A sensitivity study of BWR instability over global and regional modes at different exposures

Abstract

Boiling water reactors have unique mechanisms coupling between neutronic and two-phase flow thermalhydraulic behaviors, and may exhibit in-phase (global mode) instability and out-of-phase (regional mode) instability. In some observation modes, the regional mode instability is associated with an increase in power in one half of the core and a simultaneous decrease in power in the other half, such that the average power remains essentially constant. Yet in practice, sometimes the real situation is hidden, the neutron flux may oscillate more vigorously than expected. To investigate the stability behavior at the stability boundary from BOC (beginning of cycle) to EOC (end of cycle), fractional changes of the decay ratio are used to evaluate the parametric sensitivity of the global mode and the regional mode at different exposures. Decay ratios for regional mode oscillations are much less than those under core-wide conditions. Current studies demonstrated that for some of the parameters under particular conditions, the variation in the regional mode decay ratio exceeded that in the global mode. In this work, the thermalhydraulic parameters (such as flow rate and system pressure) exhibit a more sensitive regional variation than global. Moreover, some parameters (density reactivity coefficient and delayed-neutron fraction, for example) depend on the shape of the axial power shape; for the bottom peak axial power shape, the regional mode decay ratio variation is more sensitive than global; for the top peak axial power shape, the opposite is true.