Abstract
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 large diameter pipe (10.2 cm ID) was performed for understanding the two-phase natural circulation and flow termination during a small break loss of coolant accident in LWR. The loop design was based on the scaling criteria developed 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 vapor phase inlet section on the flow regimes and flow interruption. The loop was operated either in a natural circulation mode or in a forced circulation mode using nitrogen gas and water. Various tests were carried out to establish the basic mechanism of the flow termination as well as to obtain essential information on scale effects of various parameters such as the loop frictional resistance, thermal center, and pipe diameter. The void distribution in a hot leg, flow regime and natural circulation rate were measured in detail for various conditions. The termination of the natural circulation occurred when there was insufficient hydrostatic head in the downcomer side. The superficial gas velocity at the flow termination could be predicted well by the simple model derived from a force balance between the frictional pressure drop along the loop and the hydrostatic head difference. The bubbly-to-slug flow transition was found to be dependent on axial locations. It 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.
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