Volume 7, Issue 9 (October 2010)
Shadow Corrosion-Induced Bow of Zircaloy-2 Channels
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.