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A Large-scale simulation on two-phase flow characteristics around duel rods in a tight-lattice core

稠密炉心内燃料棒まわりの二相流特性に関する大規模シミュレーション

高瀬 和之; 吉田 啓之 ; 小瀬 裕男*; 秋本 肇

Takase, Kazuyuki; Yoshida, Hiroyuki; Ose, Yasuo*; Akimoto, Hajime

革新的水冷却炉の稠密炉心内の水-蒸気二相流構造を解明するために、稠密燃料集合体をフルサイズで模擬した体系で大規模二相流シミュレーションを行った。解析には、気液界面挙動を高精度で予測できる界面追跡法をベースにした二相流直接解析コードTPFITを使用した。大規模3次元計算には地球シミュレータやAltix3700Bx2などの超高性能スーパーコンピュータを利用した。今回の結果から、燃料棒間狭隘部や軸方向スペーサ部周りの流速,圧力,ボイド率等の3次元分布が定量的に明らかになり、稠密炉心の二相流構造に関して有益な知見が得られた。今回の結果をより大規模に拡張することによって、シミュレーションを主体とした炉心熱設計手法を確立できる見通しが得られた。

Water-vapor two-phase flow structure in a fuel bundle of an advanced light water reactor was analyzed numerically by large-scale direct simulations. A newly developed two-phase flow analysis code was used. It can precisely predict the interface behavior between the liquid and gas phase by using the interface tracking method. The present analytical geometry simulates a tight-lattice fuel bundle with 37 fuel rods and four spacers. The fuel rod outer diameter is 13 mm and gap spacing between each rod is 1.3 mm. Each spacer is installed in an arbitrary axial position in order to keeping the gap width. Water flows upward from the bottom of the fuel bundle. The inlet conditions of water are as follows: temperature 283$$^{circ}$$C, pressure 7.2 MPa, flow rate 400 kg/m$$^{2}$$s, and the Reynolds number 40,000. In the present study three-dimensional computations were carried out under the non-heated isothermal flow condition in order to remove the effect of heat transfer by the fuel rods. The average mesh size in the present numerical study was 0.15 mm. From results of a series of the numerical simulations, the following consideration was derived: (1) The fuel rod surface is encircled with thin water film; (2) The bridge phenomenon by the water film appears in the region where the spacing between fuel rods is narrow; (3) Vapor flows downward the triangular region where the spacing between fuel rods is large; and, (4) A flow configuration of vapor shows the streak structure in the vertical direction.

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