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1. Background of the formation of dent corrosion: There are a large number of gaps between the heat transfer tubes and the support plate of the steam generator, and the accumulation of mud on the tube plate provides environmental conditions for dent corrosion. Impurities brought in by leaks in the secondary loop system will continue to concentrate in these narrow gaps, accelerating the corrosion process. 2. Core formation mechanism When the secondary side water chemical treatment is changed to full volatilization treatment, the pH value drops. If chloride enters the pot water, NiCl₂ and FeCl₃ will be formed in the gap between the support plates, which will accelerate the corrosion of the inner wall of the carbon steel support plate in an acidic environment, and eventually squeeze the heat transfer tube to form dents. When the Cl⁻ content is 20μg·kg⁻¹ and the gap is 0.305mm, the predicted time for dent corrosion to begin is 2.5 years. 3. Typical actual cases: A large area of heat transfer tube deformation and rupture occurred in the steam generator of a 1300MW unit in France. After inspection, it was found that it was not caused by conventional sludge accumulation, but by residual welding rod slag and grinding iron filings from the on-site girth welding that entered the secondary side and were not completely removed before operation, ultimately inducing severe dent corrosion. 4. The main hazard is that dents will directly squeeze the heat transfer tube, causing tube wall deformation, destroying the integrity of the barrier between the primary and secondary circuits. In serious cases, it will cause the heat transfer tube to rupture, affecting the safe and stable operation of the nuclear power plant.