Abstract. The geological disposal in deep bedrock repositories is the preferred option
for the management of high-level radioactive waste. In some of these concepts,
carbon steel is considered as potential canister material and bentonites are
planned as backfill material to protect metal waste containers. Therefore, a
1D radial reactive transport model has been developed in order to better
understand the processes occurring during the long-term iron–bentonite
interaction. The conceptual model accounts for diffusion, chemistry of the
porewater and aqueous complexation reactions, mineral
dissolution/precipitation and absorption, at a constant temperature of
25 ∘C under anoxic conditions. The geometry of the axisymmetric model
reflects the canister–bentonite interface and the bentonite. The primary
phases considered are montmorillonitic smectite, quartz, muscovite, albite,
illite, pyrite and calcite. We assume that carbon steel is composed only of
iron. The potential secondary phases considered are from reported experiments,
such as magnetite, nontronitic smectite, greenalite, cronstedtite and
siderite. The numerical model results suggest that at the iron–bentonite
interface, Fe is adsorbed at the smectite surface via ion exchange in the
short term and it is consumed by formation of the secondary phases in the long
term. Furthermore, calcite precipitates are due to cation exchange in the
short term and due to montmorillonitic smectite dissolution in the long
term. The numerical model predicts the precipitation of nontronitic smectite,
magnetite and greenalite as corrosion products. Results further reveal a
significant increase in pH in the long term, whereas dissolution/precipitation
reactions result in limited variations of the porosity. Progressing bentonite
dissolution owing to the rising pH and concomitantly increasing silicate
concentrations in the porewater induce formation of Fe-silicates as corrosion
products at the expense of magnetite. A sensitivity analysis has also been
performed to study the effect of selected parameters, such as corrosion rate,
diffusion coefficient and composition of the porewater, on the corrosion
products. Overall, outcomes suggest that pH and concentration of dissolved Si
play an important role in corrosion mechanisms. The predicted main secondary
phases in the long term are Fe-silicate minerals. Thus, such phases deserve
further attention as possible chemical barriers for radionuclide migration in
the repository near-field.
The Importance of Secondary Phases in Glass Corrosion
