Alistair Garner

Understanding the in-reactor corrosion behavior of zirconium alloys is essential for optimizing the lifetime of fuel assemblies. Recent advances in available experimental methods have enabled the characterization of oxide morphology, crystallography, and chemical heterogeneity with unprecedented detail for both autoclave and reactor formed oxides. Advanced high-resolution techniques have already improved the understanding of zirconium alloy corrosion performance. However, they are carried out on small volumes of material and require preparation of thin samples, which can lead to changes in the phase distribution in the oxide and often show varied results from different regions of a single bulk specimen. The present study utilizes high-spatial-resolution electron backscatter diffraction (EBSD) performed on bulk samples to produce spatially resolved microtexture data from nanograined zirconium oxide over a large area, which has not previously been possible. This advanced method of plan-view oxide texture analysis, alongside targeted focused ion beam cross-section measurements and substrate EBSD analysis, has revealed well-defined regions of monoclinic oxide grains that exhibit different textures depending on the orientation of the substrate grain on which they have formed. The observed variations in oxide texture have significant implications on any conclusions drawn solely from methods that are limited to the characterization of small areas—especially where sampling areas are smaller than the substrate grain size. Two competing mechanisms of oxide grain growth and nucleation are discussed, and detailed EBSD analysis illustrates a correlation between local oxide texture and corrosion rate. This analysis is performed on specimens of autoclave-tested Zircaloy-2 and ZIRLO and highlights differences in oxide texture development between the two alloys, indicating the significance of material composition and thermomechanical processing on corrosion behavior.

Proton irradiation is often used as a proxy for neutron irradiation but the irradiated layer is typically in-situ loading experiments are carried out using a combination of X-ray diffraction and digital image correlation on the near surface region in order to measure stress and strain, thereby eliminating the influence of the non-irradiated volume. The second approach is to manufacture small-scale tensile specimens containing only the proton irradiated volume but approaching the smallest representative volume of the material. This is achieved by high-speed focused ion beam (FIB) milling though the application of a Xe⁺ Plasma-FIB (PFIB). It is demonstrated that both techniques are capable of recording the early stage of uniaxial flow behaviour of the irradiated material with sufficient accuracy providing a measure of irradiation-induced shift of yield strength, strain hardening and tensile strength.