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Report No.
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Measurement and analysis for rewetting velocity under post-BT conditions during anticipated operational occurrence of BWR

Shibamoto, Yasuteru ; Maruyama, Yu ; Nakamura, Hideo  

A series of experiments was performed for rewetting phenomena on dried-out heated surfaces under post-BT (Boiling Transition) conditions with high-pressure and high-water flow rate simulating anticipated operational occurrences of a BWR. An analytical model for rewetting velocity, defined by the propagation velocity of quench front, has been developed on the basis of the experimental results. The experiment was conducted within the ranges of the flow rate and the pressures covering an actual BWR plant conditions. The rewetting for the post-BT conditions is characterized by faster propagation of the quench front than that for reflood phase conditions during a postulated large-break loss-of-coolant accident. In order to provide an explanation of this characteristic, the present analytical model took the effect of precursory cooling into account by modifying the existing correlation of Sun-Dix-Tien (1975) which is based on the one-dimensional analysis in a flow direction during the reflood phase. The present model demonstrates that the precursory cooling can significantly increase the rewetting velocity by more than an order of magnitude. Appling the experimental correlation developed in the separately conducted experiment into the heat transfer coefficient in the present model at a wet and a dry region with precursory cooling, our experimental data of the rewetting velocity as well as the wall temperature profiles for the variable flow rates are successfully predicted. It is found that the effect of precursory cooling is indispensable to explain the considerably high rewetting velocity under the large flow rate condition due probably to the significant droplets cooling contribution.

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