High resolution structural analysis of irradiated zirconium alloys

This thesis is part of the MUZIC-3 (Mechanistic Understanding of Zirconium Corrosion) project, with the overall goal to understand the in-reactor corrosion and hydrogen pick-up mechanisms of zirconium alloys. Zirconium alloys are commonly used for fuel cladding and support structures in light water reactors but suffer from aqueous corrosion in service that can limit the operating lifetime and the effective burnup of the fuel. Understanding the mechanisms of corrosion, especially under irradiation, is thus of great importance for the development of accurate, physically based lifetime prediction models for cladding materials, and providing information for the design of new corrosion-resistant alloys.
Acceleration of the corrosion rate has been widely reported as a result of exposure to in-reactor conditions, but the mechanisms of this accelerating effect are still not well understood. It has been correlated with the specific in-reactor coolant chemistry, intense γ-radiation or irradiation-induced redistribution of alloying elements, but rather little attention has been paid to the irradiation-induced degradation of the corrosion protective oxide layers. In an effort to understand such mechanisms, I have used both in-situ radiation damage techniques and direct observations of ex-reactor materials to study radiation effects in zirconium oxides formed on a range of Zr alloys.
The aqueous corrosion of zirconium follows the pathway: Zr→h-ZrO→ZrO2. The transformation from Zr to h-ZrO is found to follow a displacive mechanism, and the suboxide grains can show different morphologies, equiaxed, plate-like or sawtooth-like, depending on the underlying α-Zr grain orientation, although no specific orientation relationships between h-ZrO and m-ZrO2 or α-Zr and m-ZrO2 were identified. The monoclinic-ZrO2 oxides formed in an autoclave are observed to have a similar texture on different alloys, with (10̅4)m-ZrO2 parallel to the metal/oxide interface regardless of the underlying α-Zr or h-ZrO orientations.
I report for the first time on the susceptibility to radiation damage of the suboxide phase which may influence the nucleation of new oxide grains and the transportation of oxidation species across the oxide/metal interface, and lead to enhanced corrosion rates. A monoclinic-to-cubic transformation of the bulk oxide is also observed by in-situ ion irradiation experiments, followed by irradiation-induced grain growth. The possibility of radiation-induced stabilisation of this cubic phase thus needs to be considered as a possible process that can occur at high burnups in reactors, and may further affect the corrosion rates. As a result of the in-reactor corrosion conditions, e.g. irradiation and water chemistries, the oxides formed in-flux are less well textured and with a more disrupted grain, which can contribute to enhanced corrosion rates in-flux.
My results also indicate that oxide nano-porosity plays an important role in the transportation of oxidising species throughout the oxide layer. The density of nano-porosity in the oxide corrosion layers has been successfully quantified for the first time as function of both depth in the oxide and exposure time, and a clear correlation is observed between the measured increase in hydrogen pickup fraction and the characterised increase in interconnected porosity in the oxide.
To complete the work in this thesis, a wide range of microstructural characterization techniques have been applied, like Focused Ion Beam, Transmission Electron Microscopy, Transmission Kikuchi Diffraction and high-resolution Secondary Ion Mass Spectrometry. The combination of these techniques has provided me with a wide range of information on microstructures and microchemistry, which is useful to the understanding of the degradation mechanisms in fuel cladding materials.

Keywords: Transmission electron microscopy
Zirconium oxides
Zirconium alloys
Corrosion
Transmission Kikuchi diffraction
Irradiation damage
Secondary ion mass spectrometry
Advisor: Grovenor, C
Lozano-Perez, S
Wilkinson, A