Numerical simulations of molten salt pebble-bed lattices

Abstract

The development of the Fluoride-salt-cooled High-temperature Reactor (FHR) is ongoing. The molten salt coolant provides several inherent safety characteristics that traditional water-cooled nuclear reactors lack. The first test reactor to be built in China is a pebble-bed type. Research efforts are currently focused on determining the heat transfer characteristics of such a reactor. In the past, the pebble-bed geometry has been studied for the development of the High Temperature Gas-cooled Reactor (HTGR). The previous work has indicated that the profile of the Nusselt number depends on both the lattice structure and pebble contact.In this work, a new series of simulations is sought to characterize heat transfer in an FHR pebble-bed. Due to the physical properties of molten salt and the FHR design, the Reynolds number is roughly three orders of magnitude lower than the HTGR while the Prandtl number is roughly an order of magnitude higher. Thus, the flow in an FHR lattice is expected to be unlike the HTGR counterpart. Three different lattices (Simple Cubic (SC), Body Centered Cubic (BCC), and Face Centered Cubic (FCC)) are considered.The study found that the different lattice geometries have a significant impact on the heat transfer characteristics. The SC lattice has a simplified flow that minimizes pressure-drop and Nusselt number. The BCC and FCC lattice have a complicated flow structure resulting in an order of magnitude higher pressure-drop and almost double the Nusselt number. It was concluded that heat transfer characteristics in a random pebble-bed would be highly sensitive to the mixing, and thus turbulent heat transfer, along the flow path. It was also found that while an interstitial gap allows heat transfer characteristics to be replicated, the pressure loss is significantly lower, while the computational costs increase. Correlations available in the literature (developed primarily for HTGRs) have a mixed performance in prediction of the Nusselt number. The result suggests that new experimental efforts are necessary to develop correlations that pertain to FHR conditions.