In-Core Neutron Noise Analysis for Diagnosis of Fuel Assembly Vibrations

This thesis deals with the analysis of in-core neutron noise measurements in order to diagnose fuel assembly vibrations in the PWRunit R4 of the nuclear power plant Ringhals in Southern Sweden. With the help of noise diagnostic methods the vibrational frequencies of the core and fuel components as well as the axial distribution of the neutron noise, induced by the fuel element vibrations, are determined. After evaluating the obtained auto power spectral density (APSD) plots, the maximum amplitudes of the peaks corresponding to the various vibration frequencies of interest are determined and one can plot their axial distribution. The fundamental vibration mode of the fuel assemblies was found at 1.8 Hz, their second bending mode at 6.8 Hz. The core barrel beammode vibration has a frequency of 7.8 Hz although the reliability of this interpretation suffers from a lack of information. The shell mode vibration could be found at 19 Hz which is an interesting outcome from an in-core measurement. There were at least two forced vibrations of the fuel assemblies at 9.9 and 10.8 Hz but due to too few measurement data it was not possible to identify the origin of those vibrations in more detail. Furthermore the pump frequency of the primary coolant system could be determined and was found to be 24.9 Hz, i.e. approximately 25 Hz.