Zirconium alloy corrosion and hydrogen pickup

During operation nuclear fuel rods are immersed in the primary water, causing waterside corrosion and consequent hydrogen ingress. The mechanisms of corrosion and hydrogen pickup and the role of alloy selection in minimizing both phenomena are considered, based on two principal characteristics: the pre-transition kinetics and the loss of oxide protectiveness at transition. In zirconium alloys very small changes in composition or microstructure can cause significant corrosion differences, so that corrosion performance is strongly alloy dependent. The alloys show different, but reproducible sub-parabolic pre-transition kinetics and transition thicknesses. A mechanism for oxide growth and breakup based on a detailed study of the oxide structure can explain these results. Using the recently developed Coupled Current Charge Compensation model of corrosion kinetics and hydrogen pickup, the sub-parabolic kinetics and the hydrogen fraction can be rationalized: hydrogen pickup increases when electron transport decreases, requiring hydrogen ingress to close the reaction.