Study of the Thermal and Fluid Dynamical Effects on the Heat Removal in the Vicinity of Spacers for Supporting a Fuel Rod in a Nuclear Reactor

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In order to make clear the fluid dynamical mechanisms causing the liquid film breakdown on the fuel rod in BWR, the following two kinds of fundamental investigation were made.1. The effect of the thermocapillary convection on the breakdown of a horizontal thin water layer, with its surface locally heated, was examined. We made a visual observation by using the tracer method and proposed an analytical model. In the case that the thickness of the layer is larger than 2 mm, both the observed and calculated results showed that vortices are induced in the layer and the surface profile changes transiently. On the other hand if the layer thickness is much less than 2 mm, the liquid cannot flow into the position just below the locally heated surface, causing the thinning of the layer at that position. This phenomenon, which was shown by calculation only, has the effect of promoting the film breakdown.2. To simulate a spacer supporting a fuel rod in BWR, a flat-plate-type obstacle was set in an air-water stratified flow in the horizontal and inclined ducts with rectangular cross section and the effects of the flat plate on the film breakdown was experimentally investigated. The results are as follows : The plate itself does not promote the liquid film breakdown but the projection connected on the lower side of the plate has a strong effect on it. In the case of the gas flow velocity of 60 - 70 m/s, film breakdown occurred in front of the projection due to the leading edge down-wash of the stream and due also to the rejection of water by gravitational force in the case of the upward flow in the inclined duct. It also occurred just downstream of the projection in the case of the lower gas velocity. By setting a hole in front of the projection the occurrence of the dry patch formation was delayed.