Experimental Investigation of Vibration Characteristics Induced by Decrease in Axial Preload in the Core Support Barrel of a Nuclear Reactor

Neutron noise analysis is widely used to monitor the structural integrity of nuclear reactor internals by leveraging the neutron attenuation effects associated with the cantilever bending motion of the core support barrel. Spectral analysis of ex-core neutron detector signals indicates that a decrease in axial preload is typically reflected as a shift in resonance frequency within the cantilever bending mode. Monitoring both the resonance frequency and amplitude is thus essential for effective surveillance and diagnostic procedures. In practice, however, clearly identifying the resonance frequency can be challenging, as the normalized root-mean-square value within the low-frequency band gradually increases with operational time. Consequently, understanding the dynamic behavior of reactor internals as operational time progresses is critical to ensuring accurate surveillance and diagnosis under current operational conditions. In this study, an experimental investigation was conducted to examine the changes in vibration characteristics induced by a reduction in axial preload in the core support barrel, with the aim of identifying trends in resonance frequency variation.