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A reduced-moderation light water reactor (RMWR), which is studied by the Japan Atomic Energy Research Institute, needs to the volume ratio between the coolant and fuel less than 0.2, in order to achieve 1.1 or more high breeding ratios. The RMWR fuel assembly consists of many fuel rods with a diameter of around 10 mm with the triangular tight-lattice configuration in the radial direction. A minimum gap width between adjacent fuel rods is about 1 mm. Moreover, spacers are set to keep uniformly the gap width between fuel rods horizontally. These spacers are installed at a fixed interval to the flow direction in a reactor core. As a result of this, a complicated flow is formed around each spacer because of a large flow disturbance. Then, fundamental liquid film flow behavior in a narrow channel with a three-dimensional rectangular rib was analyzed numerically using a newly developed two-phase flow analysis code. From this study the following conclusions were derived: a wake behind the spacer rib takes place easier when the initial film thickness is short and then the gas velocity is higher than the liquid velocity; and, the liquid film thickness reduces with increasing the gas velocity.

Numerical Analysis of a Water-Vapor Two-Phase Film Flow in a Narrow Coolant Channel with a Three-Dimensional Rectangular Rib