Characteristics of high temperature oxidation in Zr-based alloys

It was investigated to elucidate the high temperature oxidation behaviors of zircaloy-4 and ZIRLO cladding in the LOCA temperature ranges (700∼1200 .deg. C) using a TGA(Thermogravimetric analysis) method. As the test temperature increased, the weight gain of two claddings increased according to the paraboric rate law. But the discontinuity of KP in zircaloy-4 cladding was attributed to the microstructural changes of oxide at the temperatures of 1000∼1050 .deg. C, where the oxide structures were transformated from monoclinic-ZrO2 to tetragonal-ZrO2. These results were matched to the surface color of tested specimens and the calculation of the paraboric rate constants. And the discontinuity temperature of Kp in ZIRLO cladding was appearanced at 50 .deg. C higher than that in zircaloy-4 cladding. It was thought that ZIRLO oxidation kinetics, comparing zircaloy-4 cladding, were resulted from the stability of tetragonal-ZrO2 phase in the ZIRLO cladding due to Nb addition. Since the paraboric rate constants of ZIRLO cladding were similar to that of zircaloy-4 cladding, ZIRLO cladding would be maintained the mechanical integrity under the LOCA conditions