Evaluating the Oxidation Tendency of Alloy X-750 in Simulated Annulus Gas Environments

Abstract

Research is being conducted to determine the degradation mechanisms that govern the embrittlement observed in X-750 spacers used in CANDU reactors. While radiation damage and segregation of nanobubbles of helium are currently recognized as the primary mechanism responsible for embrittlement, it is unclear if there are contributions from the gaseous environment. Alloy 600 is well-known to undergo to internal/intergranular oxidation in reducing hydrogenated steam conditions at less than 500 °C. Since X-750 is a precipitation hardened variant of Alloy 600, spacers may be susceptible to a similar embrittlement if reducing conditions are present in the annulus CO/CO₂ environment. Experiments have been performed on Alloy X-750 using an in-house designed gas reactor to simulate reducing conditions that may be present in the annulus under transient conditions. Preliminary results suggest that similar intergranular oxidation phenomena reported for Alloy 600 in hydrogenated steam may be occurring in X-750 in reducing CO/CO₂ environment.