Destructive examination and analyses of pulled steam generator tubes

Steam generator model F in Kori 3, Younggwang 2 and Younggwang 1 as 950MWe PWR was provided by Westinghouse. Steam generator tube made of Alloy 600TT material (outer diameter 0.688'(17.475 mm), thickness 0.04'(1.016mm) in Blairsville was provided by Huntington alloys. Steam generator in Ulchin 4 as 1000MWe KHNP PWR was manufactured by Doosan heavy industry and steam generator tubes were manufactured by B and W (Bobcock and Wilcox). Alloy 600 MA was used as steam generator tubing material of outer diameter 19.05mm and thickness 1.07mm. Five tubes of Alloy 600TT which showed crack signal from non-destructive examination were pulled from Kori 3, Younggwang 2 and Younggwang 1. Two tubes which showed crack signal from non-destructive examination were pulled from Ulchin 4. For the pulled tubes, KAERI performed destructive examination. Through the destructive examination, the existence of cracks were confirmed and the cause of crack was investigated. Remedy was also suggested to mitigate the present circumstances. Stress corrosion cracking (SCC) was observed for Alloy 600TT tubes. It was recommended that the sludge content should be lowered, deleterious elements and MRI should be maintained continuously. For Ulchin 4, SCC was confirmed for Alloy 600MA tubes. It is necessary to lower and remove the sludge in the near term. In the longer term, replacement of steam generator was suggested