In the boiling water reactor safety assessment, core cooling is achieved if the core is flooded during loss of coolant accidents. It is important to accurately predict a position of water level during blowdown. The change of water level position is determined by the amount of the difference between the generated steam by depressurizing boiling and the steam flowing out from the water surface. Both the water level and void fraction are necessary to calculate the amout of steam flowing out from the water level correctly. However these data have been scarcely obtaind simultaneously in a same experiment. Both the water level and void fraction data have been obtained with the time process at the same time by the author et al. In this study, these blowdown data were used to calculate vapor velocities during blowdown. A vapor velocity correlation has been obtained by vapor velocity data under various fluid conditions. This vapor velocity correlation has been used in the LEVELY code to calculate the fluid condition during blowdown. It was confirmed that the water level was accurately predicted by the LEVELY code using this vapor velocity correlation derived in this study.
