Zircaloy-4 cladding corrosion model covering a wide range of PWR experiences

Abstract

A phenomenological corrosion model for Zircaloy-4 cladding was developed by focusing on the effect of the metallurgy of cladding and the water chemistry combined with the thermo-hydraulic conditions. The metallurgical effect was formulated by considering the Sn content in the cladding and the heat treatment of the cladding. Concerning the effect of the water chemistry, it is assumed that lithium and boron have an influence on the corrosion under the condition of subcooled void formation on the cladding surface. The developed corrosion model was implemented in a fuel performance code, COSMOS, and verified using the database obtained for the UO2 and MOX fuel rods irradiated in various PWRs. It was elucidated that the corrosion by lithium was enhanced in the case where the fuel rods were irradiated with a high linear power so that a significant subcooled void could be formed on the cladding surface. On the other hand, there was no evidence of the lithium effect even though its concentration was high enough if the void in the coolant was negligible. This result shows that the acceleration of corrosion by an increased lithium concentration occurs only when subcooled voids are formed on the cladding surface. In addition, the comparison between the measurement and the prediction for the MOX fuel rods indicates that no distinguishable difference is found in the corrosion behavior between the MOX and the UO2 fuels as expected.