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

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

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 283C, pressure 7.2 MPa, flow rate 400 kg/ms, 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.