The damped oscillator model for the post-transition oxidation kinetics of Zircaloy-4 fuel cladding

The oxidation kinetics of the zirconium alloys are governed by the coupled reaction-tensile stress evolution. Although the kinetic curve of oxidation of zirconium alloys in the post transition region can be described, more exactly in terms of cyclicity that those of oscillations, by subtracting an equilibrium line equation from the thermogravimetric curve the general kinetic equation obtained is that of a damped oscillator. Isothermal air -oxidation test was carried out at 550, 600, 650 and 700 deg. C and the kinetic equation was fitted to the experimental values. At each temperature the equilibrium line equation, the damping factor, and the frequency were obtained. The damping factor Γ, exponentially increases with the temperature. The frequency change with the oxide growth is discussed. At limits if the frequency tends to zero, the near- linear kinetic equation is obtained. (authors)