**5. Conclusion**

Failure analysis of an SG tube was performed in order to know the degradation mechanisms that occurred during the operation. Samples of sludge piles were collected during sludge lancing cleanings to understand the nature of these deposits. Several types of samples were detected: Magnetite layered, "collars" associated with Si/Al ratio, and tube scale "flakes." Sample morphology was associated with a hard sludge, for example the "collars," related to impurities intake into SG, like silicon, aluminum, and iron. Iron is ingressing in the secondary side of SG due to oxidant conditions and maintenance work and during refueling. Carbon steels and low alloy steels are the components with a greater contribution of iron at a secondary cycle by flow accelerated corrosion (FAC) process. Specific chemical conditions produced in the tube-sheet of SG on the secondary side lead to the precipitation of soluble species and solid particles performing denting mechanism by hard sludge, which is the prelude of SCC. Due to these facts, multiple intergranular cracks were formed in the OD located at 3 mm above the last contact point between the tube and the TS, practically in the whole perimeter of the tube. The fracture surface of these cracks was partially covered by a silicon-rich layer and sulfur. EDX and Auger analyses point out Ni enrichment with slight Cr enrichment and Fe depletion. These results are compatible with neutral or moderately caustic environments evidenced by comparative analyses performed for Alloy 800 tested in caustic and acid sulfate environments.
