Oxidation of Zirconium Alloys in Steam at 1000 to 1850°C

Oxygen embrittlement of fuel sheathing resulting from high temperature oxidation in steam is a potential fuel failure mechanism during a loss-of-coolant accident (LOCA). In high temperature steam, zirconium alloys form an outer layer of zirconium oxide (ZrO2) and an inner layer of oxygen-stabilized α-Zr immediately below. The metal/steam reaction for Zr-2.5Nb in the temperature range 1000 to 1600°C was studied and compared with earlier results for Zircaloy-2 and Zircaloy-4. Reaction kinetics and the rate of growth of the combined (ZrO2 + α-zirconium) layers were measured. The reaction rates for all three alloys were similar and obeyed parabolic rate laws. The reaction rates were consistently less than those given by the Baker and Just correlation for zirconium oxidation in steam. A discontinuity was observed in the temperature dependence of the rate of growth of the combined (ZrO2 + α-zirconium) layer, and this discontinuity was attributed to a change in the oxide crystal structure. The thickness ratio ZrO2/α-zirconium also varies with temperature. A method for estimating oxidation temperature and time from metallography, based on these observations, is discussed.