Oxidation behavior of Zr-2.5Nb alloy exposed to steam in the temperature range of 600-1200 °C

Zr-2.5Nb pressure tube material was subjected to isothermal steam exposures in the range 600-1200 degrees C for different durations to predict its oxidation behavior during hypothetical accident scenarios. The oxidation exponent obtained from power law fitting of the thermogravimetry (TG) curves deviated from parabolic regime. Nevertheless, oxidation kinetics was described by determining the parabolic rate constants (K-p) in two temperature ranges: K-p = 0.029 exp(-111075/RT) (kg/m(2))(2)/sec for 600-900 degrees C whereas K-p = 77652.6 exp(-254824/RT) (kg/m(2))(2)/sec for 900-1200 degrees C. In the tested durations, only the 1000 degrees C sample has shown breakaway transition, after approximately 105 min and has absorbed the highest amount of hydrogen amongst all samples. The trend in hydrogen pickup fraction (HPUF) was verified by two complementary hydrogen determination methods. Clear differences were identified in the alpha-Zr(O) layer formed on the samples below and above 900 degrees C. In the alpha + beta range of 600-900 degrees C, alpha-Zr(O) consisted of tiny beta precipitates. At and above 900 degrees C in the fully beta range, columnar alpha-Zr(O) grains were observed.