Void measurement of a vertical (4•~4) rod bundle has been conducted in a steam-water two phase flow using an advanced X-ray CT scanner and a large amount of rod bundle data was obtained. It was found from the results that the cross-sectional averaged void fraction data for a rod bundle could be correlated by the Drift-Flux model and that Zuber-Findlay correlation made it possible to appropriately predict the void data. However, this correlation underestimates data for a higher void fraction

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... on. This is because the flow pattern applicable range for Z-F correlation is not appropriate to this experimental range. Therefore, the new void correlation was developed based on the authors'data.