SYMPOSIA PAPER Published: 01 February 2018
STP159720160069

Investigating the Effect of Zirconium Oxide Microstructure on Corrosion Performance: A Comparison between Neutron, Proton, and Nonirradiated Oxides

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Scanning precession electron diffraction in the transmission electron microscope has been used to simultaneously map the phase, orientation, and grain morphology of oxides formed on Zircaloy-2 after three and six cycles in a boiling water reactor in unprecedented detail. For comparison, a region of a preoxidized autoclave-formed oxide was also proton-irradiated at the Dalton Cumbrian Facility. The proton irradiation was observed to cause additional stabilization of the tetragonal phase that was attributed to the stabilizing effect of irradiation-induced defects in the oxide. In the reactor-formed oxides, no extra stabilization of the tetragonal grains was observed under neutron irradiation, as indicated by the similar tetragonal phase fraction and transformation twin-boundary distributions between the nonirradiated and reactor-formed oxides. It is suggested that the damage rate is too low in the newly formed oxide to cause significant stabilization of the tetragonal phase. This technique also reveals that the oxide formed under reactor conditions has a more heterogeneous microstructure, and the growth of well-oriented columnar monoclinic grains is significantly reduced compared with a nonirradiated oxide. High-angle annular dark-field scanning transmission electron microscopy also revealed the development of extensive networks of intergranular porosity and eventually grain decohesion in the reactor-formed oxides. These results suggest that the tetragonal-monoclinic transformation is not responsible for the accelerated corrosion exhibited under reactor conditions. It is proposed that the usual out-of-reactor oxide growth and nucleation processes are significantly modified under reactor conditions, resulting in a more heterogeneous and randomly oriented oxide microstructure with reduced columnar grain growth. It is suggested that this disordered oxide microstructure allows for the formation of extensive intergranular porosity that could lead to accelerated in-reactor corrosion.

Author Information

Garner, Alistair
University of Manchester, Manchester, GB
Baxter, Felicity
University of Manchester, Manchester, GB
Frankel, Philipp
University of Manchester, Manchester, GB
Topping, Matthew
University of Manchester, Manchester, GB
Harte, Allan
University of Manchester, Manchester, GB
Slater, Thomas
University of Manchester, Manchester, GB
Tejland, Pia
Studsvik Nuclear AB, Nykoping, SE
Romero, Javier
Westinghouse Electric Company, Hopkins, SC, US
Darby, Edward
Rolls Royce, Derby, GB
Cole-Baker, Aidan
Rolls Royce, Derby, GB
Gass, Mhairi
Amec Foster Wheeler, Warrington, GB
Preuss, Michael
University of Manchester, Manchester, GB
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Developed by Committee: B10
Pages: 491–523
DOI: 10.1520/STP159720160069
ISBN-EB: 978-0-8031-7642-3
ISBN-13: 978-0-8031-7641-6