Project overview
Although ZrNb and Zircaloy-4 alloys have comparable in autoclave corrosion rate, it is well known that the Nb containing alloys do not show the sudden increase in oxidation kinetics at relatively high burnup that is observed in Zircaloys [1]. The main hypothesis for the absence of accelerated in-reactor corrosion kinetics of ZrNb alloys in the PI research program is that the precipitation of irradiation induced platelets/nanoclusters, and the associated change in Nb solute concentration is responsible for the low corrosion rates experienced by ZrNb alloys, resulting in lower corrosion rate at high burnup [2]. This project specifically aims at study the coupling effect of irradiation and corrosion for ZrNb metal substrate, by targeting the characterization of irradiation induced Nb redistribution before and after autoclave corrosion.
corrosion experiments of unirradiated and irradiated Zr-xNb samples in autoclave environment, (6)
characterization of corroded Zr-xNb samples with or without irradiation effect.
The overall project plan is shown in Figure 1, which involves the proton irradiation on the ZrNb metal substrate and the state-of-art characterization techniques using both scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDS) and atomic probe tomography (APT) to understand irradiation effect on the Nb distribution. STEM and EDS were used to find irradiation induced Nb-rich platelets and the compositional variations in precipitates before and after irradiation, as well as to investigate the stability of irradiation induced platelets during oxidation process. Due to low Nb concentration (less than 0.6 at.%) in the solid solution, which EDS could not quantify accurately, APT gives more quantitatively confident results. It has been shown by our APT results in [2] that irradiation indeed has induced a reduction of Nb content in the solid solution, which has partially supported our hypothesis. In the end, the irradiated samples will undergo corrosion experiment in autoclave environment to investigate (i) if irradiated samples have smaller oxide than unirradiated samples, and (ii) the stability of irradiation induced platelets in the oxide.
References:
[1] Motta, Arthur T., et al. “Corrosion of Zirconium Alloys Used for Nuclear Fuel Cladding.” Annual Review of Materials Research, vol. 45, no. 1, 2015, pp. 311–343.
[2] Yu, et al. “Irradiation-Induced Nb Redistribution of ZrNb Alloy: An APT Study.” Journal of Nuclear Materials, vol. 516, 2019, pp. 100–110.