Exploring the processes of nucleation and growth in the oxidation of zirconium alloy fuel rods

The pellets of uranium oxide fissile fuel in pressurised water reactors are contained within tubes of zirconium alloy - the fuel clad. Cladding materials present a particular opportunity for research with rapid impact, because they are amongst the few parts of a nuclear reactor that are replaced during its lifecycle.

Amongst other degradation processes, the zirconium alloy cladding undergoes oxidation in the high-temperature pressurised water in which it sits. A better understanding of the corrosion mechanism would allow for the design of safer, more efficient fuels. The pattern of corrosion involves repeated cycles of initially rapid then slower corrosion, before eventually a rapid breakaway phase, with linear kinetics, takes hold. Recent work at the University of Manchester suggests that oxidation proceeds very differently in the presence of irradiation to without. In particular, the oxide forms much more quickly, with smaller grains with more random orientations.

This modelling project will focus on disentangling the roles of oxide grain nucleation and growth in giving rise to differences in oxide growth rate and texture with and without irradiation. The working hypothesis is that irradiation enables faster nucleation of oxide grains. This gives rise to faster oxide growth leading to a weaker texture, further accelerating oxide growth and so on.

The student will develop a mesoscale model of oxide nucleation and growth on realistic length and time scales. The proposed technique will be a thermodynamically informed phase-field model, which will capture a real-space picture of the evolving oxide. The model will be flexible: it will incorporate the effects of phase transformation, grain and phase boundaries, lattice orientation effects, oxide species transport etc. and allow for the exploration of the interplay of the various different processes involved in oxide growth.

The flexibility of the model will allow the student to explore a wide range of the possible parameter space and so identify the dominant factors in oxide growth, comparing, for example the effects of: accumulation of matrix damage; the dissolution of SPPs and formation of nano-clusters; the importance of grain boundary energies and texture strength.

The chosen scale of the model will allow for direct comparison with experiment. It will also be able to draw on results from atomistic modelling and be directly compared to larger scale continuum models of oxidation. The student will work alongside an experimental PhD student and build on previous work in the Zirconium team at Manchester.