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. In conclusion: The oscillation modes in a BWR core have been examined regarding a part of the core as a local independent core. The lack of stabilization effect by flow resistance out of the core in the out-of-phase mode makes this mode more unstable than the core-wide mode. However, neutron flux change is suppressed by the neutron leakage between the regions oscillating in the opposite phases. This lowers the effective void reactivity coefficients in the out-of-phase mode compared to the core-wide mode, and stabilizes the out-of-phase mode. Whether the out-of-phase mode appears or not is determined by the balance of these two effects. Effective void reactivity coefficients have been calculated in various cores for various oscillation modes. The out-of-phase mode decreases effective void reactivity coefficients significantly. In addition, the effective coefficients are smaller for smaller region sizes to oscillate. Thus, the thermal hydraulic characteristics have to be very close to instability to cause localized oscillations in regions much smaller than a half core. The effective void reactivity coefficients for half-core out-of-phase oscillations are smaller in a smaller core. They also depend on fuel loading pattern. The loading pattern to place high burnup fuels on the core periphery decreases the effective coefficient. A method to examine oscillation modes by a frequency-domain program with point neutron kinetics have been proposed. The method has been proved to predict the oscillation modes in the actual plants successfully. The stability of the core-wide mode is highly dependent on the flow resistance out of the core. In a core with a small outer flow resistance, the core-wide mode is always more unstable than the out-of-phase mode. In a core with a large flow resistance, it is opposite. In an intermediate case, increase in void reactivity coefficient causes the core-wide mode appear first, but destabilizing thermal hydraulic characteristics leads to the out-of-phase mode. (authors)