Measurement of wall and interfacial friction factors for counter-current flow in porous bed

In the course of a severe accident, a debris bed is formed from once molten and fragmented fuel elements as observed in the Three-Mile Island unit-2 accident. The debris bed must be cooled to avoid further degradation of the core, since the degraded core releases decay heat. Even if the degraded core is in water, it cannot be judged that the degraded core would be coolable, since the degraded core may be melted again if dryout occurs. It is thus necessary to evaluate the dryout heat flux for the judgment of the coolability of the debris bed during the severe accident. Dryout phenomena in the debris bed is dominated by two-phase flow behavior in the debris bed. Especially, it is strongly affected by counter-current flow limitation (CCFL) in the debris bed. Therefore, it is important to know the flow characteristics of CCFL in the debris bed. If one hopes to analyze counter-current flow in the debris bed by using the same method as usual two-fluid model, it is necessary to know the interfacial and the wall friction factors. However, it is not clear that the correlations for interfacial and wall friction factor for a pipe can be used for counter-current flow in the debris beds. In order to determine the interfacial and wall friction factors, it is necessary to obtain wall and interfacial shear stresses. The wall and the interfacial shear stresses depend on the void fraction and differential pressure. However, at present, there is no available data for local void fraction and local pressure distribution in the debris bed at CCFL, since it is very difficult to measure the flow characteristics in the complex geometry such as the debris bed. In the present study, air-water CCFL experiment was conducted with a debris bed simulated with glass particles to measure the local void fraction and the local pressure distributions simultaneously as well as the flow rates for gas and liquid. The present experimental research succeed to measure the local void fraction under CCFL in porous bed by the combination of constant current method and quick-closing valve method. This measurement data made it possible to estimate the wall and the interfacial shear stresses as shown in Fig. A-1 and to determine the wall and the interfacial friction factors. The present paper shows the world first measurement results of the interfacial and wall friction factors under CCFL in porous bed