Nb speciation and microstructural characterization of the oxide layer formed on neutron-irradiated Zr-2.5 Nb alloy in high-temperature water

It is crucial to understand the degradation mechanisms of the alloy Zr-2.5 Nb, which make the pressure boundary in Canada Deuterium Uranium reactors. In the present study, Zr-2.5 Nb coupons were exposed for 567 days at 325 °C with and without neutron-irradiation in simulated primary water chemistry . A combination of scanning/transmission electron microscopy with energy dispersive X-ray spectroscopy, and electron energy loss spectroscopy was used to investigate the base metal and oxide . The obtained results suggest that the distribution of nanopores and the variation in Nb oxidation state likely create a gradient in the oxygen concentration and electrochemical potential through the film. • Neutron irradiated Zr-2.5 Nb alloy contains component dislocation loops and Nb-rich IIPs . • The microstructure of the irradiated oxide layer shows sporadically distributed equiaxed-columnar grains. • Nanopore density of the oxide layer plays a role on the thickness of the ZrO and OSM phases. • Nb oxidation state changes from +2 to +4 and then attains +5 oxidation sate near to oxide-water interface.