An aging assessment on ex-service Alloy 800 steam generator tubing

To support SG life extension and plant life management, an aging assessment was performed on a number of ex-service Alloy 800 steam generator (SG) tubes removed from three CANDU^®1 stations with service life spanning from 2 to 27 years. Laboratory tests and examinations were carried out to investigate the potential aging mechanisms of SG tubing. High-temperature electrochemical experiments were performed under simulated SG secondary side crevice chemistry conditions to determine the corrosion susceptibility of the ex-service tubing; metallurgical examinations were carried out to check the chemical compositions, grain size, and hardness of the ex-service tubing materials and secondary ion mass spectrometry (SIMS) analysis was performed to assess the potential surface chromium depletion and grain boundary segregation of the ex-service tubing. Based on the results from the assessment, no increase in the corrosion susceptibility or changes in metallurgical properties of the ex-service tubes resulting from aging were observed. SIMS top-down profiles did not detect any aging-related surface chromium depletion on any of the ex-service tubes. However, SIMS imaging performed on the polished cross-sections of the ex-service tubes observed boron precipitation at the grain boundaries. Since no archived tubing with the same heat number as that of the ex-service tubing is available for comparison, whether the boron precipitation at grain boundaries is attributed to aging through SG operation is not conclusive and needs further clarification. In addition, the impact of this boron precipitation on the integrity of Alloy 800 SG tubing needs further investigation.