Study on the flow distribution in prismatic VHTR core with a multi-block experiment and CFD analysis

VHTR is a gas-cooled, graphite-moderated reactor that was developed for the production of hydrogen and process heat. In the prismatic VHTR core with hexagonal graphite blocks, the bypass gap, which is an interstitial gap between the hexagonal blocks, inevitably forms due to the installation tolerance and irradiation shrinkage of the core blocks. Core bypass flow is the ineffective coolant flow that does not pass through the coolant holes within the fuel block. The core-bypass flow distribution varies according to the fuel cycle because the graphite blocks are deformed by irradiation and heating. Since the bypass flow reduces the core thermal margin, it should be evaluated precisely to guarantee reactor safety. In this regard, experimental and computational studies were carried out to evaluate the core-bypass flow distribution. A multi-block air test facility was designed and constructed in Seoul National University to measure the flow and pressure distributions according to the block combination and cross-flow gap size. The effects of the cross-flow phenomenon on the flow and pressure distribution in the core were investigated. The experimental data were used to validate a CFD model for the analysis of bypass flow characteristics in detail.