Oxidation of Zirconium During a High-Temperature Transient

Abstract

A physical model describing oxidation and oxygen diffusion under non-isothermal conditions is presented. It assumes (a) equilibrium at phase interfaces, as is usual in the treatment of diffusion, and (b) oxygen supersaturation of the β-zirconium to precipitate α-zirconium on cooling. An important consideration, not required in isothermal diffusion, is that the equilibrium oxygen concentrations in the α- and β-phases decrease with temperature. During isothermal oxidation at temperatures above 850°C, the phase distribution is zirconium dioxide (ZrO2/α/β. This model predicts that on slow cooling, oxygen in the β-region diffuses to the α/β interface leading to a depletion of oxygen in this region with a corresponding increase in the thickness of the α-layer. With more rapid cooling, α-zirconium is precipitated in the β-region near the α/β interface. The oxygen concentration of the β-region is determined not by the maximum temperature in the transient but by the lowest temperature at which appreciable diffusion is still possible.

Mechanical Behavior at High Temperatures of Highly Oxygen- or Hydrogen-Enriched α and Prior-β Phases of Zirconium Alloys

“…At HT, a significant fraction of the oxygen reacting at the metal/oxide interface diffuses through the metallic substrate. Once the solubility limit of oxygen in the βZr phase is reached, a metallic layer of oxygen-stabilized αZr phase, called αZr(O), grows from the interface between the oxide layer and the βZr phase substrate [1][2] [3]. There is a strong through-thickness gradient in oxygen concentration, from ~7 wt.% at the oxide/αZr(O) interface down to 2 wt.% at the αZr(O)/βZr interface and not higher than 0.6 wt.% in the βZr phase.…”

Study of oxide and α-Zr(O) growth kinetics from high temperature steam oxidation of Zircaloy-4 cladding

“…advance of the a/b boundary, back diffusion to the boundary and formation of incursions and islands of a-Zr(O) due to the redistribution of oxygen[26]. Experimental evidence in the investigation carried out by Swatzki et al[27] suggested that the a-Zr(O) precipitated during cooling differ from the one formed during isothermal growth in that the former are extensions of the grains in the a layer.Zirconium oxide and a-Zr(O) are inherently brittle and the resistance of the fuel cladding to fragmentation under the influence of thermal shock and post LOCA handling of the fuel bundle depends on the ductile prior b-Zr phase.…”