Sodium experiments of buoyancy-driven penetration flow into low-power subassemblies in a sodium-cooled fast reactor during natural circulation decay heat removal

自然循環時のナトリウム冷却高速炉における低発熱集合体への浮力による潜り込み現象に関するナトリウム試験

上出 英樹 ; 小林 順 ; 林 謙二

Kamide, Hideki; Kobayashi, Jun; Hayashi, Kenji

ナトリウム冷却高速炉の自然循環による崩壊熱除去機能を評価するうえで、特に炉内浸漬型の冷却器(DHX)を用いる場合、原子炉容器内の熱流動現象の評価が重要となる。DHXが作動し低温のナトリウムが炉容器上部プレナムに供給されると、高温の炉心との温度差による浮力のために特に低流速の集合体の内部に、低温ナトリウムが部分的に逆流することが考えられる。本研究ではこのような逆流現象についてナトリウム試験を行い、その発生条件を流速と温度差をパラメータとして求めた。その結果、実機で想定される条件で、このような逆流現象は集合体の上部遮蔽体内にとどまることを明らかにした。

Natural circulation has a significant role in the decay heat removal function of a sodium cooled reactor. The dipped heat exchanger (DHX) immersed in a reactor upper plenum provides cold sodium in the upper plenum during the decay heat removal operation. This cold sodium covers the top of the core under the natural circulation. Sodium experiments were carried out to find onset condition and penetration depth of such partial reverse flow driven by buoyancy force. A blanket subassembly and the upper plenum were modeled in the test section including an axial upper neutron shielding of the subassembly. The experimental parameters were temperature difference between hot upward flow in the channel and cold fluid in the upper plenum and flow velocity in the channel. The onset conditions of the penetration flow were correlated with Gr and Re numbers as well as basic water experiments. The observed penetration depths were limited in the upper axial neutron shielding of the subassembly.