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 TR AC/BF1-ENTREE has been benchmarked for predicting decay ratio and limit-cyc le amplitude of the regional instability The core was mapped into fewer CHA N 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-mod al reactivities were evaluated based on the higher modal flux shapes derive d by ACCORD-N.