Mathematical modeling of large-scale nonuniform corrosion: Coupling of corrosion, transport, and geochemical processes in nuclear waste isolation

Abstract

Understanding of container corrosion is fundamental to long term isolation of nuclear waste. One aspect of corrosion which is difficult to examine experimentally is the formation of large scale corrosion cells. Initiation of the phenomenon in environments where diffusional transport dominates requires very long time periods, generally longer than experimental time frames. A mathematical model is developed to evaluate formation of large scale corrosion cells surrounding steel containers. The model includes transport of nine species by diffusion and electromigration with simultaneous chemical reaction. The model can be applied in partially or fully water saturated environments. The governing equations are first examined in dimensionless format to illustrate some of the important factors controlling cell strength. Numerical solutions to two general classes of problem are examined (a) differential aeration, and (b) spatial variation in passivation current. The focus is on whether predicted changes in solution composition attributable to the cells are large enough to significantly impact further corrosion and/or radionuclide release rate subsequent to localized container breach. The analysis suggests that large scale cells are likely to be an important factor in waste isolation.