The effect of nitride formation on the oxidation kinetics of Zry-4 fuel cladding under steam-air atmospheres at 1273–1573 K

Abstract The study deals with the oxidation behavior of fuel cladding under mixed steam-air atmospheres. Oxidation tests of Zry-4 were carried out at temperatures of 1373 and 1573 K. Post-test weight gain measurement along with metallographic examination were conducted to study separately the kinetics of the region where nitrides formed and the nitride-free region. The results of oxidation tests at 1273 and 1473 K published previously by the same authors are also addressed in the discussion. The weight gain coming from the nitride-free region was estimated employing one-dimensional finite difference oxygen diffusion model and measured thicknesses of the metallic part of the oxidized specimen, the columnar oxide and the oxygen stabilized α-Zr(O) as well as the fraction of the columnar oxide at the oxide/metal interface. Consequently, the weight gain related to formation of the porous mixed ZrN–ZrO 2 regions has been assessed. The results suggested that the nitrides formed beneath the protective columnar oxide layer and that a major part of the weight gain related to formation of the porous mixed ZrN–ZrO 2 regions was associated with the formation of ZrO 2 from α-Zr(O).