Swelling Model of Two-Phase Mixture in Lower Plenum at End of Blowdown Phase of PWR-LOCA

The swelling of the two-phase mixture in the lower plenum governs the Emergency Core Cooling (ECC) water bypass and the refill of the lower plenum at the end of blowdown phase of a PWR-LOCA. In order to develop an analytical model of the swell behavior, a visual air-water experiment was performed by simulating the flashing in the lower plenum. In the experiment, two layers, i. e. the froth at the lower portion and the liquid deficient layer at the upper portion were observed in the lower plenum. And it was observed that the liquid drops were ejected from the froth surface and carried to the downcomer by the ascending air flow. Based on these results, a new correlation was developed for the overall averaged void fraction of the lower plenum. This correlation was applied to the CREARE flashing experiments and good comparisons were obtained between the calculated and the measured mass transients in the lower plenum.