Two-Phase Flow Distribution in 5 × 5 Rod Bundle During Boil-Off Process

Abstract Boil-off may occur in light-water reactors with a stagnant core flow when a long period passes after the initiation of an accident. Improving the predictability of the two-phase mixture water level and void behavior during the boil-off process can effectively evaluate the long-term core coolability in accident progression analyses. Thus, this study investigated the void behavior transition during the boil-off process in a 5 × 5 heated rod bundle, which partially simulates the actual fuel assembly, at atmospheric pressure. Heat fluxes of 3, 6, and 9 kW/m2 were imposed to simulate the decay heat in long-term core cooling during an accident. The influence of collapsed water level on the distributions of void fraction, bubble chord length, and phase velocity was quantified using subchannel void sensors mounted within the rod bundle. The void fraction increased as the collapsed water level decreased because of the decrease in hydrostatic head. The measured three-dimensional distribution of phase velocity indicates the occurrence of inverse flow and the presence of global flow in the bundle during the boil-off process. The obtained results can contribute to the understanding of the multi-dimensional behavior of boil-off phenomena.