Study on coolant accumulation in sg U tube upflow side during natural circulation reflux cooling condition of small break loss-of-coolant accidents of pressurized water reactors

PWRにおける小破断冷却材喪失事故時の自然循環リフラックス冷却状態下の蒸気発生器U字管上昇流側蓄水挙動に関する研究

小泉 安郎; 山路 達也*; 山崎 康平*; 大竹 浩靖*; 長谷川 浩司*; 大貫 晃*; 金森 大輔*

Koizumi, Yasuo; Yamaji, Tatsuya*; Yamazaki, Kohei*; Otake, Hiroyasu*; Hasegawa, Koji*; Onuki, Akira*; Kanamori, Daisuke*

PWR小破断冷却材喪失事故時の還流冷却状態において、過渡的な炉心水位低下をもたらすことが懸念される、蒸気発生器上昇流側U字管内冷却水滞留を調べることを目的として、凝縮を伴う垂直管内気液対向流の蓄水挙動に対して熱流動実験を実施した。実験は、内径18mm、長さ4mの垂直円管を試験流路とし、作動流体として蒸気と水を用いて、圧力0.1MPaの条件で行った。実験では、可視化を主目的として透明管による試験体、及び、蓄水量の定量評価を目的とした真鍮管を用いた試験体を用いた。流路下端入口で液が流下できない条件であっても、流路内の上昇に従う凝縮により蒸気の流速が減少するため、管路上方で凝縮液流下の状態となる。これにより、管路内に二相混合水位が形成され、管路内に凝縮水滞留を生じる事を実験的に把握した。これらの現象把握や、蓄水量の定量評価結果をまとめ、蓄水量評価モデルを導出するとともに、実験結果との比較により蓄水挙動を適切に表現できることを確認した。

Experiments of condensing counter-current two-phase flow in a vertical pipe were performed. This study was intended to examine water accumulation in the up-flow side of steam generator U-tubes of a PWR during the reflux cooling stage of a small break LOCA. It has been apprehended that the water accumulation may result in temporary core liquid level depression. The inner diameter and the length of a test flow channel used in the experiments were 18 mm and 4 m, respectively. The experiments were performed by using steam and water at 0.1 MPa. Two kinds of experiments were conducted; visualization experiments by using a transparent test section and quantitative water accumulation evaluation experiments by using a brass test section. Even if water on the inner surface of the test pipe could not flow downward at the lower portion of the test pipe, a part of water became to flow downward at the upper portion of the test pipe since steam velocity decreased because of condensation. Thus, two-phase mixture level was formed in the upper portion of the test pipe, which resulted in the water accumulation in the pipe. The model to predict the water accumulation was proposed. It predicted the water accumulation reasonably well.