CFD Assessment of the Local Hot Core Temperature in a Pebble-Bed Type Very High Temperature Reactor

Assessment of the local hot core temperature during normal operation in a pebble-bed type of Very High Temperature Reactor (VHTR) has been carried out by using the Computational Fluid Dynamic (CFD) method for which the boundary conditions were obtained from the results of a macroscopic analysis of the core using a system thermal analysis code, GAMMA. Three pebble arrangements are selected, which are Simple Cubic (SC), Body-Centered Cubic (BCC), and Face-Centered Cubic (FCC). Results showed that the SC arrangement having the lowest porosity gives the highest fuel temperature of 1237°C but still below the normal operational fuel limit of 1250°C. Comparison of the CFD results with an empirical correlation was made for the pressure drop and the Nusselt number but there were large differences between them. The benchmark calculation of a pressure drop for packed particles in a square channel indicated that the correlation for the full core used in the system code is not appropriate for the prediction of a local thermal fluid behavior.