Evaluating the oxidation behaviour of alloy X-750 in reducing annulus gas environments

Alloy X-750 spacers employed in CANDU reactors are known to experience degradation of their mechanical properties in service. Currently, radiation damage and resulting segregation of helium nanobubbles are recognized as the primary mechanism responsible for this embrittlement, but it is unclear if there are contributions from the gaseous environment of exposure. Alloy X-750 is susceptible to primary water stress corrosion cracking (PWSCC) under a mechanism of internal/intergranular oxidation. Spacers reside in the Annular Gas System within a CO₂-O₂ mixture, but transients that result in reducing conditions may happen during their lifetime. Hence, spacers could be susceptible to a similar embrittlement observed in PWSCC, if reducing conditions are present in the annulus environment. Experiments have been performed on Alloys 600 and X-750 using an in-house designed gas reactor to simulate reducing conditions resultant from possible transients. Results suggest that similar intergranular oxidation phenomena reported in primary water could occur in X-750 in reducing CO/CO₂ environments. (author)