BWR Regional Instability Analysis by TRAC/BF1-ENTREE - II: Application to Ringhals Unit-1 Stability Test

Based on the Ringhals unit-1 stability test results, the coupling system TRAC/BF1-ENTREE has been benchmarked for predicting decay ratio and limit-cycle amplitude of the regional instability. The core was mapped into fewer CHAN groups based on the first azimuthal mode flux shape and the guidelines to minimize the numerical interregion stabilizing interaction. The system was further applied to detailed phenomenological studies. A symmetric pattern of the first azimuthal mode gave a dynamic boundary condition for ideal out-of-phase flow oscillations and lowers the regional instability threshold. The intermode interaction between the fundamental and first azimuthal modes was demonstrated under postulated large oscillations. Self- and mutual-modal reactivities were evaluated based on the higher modal flux shapes derived by ACCORD-N.