Effect of dissolved oxygen on oxidation and hydrogen pick up behaviour-Zircaloy vs Zr-Nb alloys

Upcoming advanced nuclear power reactors contemplate partial boiling inside the core and it is important to establish the effects of partial boiling on oxidation and hydrogen pick up for sufficiently long periods of time. The effect of partial boiling on oxidation behaviour was assessed in this study in terms of change in the level of dissolved oxygen (DO) in static autoclaves. The results clearly demonstrated that the oxidation behaviour of Zircaloy-2 is quite immune to the level of dissolved oxygen, while Zr–Nb alloys showed very different oxidation behaviour with change in the dissolved oxygen level. The weight gain due to oxidation in Zr–Nb alloys during long term exposure was almost doubled with increase in the DO level. Nevertheless, in case of all the three alloys, the corresponding hydrogen pick up was found to be lower for materials oxidized at higher DO levels irrespective of the effect of DO level changes on weight gain. This is attributed to the preferential reaction of the metal with free oxygen rather than water to produce the oxide when the DO levels are high. Further, the oxidation behaviour of Zr–1 wt%Nb alloy, as a function of dissolved oxygen content in lithiated water, was monitored in situ by electrochemical impedance spectroscopic (EIS) measurements using controlled displacement electrochemistry (CDE) technique in a recirculation loop at reactor operation temperature. The oxidation behaviour (for upto 90 h duration at 310 °C) in high DO content of water was in good correlation with the results obtained from accelerated oxidation experiments during short term exposure (up to 24 h at 400 °C) in static autoclaves.