KWU-results on waterside corrosion of Zircaloy in PWRs and BWRs

The incentive to increase the design burnup of LWR fuel has again initiated research on the waterside corrosion of the Zircaloy cladding. Thereby it has to be considered that the corrosion rate of Zircaloy increases with increasing temperature at the metal/oxide interface, and thus the temperature raise over the growing oxide may have a certain ''feed-back'' effect. The in-reactor corrosion of Zircaloy is significantly influenced by the oxygen content of the coolant. In PWRs, where oxygen is suppressed by hydrogen addition, the corrosion rate was found to be the same as out-of-reactor up to an oxide thickness of 2 to 5 μm. At a higher thickness the corrosion rate is enhanced. In BWRs, under oxiginated conditions, the corrosion is enhanced especially early in time. Furthermore, a localized corrosion phenomenon, the so-called nodular corrosion, is often found after exposures of >100-400 EFPD. Lot to lot - and reactor to reactor-variations were observed in both, PWRs and BWRs. For mechanistic studies, electron microscopic investigations, impedance-, surface area- and porosity- measurements have been performed on oxides from various sources. (author)