Atomic-scale damage induced in zirconium alloys by neutron irradiation, and the effect on in-reactor corrosion

Thesis

This thesis aims to understand the mechanisms of irradiation damage accumulation and inreactor (in-pile) corrosion of cladding Zr alloys used in light water reactors using atomic- to µm-scale characterisation techniques, mainly atom probe tomography. These degradation mechanisms have been carefully studied in a range of valuable Zr alloys in the baseline, autoclave-corroded, irradiated, and in-situ irradiated and corroded conditions provided by collaborating company Westinghouse.

This thesis presents novel results that contribute to a deeper understanding of the degradation processes in Zr cladding materials in service. Direct evidence has been shown that both O and H segregate at and preferentially diffuse along the oxide grain boundaries, likely in the form of hydroxyl groups. During irradiation, Fe, Cu and other impurities are released from segregated sinks (second phase precipitates and grain boundaries), forming clusters and banding patterns in the bulk alloy. In-solution Sn and Nb are depleted to form Sn banding structures that are spatially out of phase with Fe bands, and growing Nb clusters, which I have shown to saturate at 40% Nb. At the same time, the Nb concentration in the cores of pre-existing β-Nb SPPs decreases from 90% to below 40% due to the intermixing of Zr atoms. The dose rate of neutron irradiation also affects the kinetics of these microchemistry changes, and for the first time it is observed that some of the Zr and Nb atoms transmute into Mo isotopes after severe irradiation, and that Li from the water chemistry can penetrate into the underlying metal and become trapped at irradiation-induced voids. During in-reactor corrosion, the oxidation front consumes the irradiated metal substrate. Most of the nanoscale solute segregation identified become randomly distributed once incorporated into the Zr(O) region, whereas Nb clusters can persist longer even after full oxidation, showing a significant delayed oxidation behaviours like the pre-existing Nb-rich second phase precipitates.

These new results offer insights to the industrial partners developing of new alloys, and so contribute to the safe and more cost-effective operation of nuclear reactors. The sample preparation and data analysis methods developed in this thesis offer new approaches in the analysis with laser-mode atom probe tomography of H distributions in oxide materials and cluster analysis on peak-overlapping signals.

Authors: Zhang, W

• DOI: 10.5287/ora-yqzjrpx0e
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: corrosion; zirconium alloys; irradiation; nuclear materials; atom probe tomography