Abstract
Zirconium-based alloys are widely used for fuel claddings particularly in pressurized water reactor. This study investigates the oxidation characteristics of Zr-4 and Zr-Sn-Nb alloy in steam, oxygen and air atmosphere respectively in temperature range of 650–1250 ℃ by a modified thermo-gravimetric analyzer. The oxidation rate exponents and oxidation kinetic correlations are calculated. The rate transitions of the kinetic oxidation for Zr-Sn-Nb and Zr-4 occur both at 1100 ℃ in steam, both 950 ℃ in air, and 1200 ℃ and 1150 ℃ in oxygen, respectively. The oxidation kinetics of both zirconium alloys in air is faster than in steam and oxygen. The maximum thickness of the oxide layers for both zirconium alloys is reached at 1150 ℃ in steam due to the phase transformation of the zirconium from monoclinic to tetragonal. The oxide layer formed in air is porous, heterogeneous with a multi-layer structure of oxide-nitride-oxide because of the formation and re-oxidation of zirconium nitride in oxygen starvation environment. The oxidation resistance of the zirconium alloy is related to the integrity and compactness of the oxide layer. Gas diffusion through the imperfection in the oxide scale may accelerate subsequent oxidation.
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