Neutron noise to detect BWR-4 in-core instrument tube vibrations and impacting

Noise analysis was used to investigate in-core instrument tube vibrations in boiling water reactor-4 (BWR-4)-type reactors. Neutron noise signals from in-core fission chambers and acoustic noise signals from externally mounted accelerometers were used in these studies. The results show that neutron noise can be used to detect vibration and, more important, impacting of instrument tubes against adjacent fuel channel boxes. Externally mounted accelerometers detect impacting but not rubbing of instrument tubes against fuel channel boxes. Accelerometers can monitor impacting only on the particular instrument tube where the accelerometer is mounted. Surveillance for instrument tube impacts can be accomplished using standard BWR-4 in-core power range neutron flux detectors at all instrument tube locations containing these detectors. Ex-vessel accelerometers can then be used to monitor instrument tubes that lack power range neutron flux detectors. However, noise on axial flux profiles obtained with movable in-core detectors is not a reliable indicator of impacting because the recorder used to plot the flux profiles does not respond adequately to the noise frequency generated by impacting. Neutron noise signatures show that modification of the bypass cooling by plugging preexisting holes and drilling new holes in the fuel assembly lower tie plate greatly reduces instrument tube vibration and eliminates impacting of instrument tubes against fuel boxes