Reactive transport models of the geochemical interactions at the iron/bentonite interface in laboratory corrosion tests

Abstract

Carbon steel and compacted bentonite have been proposed as candidate materials for the overpack and buffer, respectively, of the multi-barrier system of a geological high-level radioactive waste repository. Carbon steel corrosion may impair bentonite properties. The interactions of corrosion products and bentonite are analyzed with laboratory corrosion tests. Here coupled thermo-hydro-chemical-mechanical (THCM) models of two types of heating and hydration tests performed on compacted bentonite in contact with Fe powder are presented to study the iron-bentonite interactions at representative repository conditions. Tests on small cells (SC) were performed under unsaturated non-isothermal conditions in 25 mm long columns containing 21 mm of bentonite and 4 mm of Fe powder. Tests on medium-size cells (FB) were performed under unsaturated non-isothermal conditions in 99.8 mm long columns containing 86.8 mm of bentonite and 13 mm of Fe powder. Model results for the SC tests showed that magnetite and Fe(OH)₂(s) were the main corrosion products which compete for Fe²⁺ precipitation. Computed corrosion products precipitate mainly in the Fe powder, penetrate a few mm into the bentonite and reproduce the measured iron weight data. Model results of the FB tests showed that magnetite precipitates throughout the Fe powder interface and reproduce the main trends of the corrosion products. Model results of these corrosion tests will be of great relevance for the performance assessment of engineered barriers of radioactive waste repositories.