Root Cause Analysis of Vibration Exceeding and Design Optimization of Stagnant Branch of Reactor Coolant Loop

During the hot function test of nuclear power plant, it was found that the vibration of a stagnant branch of reactor coolant loop (RCL) exceeded the limit. According to the natural modal analysis, acoustic modal analysis and the variation trend of measured vibration with temperature, it is inferred that the acoustic vibration of the fluid in the pipeline causes the resonance of the pipeline. According to the calculation of vortex shedding frequency and acoustic vibration frequency, it is inferred that the acoustic vibration and the vortex shedding frequency of fluid at the tee are locked, which leads to the amplification of acoustic vibration excitation. Based on the analysis result and measured data above, it is determined that the cause of the vibration exceeding is the fluid-acoustic-structure coupling vibration. The pipe supports were optimized to avoid resonance frequency of acoustic structure coupling. The fillet at the tee was used to weaken the vortex shedding, so as to reduce the amplitude of flow-excited acoustic resonance. The vibration of the optimized branch will be significantly reduced to ensure the safety of the pressure boundary.