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Measurement and analysis for rewetting velocity under post-BT conditions during anticipated operational occurrence of BWR

BWRの異常な過渡変化におけるpost-BT時リウェッティング速度の計測と解析

柴本 泰照 ; 丸山 結 ; 中村 秀夫  

Shibamoto, Yasuteru; Maruyama, Yu; Nakamura, Hideo

BWR運転時の異常な過渡変化を想定した高圧・高質量流速条件において、post-BT時のドライアウト伝熱面上の液膜リウェット現象に関する一連の実験を行った。これらの実験結果に基づき、液膜の先端進展速度で定義されるリウェッティング速度の予測モデル式を開発した。リウェッティング速度は大破断LOCA再冠水過程を対象としてこれまでにさまざまなモデルが提案されているものの、post-BT条件においては予測モデルとして適用可能なものがなかった。実機異常過渡条件では再冠水過程と異なり、液滴流密度が非常に大きく、このためリウェット速度も非常に速いという特徴がある。本研究では、Sun-Dix-Tienによる先行冷却モデルを改良することで、このような速いリウェット速度を広範囲の条件で予測できる相関式を提案する。

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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