Comprehensive presentation of extended Zircaloy-4 steam oxidation results (600-1600 deg. C)

Abstract

The results of a detailed analysis of the long-term and high-temperature oxidation behaviour of Zircaloy-4 in steam (600-1600 deg. C) may be summarized as follows: During an introductory phase of ''normal'' oxidation up to about 30 min, an adherent oxide layer was formed. More specifically: a cubic rate law governed the reaction kinetics from 600 to 1000 deg. C; a parabolic rate law applied from 1000 to 1600 deg. C. After an extended exposure for up to 25 hours, three individual types of oxidation behaviour were apparent: from 600 to 800 deg. C and at 1000 deg. C, a transition from cubic or parabolic to a linear oxidation rate resulted from oxide scale cracking, the so-called breakaway effect; from 850 to 950 deg. C, that is in the two-phase (..cap alpha..+..beta..)-Zr region, a change from cubic to parabolic oxidation was caused by a moderate breakaway effect; at and above 1050 deg. C, no change in rate law towards an accelerated oxidation was detected. The oxide scales remained adherent and protective during the whole exposure. At temperatures above 1500 deg. C increased parabolic oxidation rates result from formation of cubic oxide below the tetragonal modification. The Baker-Just correlation may be used to describe the oxidation kinetics in this temperature range. The application of these results to severe core damage analysis is given, when under accident conditions flowing steam is present. In the case of limited steam supply and/or presence of steam/hydrogen mixtures oxidation kinetics can be different and superimposed by effects of hydrogen to the integrity of the cladding.