Long-Term Corrosion and Deuterium Uptake in CANDU-PHW Pressure Tubes

Abstract

Recent examinations of cold-worked Zircaloy-2 and cold-worked Zr-2.5Nb pressure tubes from CANDU-PHW reactors have extended the corrosion and deuteriding data for these materials up to 5600 effective full power days (efpd), the longest published exposure for these materials in reactor. The examinations have demonstrated that short-term tests to investigate irradiation and water chemistry effects must be supplemented by extended exposures in order to better understand the long-term corrosion and deuteriding behavior of zirconium alloy components with thick oxide films. In-reactor corrosion and deuteriding of Zircaloy-2 was found to accelerate with time once the oxides exceeded a critical thickness of 15 to 20 µm. Radiolysis within these thick porous oxides, leading to a highly localized water chemistry near the metal-oxide interface, independent of the bulk water chemistry, is suggested as the probable explanation for the acceleration in kinetics. In conformance with this hypothesis, Zircaloy-2 pressure tubes developed large maxima in oxide thickness and deuterium concentration towards the outlet end where the combined effect of neutron flux and temperature was most severe. Corrosion of Zr-2.5Nb tubes with a similar operating history was about one third that for Zircaloy-2, with no evidence of accelerating kinetics under normal operating conditions. Deuterium uptake was remarkably low, being 2 to 4% of that measured for Zircaloy-2. After 3500 efpd irradiation, the average deuteriding rate in Zr-2.5Nb tubes was only 1.5 mg/kg per year. The observations are discussed in terms of the current understanding of corrosion and deuteriding processes.