Chemical composition analysis, microstructure analysis, macro and micro fracture morphology analysis were conducted on a cracked leak off tube in primary heat transport and heavy water collection system of a CANDU6 unit. The results show that the material conforms to the original design requirements. The leaked fluid vaporized at the penetration and stress corrosion crack initiated because of cl-concentration. The leaked fluid drains periodically and lead the wall surface temperature periodically changes. The SCC crack transformed to corrosion fatigue crack and the tube finally fractured. In order to prevent the similar tubes crack again, preventive maintenance strategies were given in this paper.

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