The influence of the air fraction in steam on the growth of the columnar oxide and the adjacent α-Zr(O) layer on Zry-4 fuel cladding at 1273 and 1473 K

The growth kinetics of the columnar oxide and α-Zr(O) layers on Zry-4 under mixed steam-air conditions at temperatures of 1273 and 1473 K were investigated in this study by means of post-test metallographic measurements. The hydrogen uptake was also determined by the hot extraction method. The kinetics of the columnar oxide layer obeyed a parabolic law for all air fractions at both temperatures. The parabolic oxidation rate constant of the columnar oxide increased with increasing air fraction, whereas the parabolic oxidation rate constant of α-Zr(O) layer seemed to be independent of the air fraction. At 1273 K, the kinetics was assessed until the separation of the columnar oxide from α-Zr(O). The columnar oxide lost its protectiveness after the separation. As a consequence of the separation, nitrides along with the porous oxide formed underneath the columnar oxide in higher extent. Nitrogen in the atmosphere was found to influence the origin of the separation and the formation of nitrides and the porous oxide considerably. Mixed steam-air conditions enhanced hydrogen absorption substantially.