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I. Indicators at the operational monitoring level: An abnormal increase in radioactivity in the primary loop – a rapid rise in the activity of isotopes such as krypton, xenon, and iodine in the coolant; the specific activity of fission products like 133Xe and 131I can increase by more than a hundred times; a ratio of A(131I)/A(133I) greater than 0.2 – are the key operational indicators indicating the presence of erosion cracks. Abnormalities in cesium isotope ratios: During transient changes in the reactor’s power, cesium isotopes such as 134Cs and 137Cs are released in large quantities from the damage site. The ratio of their activities can help determine the burnup level of the damaged fuel, thereby indicating the occurrence of wear failure. II. Indicators related to the appearance and structure of the cladding: Characteristics of scratches on the cladding surface: Wide and shallow pits appear on the surface of the zirconium alloy cladding; there are obvious plow-like grooves and accumulations of oxide particles in the scratched areas. The wear mechanisms involved are mainly abrasive wear and oxidative wear, and in severe cases, the tube wall can be worn through, resulting in small holes. Hydrogenation-related characteristics: When primary loop water enters the erosion crack, a zirconium-water reaction occurs; the hydrogen generated diffuses to the outer wall of the cladding, resulting in a large number of hydrogenated bubbles in a “sunburst” pattern. Further development leads to the formation of secondary cracks. III. Indicators related to the performance of fuel assemblies: Local overheating of fuel rods: Scaling resulting from abrasion causes an abnormally high local heat flux density in the cladding, leading to a significant increase in the temperature of the fuel surface; this results in overheating conditions similar to those seen in deviation from nucleate boiling, thereby accelerating the process of failure. Fuel pellet loss: As the cracks caused by erosion continue to expand, the matrix material of the fuel pellets inside the damaged fuel rods is lost, resulting in a significant decrease in the levels of 134Cs and 137Cs at those locations; this is a typical characteristic of erosion failure in its advanced stages.