Quantitative Indicators of Detector String Impacting

Abstract

Detecting the vibration and impacting of neutron detectors in boiling water reactor cores is usually attempted from the detector signals. Two such indicators used or suggested earlier are the widening of the vibration peak in the detector noise auto-power spectral density and the deviation from Gaussian (=“distortion”) of the signal amplitude probability distribution (APD). Quantification of both methods is hindered by the presence of a strong, Gaussian background; thus, it was thought that band-pass filtering around the vibration peak would improve the performance of the methods. This suggestion has been investigated. It turns out that filtering reduces the background, but it also distorts the vibration component of the signal. For good performance, this latter effect must be compensated for. Such methods are elaborated and applied to both peak widening and APD distortion techniques. It was found that application of such techniques makes the kurtosis and the decay ratio associated with the signal suitable to be used as quantitative indicators of impacting. The methods elaborated were also checked by numerical simulations and real measurements with positive results.