Comparison of the High-Temperature Steam Oxidation Kinetics of Advanced Cladding Materials

Isothermal and transient steam oxidation kinetics of the fuel rod cladding materials Duplex, M5™, E110, and Zircaloy-4 (Zry-4) were determined in separate-effect tests at various temperatures between 1073 and 1673K. All materials show parabolic time dependence at all temperatures, at least at the beginning of the oxidation. The temperature dependence of the oxidation rate is Arrhenius-like. All materials investigated show changes in the activation energy of the steam oxidation connected with the tetragonal-monoclinic phase transformation in the oxide. The temperatures of these changes differ between the Zr-Sn (Zry-4, Duplex: 1223 to 1273 K) and the Zr-Nb alloys (M5™, E110: 1273 to 1323 K). At temperatures below this phase transition, parts of the oxide layer can spall after longer oxidation times. It is known as the so-called “breakaway effect.” This effect occurs in Zry-4 and E110, whereas it was not detected in Duplex and M5™. The breakaway effect results in nearly linear oxidation kinetics. The width of the temperature range and the morphology of the spalled oxide parts differ significantly between Zry-4 and E110. For Zry-4, the breakaway effect was found only at temperatures between 1233 and 1313 K. The spalling of the oxide layer at E110 was detected between 1073 and 1313 K. This wide temperature range also affects the transient steam oxidation behavior. For heating rates below 0.1 K/s, a stronger oxidation was found than expected for parabolic oxidation behavior. The oxide parts spalled from E110 specimens are much finer than the particles after breakaway from Zry-4.