Influence of Alkali Metal Oxides and Alkaline Earth Metal Oxides on the Mitigation of Stress Corrosion Cracking in CANDU Fuel Sheathing

Abstract

This work investigates strategies to mitigate stress corrosion cracking (SCC) in Zircaloy-4 sheathing materials. The CANLUB coatings currently used in CANDU reactors contain both alkali metal and alkaline earth metal impurities, which can exist as oxides (e.g., Na₂O and CaO). It is believed that when the corrosive fission product iodine reacts with these oxides, the iodine can be sequestered through the formation of an iodide (e.g.,NaI and CaI₂). The subsequent O₂ release may repair cracks in the protective ZrO₂ layer on the sheathing,shielding the Zircaloy-4 sheathing from further corrosive fission product attack. For this investigation, O₂ gas, Na₂O, and CaO were separately introduced into an environment wherein slotted Zircaloy-4 rings endure mechanical stresses in iodine vapour at high temperatures. Controlled additions of O₂ gas created a slight reduction in the corrosive attack on Zircaloy-4 sheathing, while the inclusion of Na₂O and CaO lead to greater reductions.