Oxidation mechanisms in zirconium alloys for nuclear applications

Thesis

The oxidation of zirconium alloys under aqueous conditions has been studied for more than 50 years in relation to its role as a fuel cladding element in water-cooled nuclear reactors.

The most well-known and interesting phenomenon associated with the corrosion of zirconium alloys is the cyclic oxidation rate. Oxidation initially proceeds rapidly, forming an oxide film. As this film thickens, the rate of oxidation slows before transitioning, and begins oxidising at the higher rate again. This effect has been well studied, although the cause of rate transition is not understood.

Using samples oxidised in conditions very closely matching those of a nuclear reactor, although without radiation, several techniques were used to characterise the oxide before and after the transition in oxidation rate. Comparison of these results allows some insight into the mechanism operating.

Doping samples with isotopic tracers during different points in the oxidation cycle and subsequent analysis with NanoSIMS gave insight into the porosity and protectiveness of the oxide film before and after transition. Before transition, the entire oxide film acts as a barrier, rather than a small sublayer. After transition, the whole layer of previously formed oxide became porous and the oxidising media was admitted directly to the metal/oxide interface. Examining samples of differing oxidation times showed each subsequent transition was a close repeat of the first. Oxide thickness, rather than time under corrosion conditions, was identified as the best predictor of whether a sample would undergo transition. Oxide porosity (and thus rate transition) was demonstrated to be a local process on scales as small < 1μm.

Residual stress in the oxide and underlying metal substrate have been studied for zirconium alloys for some years. Here, a new technique is applied that largely confirms previous measurements made by XRD. Testing pre- and post-transition oxides revealed a large drop in the compressive stress after transition, showing the post-transition oxide is both porous and contains much lower compressive stresses.

Authors: Yardley, S

• DOI: 10.5287/ora-zrkxwq4gx
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford