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BWR fuels are covered with liquid water under normal operation conditions but in such events as pump trip, supply of water decreases and fuels may touch on vapor directly. Such condition is called dryout. An occurrence of dryout results in increase of fuel surface temperature, which may cause a severe accident. Therefore it is important from a view point of nuclear safety to make an accurate prediction of surface temperature. Though the final target of this study is to develop a prediction method of surface temperature for actual geometry and conditions of BWR fuel assembly, in this study we focused on the heat transfer phenomena for upward flow in a vertical tube as a first step. The prediction was carried out in order of (1) dryout point location, (2) heated surface temperature in pre-dryout region and (3) in post-dryout region. For prediction of dryout point location, a conventional model of liquid film dryout was used. We attempted to improve the prediction accuracy by adopting equations considering the traction at the vapor-liquid interface. In pre-dryout region, conventional equations for subcooling and nucleate boiling were applied. In post-dryout region, surface temperature prediction with a conventional method showed low accuracy at low flow rate. We specified the cause of low accuracy with statistical method and changed the prediction model. Major conclusions are as follows: (1) Prediction results were compared with various experimental data which cover the conditions of actual reactor to show that the developed method provided dryout location in good agreement but didn’t bring a significant improvement compared with conventional model. (2) Developed model can accurately predict surface temperature including the range of high temperature ( >650 ◦C) where conventional model showed low accuracy. (3) Calculation results were compared with experimental data of several researchers to confirm that the dependence on experiment geometry is small enough. 1. 緒 言 沸騰水型原子炉(BWR)の燃料棒表面は通常運転時には常に水で覆われている.しかし,ポンプの不具合や配 管の破断等の異常時には燃料棒表面が水で覆われず直接蒸気と触れ合うドライアウトが発生する場合がある.蒸 ∗1 学生員,早稲田大学大学院先進理工学研究科共同原子力専攻(〒 169-8555東京都新宿区大久保 3-4-1) ∗2 正員,フェロー,早稲田大学大学院先進理工学研究科共同原子力専攻 垂直円管内上昇流における沸騰遷移点と伝熱面温度の予測 (燃料棒表面温度予測手法の検討と改善) *1,*2 Waseda University 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan Received 5 August 2014

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Prediction of a boiling transition location and heated surface temperature for upward flow in a vertical tube (Investigation and improvement of prediction method for fuel rod wall temperature)