Oxidation of zirconium alloys in mixed atmospheres containing nitrogen

Martin Steinbrück, Karlsruhe Institute of Technology, Germany

Zirconium (Zr) alloys are widely used in nuclear and chemical industries because of their low neutron absorption and their excellent mechanical and corrosion properties under operational conditions. One drawback of these materials is their high oxidation potential at elevated temperatures. Oxidation of Zr in steam causes embrittlement and is a considerable source of hydrogen and heat.

The oxidation and reaction behavior of Zr alloys at high temperatures, relevant for nuclear design-basis and beyond design-basis accident scenarios, has been under investigation at Karlsruhe Institute of Technology for many years. Recently, these activities have been intensified again in the frame of the worldwide research on advanced cladding alloys, new LOCA embrittlement criteria and air ingress scenarios.

The most prototypic atmosphere during nuclear accidents is steam; but scenarios with ingress of ambient air or nitrogen, which is used for containment inertization or emergency water pressurizing systems, are under discussion, too.

The oxidation kinetics of Zr in steam is mainly determined by the formation of a more or less protective oxide scale. A parabolic rate law describes this behavior and corresponding correlations are included in most computer codes for simulation of nuclear accidents. Nitrogen strongly affects the oxidation mechanism and kinetics by the formation of zirconium nitride at the metal-oxide phase boundary and its re-oxidation with progressing reaction. Due to the significantly different densities of the involved phases the oxide scale becomes porous and non-protective and the oxidation kinetics switch from parabolic to linear ones. As a result, the degradation of the cladding tubes is strongly accelerated and their barrier effect against the release of fission products is lost much earlier.

This paper summarizes extensive KIT work in this field, compares the reaction of Zr alloys in various atmospheres and briefly discusses the different reaction mechanisms.