Noise source estimation of boiling water reactor power fluctuation by autoregression

Some attempts have been made to investigate noise sources in a boiling water reactor (BWR) by multivariate random data analyses. Autoregression and multivariate coherency such as partial and/or multiple coherency have been introduced to the analysis of time series data gathered from a medium-sized BWR plant (BWR-3) of 460-MW electric power to evaluate linear relations among multiple inputs and outputs that are coupled with each other by sophisticated feedbacks. Through these attempts, the main local disturbance that leads to the peak in the spectrum of reactor power noise and is classified as global noise has been concluded to be caused by noise sources originated, not outside the reactor core, but inside the reactor core itself. Furthermore, the noise sources in the core have been found to be the turbulence of bubble generation and extinction in the lower region of coolant flow channel. It is found that the noise sources have different resonant frequencies that depend on the running speeds of coolant flows in fuel assemblies near the bottom local detector. It can also be shown that pressure waves induced by the local disturbances propagate into the coolant water in the lower core plenum, where they are mixed together into a single-pressure wave whose resonant frequency corresponds to the peak frequency in the spectrum of reactor power noise