Three-dimensional numerical simulation of flow and heat transport in high-temperature nuclear reactors

In next generation nuclear High-Temperature Reactors an annular nuclear core consisting of a central column of graphite spheres and a surrounding ring of fuel pebbles is employed. Due to the complex feeding and shutdown mechanisms three-dimensional effects of heat production, gas flow and heat transport may become important for safety analysis. To simulate flow and heat transport in the core and the surrounding graphite reflector a new code system based on CFX-4 has been developed and run on the NEC-SX4 and NEC-SX5 supercomputers. The simulations are performed with the Heterogeneous Model of porous media. The program has been verified by comparison with two-dimensional simulations of the HTR-MODUL using the well established thermal analysis code THERMIX. A sensitivity study of several models for pressure drop and heat transfer on a simplified model of an HTR-MODUL is performed. Additionally the influence of the variation of the volume porosity near walls on flow and heat transport is analysed. In order to demonstrate the simulation of three-dimensional effects the influence of a package of fuel pebbles located asymmetrically in the central column is investigated. A significant influence on the temperature distribution and the maximum temperature core is found.