Effect of surface oxidation on the cesium chemisorption behavior of SS 304, Inconel 600 and X-750

Cs chemisorption of structural materials is important for evaluating source terms. Stainless steels and Ni-based alloys utilized in light water reactors, exhibit similar oxidation films during normal operation. However, the presence of minor elements and temperature changes during accidents can alter surface structures and Cs chemisorption characteristics. The oxidation effects of three materials used in modern nuclear power plants were compared. Specimens with different pre-oxide films were prepared from well-polished SS 304, Inconel 600, and X-750, which were then coated with CsOH and treated at 300-1050 ℃. They were soaked in water after analysis to remove CsOH residues. CsOH residues, including Cs₂FeO₄ and Cs₂CrO₄, dominated pre-soaked specimens, with higher concentrations in the oxidized specimens, but disappeared after soaking. Some Cs particles enriched with Ti, Al, and Si were retained in post-soaked specimens. Cs, Si, and O enrichment between outer and inner oxide layers was detected from cross-sectional observations with larger Cs amount on SS 304, while lesser Cs which is also related to Ti located in the thinner outer layer of Inconel 600 and X-750. Cs enrichment on the inner layers is likely to form on the pre-existing layer and then overlap with the outer oxide layer growth. (author)