Microstructural analysis and XPS investigation of nodular oxides formed on Zircaloy-4

In order to study nodular oxidation behavior under LOCA conditions, Zircaloy-4 cladding tubes were oxidized in high-temperature steam ambience and the growth evolution and microstructural properties of the oxides formed on the Zircaloy-4 surfaces were then investigated. Optical metallography showed different growth behaviors of uniform black oxide and localized whitish nodular oxide. The changes in composition and chemical states of the oxides formed on the Zircaloy-4 surface were investigated using X-ray photoelectron spectroscopy (XPS). Anion-deficient and non-stoichiometric ZrO was the main species in initial black uniform oxide surface, while stoichiometric ZrO2 was the main oxide species in the whitish nodular oxide. A stoichiometric composition in the nodular oxide resulted in a decrease in plasticity of the oxide layers. Unlike black uniform oxide, XPS spectrum from the nodular oxide showed clear Sn photopeak, which indicates that Sn species were observed in the nodular oxide only. As a result, it is concluded that the decreased plasticity and localized Sn additives may be the causes of nodular oxide initiation under LOCA conditions.