For blowdown phenomenon that would appear in primary stage of large break LOCA (Loss Of Coolant Accident), there exists downward misty flow over fuel rods. The behavior of droplets in such flow has effects on clad temperature because droplet removes heat from a reactor core by its direct contact on fuel rods and its evaporation at the surface and in steam flow. Droplets are generated from liquid film flowing with flashing on surface of super-heating clad or flow directory from upper plenum when coolant flows into fuel rods from upper plenum of a reactor vessel. Droplet flows in steam flow with integration and disintegration, and finally becomes steady droplet, i.e. droplet flows in the fuel rods without changing its diameter. For estimation of thermal hydraulic behavior of coolant, LOCA analysis code, MCOBRA/TRAC was developed. In order to estimate peak clad temperature (PCT) over LOCA phenomenon especially reflooding phase following blowdown phase, droplet diameter in actual plant condition must be clarified. We carried out the experiment to measure a diameter of stable droplet simulating actual condition of large break LOCA with two kinds of fluids: Sulfur Hexafluoride (SF6) and ethyl alcohol (C2H5OH) as gas and liquid phase, respectively. Those fluids can simulate a high pressure and temperature steam-water system of 6MPa, and 550K at the condition of the lower pressure and atmospheric temperature of 0.5MPa and 300K, respectively. The experiment was focused on investigating the Weaber number of steady droplet in the blowdown phenomenon of large break LOCA. It was clarified that the minimum distance was under 300mm, and the Weaber number was 3.1.

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