The Effect of Irradiation on the Corrosion of Zirconium LWR Cladding

Abstract

The research presented in this thesis was directly funded by Rolls Royce as part of the MUZIC- 3 consortium, which is a project involving a number of universities and industrial sponsors. The overall goal of MUZIC-3 is to understand the corrosion mechanisms of zirconium alloys specifically in the presence of irradiation. The research carried out as part of this project aimed to give an insight into the corrosion behaviour both in autoclave testing and in-service through the use of a number of state of the art techniques that allowed for analysis of the nanoscale microstructure of zirconium oxide corrosion films. The effect of focused ion beam (FIB) sample preparation was initially investigated in order to understand the potential for introduction of FIB-induced microstructural artefacts in the FIB foils typically used for characterisation of zirconium oxide films. An oxidised Zircaloy-2 specimen was exposed to both Ga+ and Xe+ ions. At energies of 5 keV and above extensive phase transformation was observed from the monoclinic to tetragonal phase however at energies below this no effect was observed. This study not only gave emphasis to the importance of low energy cleaning steps during FIB foil preparation but it also gave an insight into the potential stabilisation mechanisms for the tetragonal phase during oxidation. Electron backscatter diffraction in the scanning electron microscope has been used for the first time to reliably map the orientation of zirconium oxide monoclinic grains. The oxide was observed to form in macrozones that are shown to have a direct orientation relationship to the metal grains they grow from. Results show that depending on the orientation of the substrate grain, oxides can either form by lattice matching, where they show increased disorder and faster corrosion or by a stress dominated scenario where a much more well orientated, protective microstructure forms. This clearly shows that the metal orientation has a huge effect on the oxide nucleation and subsequent growth of the oxide grains and also highlights the importance for increased care when using high-resolution techniques to compare oxide microstructures from different materials. Scanning precession electron diffraction (SPED) in the transmission electron microscope (TEM) was used to map the phase, orientation and grain morphology of Zircaloy-2 after 3 and 6 cycles in a BWR in fine detail. For comparison a pre-oxidised Zircaloy-2 specimen was proton-irradiated ex-situ. Stabilisation of the tetragonal phase was observed in the protonirradiated case however for both reactor-formed oxides no extra stabalisation was observed. This was attributed to the lower damage rate in the newly formed oxide suggesting that the tetragonal to monoclinic phase transformation is not responsible for the accelerated corrosion observed in reactor. It was also observed that the reactor-formed oxide had a larger spread of orientations present in the region studied resulting in an increased number of high-energy grain boundaries, which also correlated to increased intergranular porosity and a disordered microstructure. All findings point to irradiation inducing changes in the nucleation and growth processes typically observed in autoclave grown oxides. The suitability of using in-situ proton irradiation to mimic the effects of neutron irradiation was assessed with the aim of obtaining an increased mechanistic understanding of the corrosion mechanism in reactor. An experiment conducted at Michigan ion beam laboratory allowed for a Zircaloy-4 specimen to undergo simultaneous proton irradiation and aqueous corrosion. SPED in the TEM was used to investigate the microstructure and compare it to an oxide formed after 5 cycles in reactor. Although the corrosion rate in the proton-irradiated case was approximately 10 times faster, and more similar to the rates typically observed in-service post transition, a completely equiaxed microstructure was observed that exhibited sim

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philipp Frankel (Supervisor) & Michael Preuss (Supervisor)