Oxidation of Zircaloy-4 under simulated accident conditions – kinetics and microstructural developments

Abstract

Zircaloy-4 cladding was exposed to steam under isothermal conditions at temperatures ranging from 600 to 1200 °C in a thermogravimetric (TG) setup. Differential thermogravimetry (DTG) curves were analysed to identify kinetic transition points and the breakaway regime. Cyclic kinetic transitions were observed in all isothermal tests within the range of 600–1000 °C, while breakaway was only seen at 600 °C, 700 °C, and 1000 °C. Electron backscattered diffraction (EBSD) analysis of the oxide formed at 900 °C revealed fragmented monoclinic grains in certain regions. A possible mechanism for this grain refinement in the oxide, along with its implications for cyclic transitions, is discussed. Additionally, various phases were identified in the base metal microstructure, including oxygen-stabilized α-Zr(O), α′ platelets, hydrides, and Fe-Cr-rich intermetallic precipitates. The distribution and fraction of these phases varied with temperature. The differing susceptibility to breakaway at the examined temperatures was correlated with the observed phase and microstructural features. The hydrogen pickup fraction (HPUF) for samples oxidized at 600 and 700 °C, where breakaway occurred, was notably high (>40 %). In contrast, the HPUF was much lower in the pre-transition 600 °C sample and the 900 °C sample, which only exhibited cyclic transitions. Qualitative stress assessment, based on Raman position shifts of monoclinic oxide peaks, indicated the presence of compressive stresses in the oxide before a transition. These stresses were relieved through cracking or grain fragmentation in the oxides that underwent either cyclic transitions or breakaway.