Piping systems in nuclear power plants are affected by degradation mechanisms such as FAC (Flow-Accelerated Corrosion), Cavitation, Flashing, LDI (Liquid Droplet Impingement) and the like. FAC is well-known degradation mechanism. Most nuclear power plants, therefore, implement inspection programs to detect and control damages by FAC. On the other hand, various erosion mechanisms occurring on piping lines and local components lead to wear, leaks, and failures on piping systems. This paper describes intensive wear phenomenon on pipes, elbows, and tees located in upstream of heat exchangers. In the beginning of the study, it appeared to be flow cavitation damages. This is because the orifice is installed in the upstream section of a damaged elbow. After reviewing the damaged surface, it turned out that flow cavitation was not the key issue in this case. It is supposed that shock focusing effect, related to the bubbles that are created by vibration of a pump impeller, is the main reason of this intensive wear. The phenomena of shock focusing effect are known to have resulted from the pressure concentration due to collapsing bubbles in liquid.

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