of this material is injected.Dans le concept actuel de stockage de déchets radioactifs en couche géologique profonde tel qu’envisagé par l’Andra (projet CIGEO), les colis de déchets sont placés dans des alvéoles creusées dans l’argilite du Callovo-Oxfordien (COx) dont le chemisage est en acier faiblement allié. Le COx, et en particulier les pyrites qui le composent, peuvent s’oxyder lors du creusement. Grâce à un montage expérimental original, impliquant plusieurs mélanges pyrite/minéral et un réacteur couplé à un micro chromatographe en phase gazeuse, il est démontré, sur la base de la consommation d'O2 mesurée, qu'à 100°C en présence de calcite, moins de 50% de la pyrite est oxydée. Cette oxydation implique une acidité susceptible de corroder le chemisage des alvéoles. L’Andra a donc développé et breveté un coulis cimento-bentonitique bas pH (9<pH