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 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 pass through the break in lieu of flooding the reactor. 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 regions: at the beginning of oscillation, at the full wave, and at the wave peak (droplet separation). The beginning of oscillation normally occurs as a result of the Helmholtz instability, which is defined in terms of the difference between the gas and the liquid velocities. The horizontal water surface is waved greatly before the gas flow reaches the critical point of droplet detachment. In the full-wave region, the droplets from the rough wave are swept into the gas flow and driven to the break. The water stream and droplets near the wave-peak region pass through the break at extremely high velocities.

In view of these observations this paper investigates the relation between the gas flow rate and the amount of bypass as a function of time. The test facility was constructed in a 1/10 linear scaled-down model from the 1400-MW(electric) Advanced Power Reactor 1400 (APR1400), which has four direct vessel injection lines, four cold legs, and two hot legs. The air was injected through the three intact cold legs and passed through the broken cold leg. The sweepout was visualized from the acrylic test vessel. When the water level was located at the bottom of the break nozzle, the amount of bypass increased at the high Reynolds number of the gas. In the test the downcomer water level rapidly decreased for the initial minute. Then, given the Reynolds number of the gas, the sweepout hardly occurred as the water level approached the critical point 10 min into the test. So far, the experiment and the analysis for the sweepout have been limited to small annuli, flat plates, and T-junctions, which yielded the two-dimensional flow field. The current experimental results shed light on the flow mechanism and the semiempirical relations for the three-dimensional sweepout in a large-diameter annulus such as the reactor downcomer. The sweepout and entrainment are physically understood by visual inspection of flow in the downcomer. An engineering correlation is developed to predict the multidimensional sweepout and entrainment in the annular downcomer.