Effect of inlet geometry on hot-leg U-bend two-phase natural circulation in a loop with a large diameter pipe

In relation to nuclear reactor accident and safety studies, experiments on hot-leg U-bend two-phase natural circulation in a loop with a relatively la rge diameter pipe (10.2 cm ID) was performed for understanding the two-phas e natural circulation and flow termination during a small break loss of coo lant accident in LWR. The loop design was based on the scaling criteria dev eloped under this program and a horizontal section was inserted between the gas injector and the hot leg in order to investigate the effect of the vap or phase inlet section on the flow regimes and flow interruption. The loop was operated either in a natural circulation mode or ill a forced circulati on mode using nitrogen gas and water. Various tests were carried out to est ablish the basic mechanism of the flow termination as well as to obtain ess ential information on scale effects of various parameters such as the loop frictional resistance, thermal center, and pipe diameter. The void distribu tion in a hot leg, flow regime and natural circulation rate were measured i n detail for various conditions. The termination of the natural circulation occurred when there was insufficient hydrostatic head in the downcomer sid e. The superficial gas velocity at the flow termination could be predicted well by the simple model derived from a force balance between the frictiona l pressure drop along the loop and the hydrostatic head difference. The bub bly-to-slug flow transition was found to be dependent on axial locations. I t turned out that the inlet geometry affected the flow regime at the inlet of the hot leg, namely the void distribution in the hot leg. (C) 2001 Elsev ier Science B.V. All rights reserved.