Investigation of Sweepout Mechanism and Critical Void Height in Annular Downcomer

Abstract

The APR1400 (Advanced Power Reactor 1,400 MWe) has adopted the direct vessel injection (DVI) in lieu of the conventional cold leg injection for its emergency core cooling system (ECCS). In this reactor, sweepout from the water surface by gas (vapor or air) flow plays an important role in analyzing the mass and momentum transfer in the reactor downcomer of multidimensional geometry during a loss-of-coolant accident (LOCA) by decreasing the water level in the downcomer. The core water level will tend to decrease rapidly if a considerable amount of the entrained water stream and droplets bypasses through the break. The amount of entrained water is mostly determined by the interacting gas flow rate, the geometric condition, and the interfacial area between the gas and the water. The sweepout is observed to take place in three rather distinct steps: the beginning of undulation, the full wave and the wave peak (droplet separation). In view of these observations we investigated the relation between the gas flow rate and the amount of bypass as a function of time. The current experimental results shed light on the flow mechanism and the semi-empirical relations for the three-dimensional sweepout in a large-diameter annulus such as the reactor downcomer. A physico-numerical model is being developed to predict the multidimensional bypass flow rate resulting from the sweepout and entrainment in the downcomer.