Effects of γ-radiation on the interfacial chemistry of spheroidal graphite cast iron in contact with Wyoming bentonite slurry

Abstract

In most concepts for the disposal of high-level radioactive waste, cast-iron canisters containing spent fuels are encapsulated by bentonite that hydrates on contact with inflowing geological waters. This triggers the corrosion of cast iron, which can be turned worse under the exposition to significative levels of γ-radiation in unforeseeable scenarios. Such effects were examined by contrasting the chemistry of the corroded surface of spheroidal graphite cast iron in contact with Wyoming bentonite, fully hydrated with natural aerated Opalinus-Clay water, after 220 days of γ-ray exposition with a dose rate of 130 Gy h^-1 at 50 °C and that observed after a similar experiment without radiation. SEM-EDX, XPS, XRD, and ICP-OES were applied. The growth of patches of calcium hydroxide and ferrous hydroxide crystals as well the presence of iron silicate layers adhered to the cast iron surface as predominant corrosion products are a fingerprint for a strong interfacial alkalization and the creation of a rather reductive milieu by radiation. These chemical transformations reveal an enhanced cathodic electrocatalytic activity of graphite nodules for water reduction under radiation which increases the anodic dissolution of the iron matrix and triggers the formation of a silicate diffusion barrier by dissolution-precipitation reactions. The radiation-induced cathodic activity may constitute an additional, more critical factor than the generation of oxygen for the container integrity, even after having reached the anoxic phase.