Photoelectrochemical Investigation of Radiation-Enhanced Shadow Corrosion Phenomenon

During operation, nuclear fuel rods are immersed in the primary water, causing waterside corrosion and consequent hydrogen ingress. In this review, the mechanisms of corrosion and hydrogen pickup and the role of alloy selection in minimizing both phenomena are considered on the basis of two principal characteristics: the pretransition 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, subparabolic pretransition kinetics and transition thicknesses. A mechanism for oxide growth and breakup based on a detailed study of the oxide structure can explain these results. Through the use of the recently developed coupled current charge compensation model of corrosion kinetics and hydrogen pickup, the subparabolic kinetics and the hydrogen fraction can be rationalized: Hydrogen pickup increases when electron transport decreases, requiring hydrogen ingress to close the reaction.

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Shadow Corrosion-Induced Bow of Zircaloy-2 Channels

Mahmood¹,

Cantonwine

Lin

et al. 2012

Zirconium in the Nuclear Industry: 16th International Symposium

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For the past 10 years, the frequency of channel-control blade interference has been greater than before because of shadow corrosion-induced bow. Shadow corrosion-induced bow is caused by differential hydrogen concentration on channel sides adjacent to and away from the control blade. The differential hydrogen concentration is a result of increased shadow corrosion on the channel side adjacent to the control blade, hence the description of this mechanism as shadow corrosion-induced bow. Channel bow measurements and post-irradiation examinations (PIEs) confirmed that shadow corrosion-induced bow occurs independent of fluence gradient-induced bow. Specifically, bow of channels that experienced early-life control (i.e., shadow corrosion) was significantly greater than expected assuming only fluence gradient-induced bow. In addition, it was found that plants with smaller channel-to-channel gaps had greater channel bow, consistent with the gap dependence of shadow corrosion. PIE revealed that the elevated hydrogen concentrations in the metal were found only on those channel sides and elevations that experienced shadow corrosion. A model was developed that related this measured differential hydrogen to the observed channel bow that supports the increased volume of hydrides as the underlying cause. However, questions about the time/exposure dependence of shadow corrosion-induced bow remain. The PIE of shadow corrosion-induced bow at low exposure (one cycle) indicates that little shadow corrosion-induced bow occurs after the first cycle even though shadow corrosion is observed on the side adjacent to the control blade. The differentials in oxide thickness and hydrogen concentration between opposite channel sides are considerably less than those measured later in service lifetime (i.e., after additional cycles of operation). The data indicate that the larger differential hydrogen at high exposure/residence time is a result of a larger difference in oxide thickness and a corresponding increase in percent hydrogen pick up that developed at higher exposures/residence times.

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Photoelectrochemical Investigation of Radiation-Enhanced Shadow Corrosion Phenomenon

Cited by 7 publications

References 25 publications

Corrosion of Zirconium Alloys

Corrosion of Zirconium Alloys

Corrosion of Zirconium Alloys Used for Nuclear Fuel Cladding

Shadow Corrosion-Induced Bow of Zircaloy-2 Channels

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