Preliminary Study of Bypass Flow in Prismatic Core of Very High Temperature Reactor Using Small-Scale Model

Very high temperature reactor (VHTR) is one of the candidates for Generation IV reactor. It can be continuously operated with average core outlet temperature between 900?C and 950?C, so the core temperature is one of the key features in the design of VHTR. Bypass flow in the prismatic core of VHTR is not a designed feature but it is inevitable due to the combination of several causes and considerably affects the core temperature. Although bypass flow has been studied extensively, the current status of research on thermal/hydraulic core flow of VHTR is far from completion. Present study is the starting of bypass flow characteristic investigation using small-scale model that will fulfill understandings of bypass flow in the prismatic core of VHTR.

Bypass flow experiments are conducted by using three small-scale models of prismatic blocks. They are stacked in a test section to form bypass gaps of single-layer blocks as exist in prismatic core of VHTR. Three bypass gap widths set in air and water flow experiments are 6.1, 4.4 and 2.7 mm. Experimental data shows that bypass flow fraction depends on bypass gap width and downstream condition of prismatic blocks, while pressure drop of flow through bypass gaps depends on bypass gap width only.

Bypass flow simulations are performed by using STAR-CCM+ software after meshing parameters were determined from simulation exercises and grid independent study. Three turbulence models are employed in all bypass flow simulations which are stopped at physical time of 100 seconds marching by implicit unsteady scheme. Bypass flow fraction, coolant channel Reynolds number and bypass gap Reynolds number from air flow and water flow simulations with 6.1-mm bypass gap width are very close to experimental data. This is because bypass flow fractions from experiments at this bypass gap width are matched in construction of the simulation models. Discrepancies between results from simulations and experiments for remaining gaps increase when bypass gap width becomes smaller. Finally, guidelines for bypass flow experiments and simulations are drawn from the data in present study to improve bypass flow study in the future.