ON THE MICROSTRUCTURE OF ALLOY 600 SCC CRACKS OBSERVED BY TEM ON PWR SG PULLED TUBES AND ON LABORATORY SPECIMENS

摘要

Secondary side corrosion cracking of steam generator tubes in Mill Annealed Alloy 600 occurs in flow-restricted areas where impurities get concentrated under heat flux. During spring 2009, eddy current test showed a circumferential indication (tube support plate elevation) in Bugey-3 unit for the first time on French nuclear plants in which very little PbO was detected in deposits in the late 80's after 67000h of service. The corresponding tube was therefore removed. IGSCC cracks and outer oxides layers formed on these pulled tubes were examined by ATEM. The results are compared to previous ones obtained on a tube pulled out from another unit (Dampierre-4) where Pb was detected during ATEM observations and not suspected to be at the origin of IGSCC. Then, results are compared to those obtained on specimen tested in laboratory environment all leading to SCC rates comparable to secondary side corrosion cracking rates observed in the field. The oxides formed were compared to identify the typical environment responsible for the degradation observed on the pulled tubes. It appears that the best laboratory environment reproducing oxides morphology observed on the selected pulled tubes was a NaOH with Pb environment even thought Si pollution was sometimes detected in outside oxides layer.