The effects of crossflow gap and axial bypass gap distribution on the flow characteristics in prismatic VHTR core

In prismatic VHTR design, the occurrence of the bypass gap between the core blocks is inevitable for installation and refueling. Since the core bypass flow affect both the reactor efficiency and core thermal margin, it is important to evaluate the bypass flow distribution accurately. The gap size varies with the reactor fuel cycle as the dimension of graphite block varies due to the fast-neutron induced shrinkage and thermal expansion. In particular, the dimensional change of graphite block is different from each location in the core, since the neutron fluence of the blocks is different locally. In addition, fuel and reflector blocks are rearranged by axial shuffling for refueling, so that bypass gap distribution is not axially uniform. Therefore, the consideration of axial profile of the bypass gap is important for accurate evaluation of bypass flow but most of the previous studies assumed uniform bypass gap size. In present study, the multi-block air test experiment was carried out to evaluate the bypass flow distribution with the consideration of axially varying bypass gap profile and the crossflow effect. Axial bypass gap profile of 6 mm, 2 mm, 4 mm, 2 mm from top to bottom layer and the crossflow gap of 2 mm were tested in the experiment. In addition, comparative computational fluid dynamics (CFDs) analysis for the multi-block experiment was carried out to supplement the experimental limit and to analyze the local flow characteristics in detail. The results suggest that the CFD code, CFX-12 has a sufficient capability to analyze the VHTR core and the prismatic VHTR core should be modeled as the flow network system. The experimental data and CFD simulation results could be used to validate the thermal–hydraulics system code for VHTR system.

► We conducted the multi-block experiment and CFD analysis to investigate flow characteristics in prismatic core. ► The effects of crossflow and uniform and non-uniform distributions of bypass gap were evaluated. ► Ratios of the bypass flow rate to total flow rate in bypass gap of 2 mm and 6 mm were 16% and 50%, respectively. ► In non-uniform bypass gap test, 39% bypass flow ratio at the entrance decreased to 22% at the exit due to the crossflow.