Examination of nuclear thermal hydraulic oscillation modes in BWR core

Nuclear thermal hydraulic oscillation modes in a BWR core have been examined. A hypothesis has been produced that the factor to stabilize the core-wide mode is the flow resistance out of the core, and that the factor to stabilize the local or regional out-of-phase mode is neutron leakage from the region to diminish void reactivity feedback. Effective void reactivity coefficient of a region has been calculated by adding a void fraction perturbation to the region regarding it as an independent core. It has been found that a smaller region has a smaller effective void reactivity coefficient. Stability analyses of actual plants have been performed using the effective void reactivity coefficient and taking into account the lack of stabilization effect by flow resistance out of the cores. The analyses have successfully predicted the observed oscillation modes. A mode map has shown that if the flow resistance out of the core is small the core-wide mode appears before the out-of-phase mode, and that it is opposite in a core with a large flow resistance out of the core. In an intermediate case increase in void reactivity coefficient leads to the core-wide mode first, but increase in two-phase pressure drop causes out-of-phase mode before core-wide mode