High Temperature Oxidation of Zircaloy-4 in Diluted Steam

Abstract

Isothermal oxidation tests of Zircaloy-4 have been performed in steam/argon mixtures at the temperatures of 1,000–1,490°C to study the oxidation behavior of fuel cladding under severe accident condition in LWRs. The change in mass increase of oxidized specimens was related to the concentration of steam in the atmosphere, f(H₂O) (^mol/₀). Thereby the critical value of f(H₂O) above which the mass increase settles on a constant level was estimated at each test temperature. It was of the order of several percent, increasing with the temperature. This showed that the steam starvation for the Zircaloy-steam reaction took place only in the extremely diluted condition. Under very low steam fraction condition below the critical value, the rate determining step of the reaction was the surface processes which allowed oxygen to reach ZrO₂ surface. High concentration hydrogen of several hundred wt. ppm could be absorbed in Zircaloy during oxidation in the steam/argon mixture of very low steam fraction.

KEYWORDS:

• severe accident
• Zircaloy-4
• steam starvation
• very high temperature
• temperature dependence
• oxidation
• reaction kinetics
• hydrogen