Oxidation behavior of Zr-alloy cladding candidates for the TrEat reactor

The TREAT facility, located at the Idaho National Laboratory, is an air-cooled test facility designed to simulate rapid reactor transient conditions up to 600 °C. Zirconium and its alloys are known to undergo a transition from parabolic to linear oxidation kinetics, termed “breakaway.” After breakaway, the oxide formed is porous and non-protective which significantly decreases its mechanical stability as a cladding material. In this work, the oxidation behavior of Zircaloy-3, Zircaloy-4, and Zr-1Nb is evaluated in isothermal and rapid transient conditions (air at ≤ 820 °C). Separate effects studies were done on plate samples in air (80% N₂+O₂) and oxygen (80% Ar+O₂) to characterize the effect of nitrogen on the oxidation behavior during isothermal oxidation using thermogravimetric analysis. It was observed that the three alloys exhibit decreased resistance to breakaway in air. In addition, the effects of chamfering and welding on the oxidation behavior were investigated during isothermal and rapid transient oxidation. All alloys were affected by welding, however only the Fe/Sn containing alloys were affected by plastic deformation. Advanced characterization techniques (scanning Kelvin probe force microscopy and Raman spectroscopy) were performed on cross sections of oxidized specimens to further investigate the effects of oxide phase and composition on breakaway.