Understanding in-reactor formed oxides on cladding materials by X-ray diffraction analysis

Zirconium alloys are used for fuel rod cladding tubes and fuel assembly structural components in civil nuclear reactors. Oxidation and hydrogen pickup of the zirconium alloy cladding during operation have become limiting factors for the economical and safe operation of nuclear fuel. For this reason it is critical for the industry to understand the fundamental mechanisms involved, aiming to increase the operating margins to offset the ever increasing demands on fuel duty and aggressive in-reactor exposure conditions (temperature, burnup, residence time, cycle length, coolant chemistry, etc.).
This PhD project will investigate key factors affecting the corrosion performance of Zr alloys in-reactor. This will include:
The impact of alloy chemistry on in-reactor performance
The influence of hydrogen content on corrosion

A carefully selected set of irradiated samples from two alloys with different response to long-term in-reactor operation have been made available. These samples were fabricated from the same set of materials, exposed to the same irradiation conditions in the Vogtle PWR (Pressurized Water Reactor). This project will use X-ray diffraction to study the evolution of oxide phase fraction, texture and residual stresses in the different materials and relate these to the corrosion performance of each condition. Extensive XRD studies on autoclave tested samples have shown the value of this technique, but there are very few systematic studies on reactor formed oxides, such as the ones available here. The project will complement another PhD project which will focus on the same materials using high resolution techniques (TEM, TKD, APT). While these techniques provide unique insight into the local structures in each oxides, they require the extraction of very small specimen and are not able to provide good statistics about the overall oxide structure. The detailed analysis of the same samples by XRD ( both lab XRD and Synchrotron XRD) will enable the evaluation of any sample preparation artefacts in the microscopy studies and provide non-destructive analysis of the bulk oxides. Together, the two projects will significantly increase our understanding of the corrosion and hydrogen pickup mechanisms, thereby enabling the development of improved modelling tools and so the confidence to predict ways to achieve improved utilization of expensive fuel while still increasing the margins to the applicable design limits.
The PhD project will be linked to the EPSRC Programme grant MIDAS (Mechanistic understanding of Irradiation Damage in fuel Assemblies; EP/S01702X/1). The project is planned to be performed in the Manchester Materials Department at the Royce Institute, supervised by their team of experts on zirconium based materials in combination with industrial supervision by Westinghouse's key specialists.