Chemical evolution of the full glass/steel/cement/claystone system in a HLW disposal cell
Résumé
The Belgium, Dutch and French concepts for the disposal of vitrified high-level waste (HLW) are based on underground disposal facilities located in clay formation. The first two ones rely on supercontainers containing HLW and carbon steel overpack encased in a concrete buffer to keep steel passivation. In the French concept, the annular gap between carbon steel sleeve and host rock is filled with a low-pH/bentonite grout that imposes a corrosion-limiting environment during the operating period but has to be readily neutralized afterwards. The HYTEC code was used to assess of the chemical evolution of the full system including the “vitrified waste – steel overpack – concrete buffer – clay host rock” over 100,000 years. The modelling of the full system showed that the driving force was the chemical destabilization of the cement phases of the concrete buffer by the clay rock. This slowly propagated by diffusion towards the steel overpack with a pH decrease. Therefore, the durability of the cement buffer would strongly affect the duration of the containment properties of the complete multi-barrier system as a whole. As long as the high-pH buffering capacity lasted, steel generalized corrosion was minimized and magnetite was the main corrosion products. The loss of such high-pH conditions led to the speed-up of the corrosion with Fe(II)-silicate formation. The important formation of Fe-silicates corrosion products at the glass/overpack-steel interface sustained glass dissolution.