Detection of flow-induced vibration of reactor internals by neutron noise analysis

Flow-induced vibrations of reactor core components have been a major cause of failure of reactor internals in many nuclear power plants. To address this issue international standards (viz., ASME-OM-05) require monitoring of structural integrity of reactor core in nuclear power plants. A reactor internals vibration monitoring system (IVMS) has been developed for nuclear power plant surveillance. The system detects the core barrel motion and flow-induced vibrations of reactor internals by analyzing the inherent fluctuations (reactor noise) present in the neutron flux signals from ex-core neutron detectors. Before application of the IVMS in nuclear power plants the system hardware and methodology/software has been extensively tested on the research reactor through a series of reactor noise measurements. In the noise measurements the neutron flux signals were correlated with the signals from vibration sensors mounted on reactor structure and control rods drive mechanisms. The frequency spectra of reactor power signals obtained with IVMS had shown small oscillations in the neutron flux signals at well-defined frequencies. There was a strong correlation of these fluctuations with the establishment of coolant flow through the core. With the help of elaborate experiments using the IVMS the cause of oscillations in neutron flux signals was attributed to the flow-induced vibrations of control rods, and the particular vibrating control rod was identified. The magnitude of the displacement of vibrating control rod was calculated from the measured power spectral density of neutron power signals. It has been shown that ex-core neutron noise measurements are more sensitive in determining the dynamic behavior of reactor internals than the vibration sensors mounted at remote, out-of-the core locations.