A CFD Assessment of a Bypass Flow in a Cooled-Vessel Design of a Prismatic VHTR

Due to higher operating temperatures, design of a reactor pressure vessel (RPV) is very important in Very High Temperature Reactors (VHTR). Recently KAERI has developed a cooled-vessel concept for a prismatic VHTR that allows the use of a SA-508/533 vessel as used in the commercial light-water reactors by maintaining the vessel temperature below its operating structural limit. A parametric study was conducted for the vessel cooling flow rate and the location of the riser holes in the reflector. Although the cooled-vessel design allows us to use the conventional reactor vessel, the bypass flow, one of the related issues, has not been evaluated. The purpose of this study is to assess how much bypass flow occurs due to routing the flow path into the graphite structure in the cooled-vessel design. A model to assess the bypass flow is developed by using the ANSYS CFX code. The analysis is performed to examine the effect of the gap size of the graphite blocks on the bypass flow. In addition, the effect of the block arrangement in a vertical direction is also investigated