Analysis of Neutronic/Thermal-hydraulic Coupling Flow Instability in Reactor Core under Rolling Condition Using FFT Method

Based on the developed neutronic/thermal-hydraulic coupling flow instability code for reactor cores under ocean conditions, the fast Fourier transformation (FFT) method was used to analyze oscillation curves of reactor core channels, and spectrum characteristics of reactor core channels occurring flow instability under static and rolling conditions were obtained. The results show that one spectrum peak can be found when the flow instability happens under static conditions, and the corresponding spectrum is called inherent spectrum. For the flow instability occurred under rolling conditions, all channels of the reactor core are affected by both rolling conditions and neutronic/thermal-hydraulic coupling effect, and the inherent spectrum is dominant in the highest power channels which means that the flow instability is firstly induced in those channels. The characteristics of the complicated oscillation curves of mass flow rate can be analyzed precisely by FFT method to estimate whether the neutronic/thermal-hydraulic coupling flow instability under rolling conditions occurs.