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

Abstract

The next generation of High-Temperature Reactors, an annular nuclear core consisting of a central column of graphite spheres (or pebbles) and a surrounding annulus of fuel pebbles, may be employed. Due to this geometry as well as due to the complex feeding and shutdown mechanisms of such a configuration, three-dimensional nuclear heat production, gas flow and heat transport mechanisms have to be investigated. To simulate the flow and heat transport within the core and the surrounding graphite reflector, a new code system based on the three-dimensional computational fluid dynamics package CFX-4 has been developed. As a validation of the new program, steady state and transient simulations of an experiment involving a pebble bed (SANA test rig at the Jülich Research Center) are performed. Extensive further code verification has been done by comparison axisymmetric simulations with known results obtained with the well established thermal analysis code THERMIX, which is formulated in two-dimensional cylindrical coordinates. In order to demonstrate the feasibility and the computational effort of three-dimensional nonaxisymmetric simulations, an assumed influence of a misplaced package of heat-generating fuel pebbles located asymmetrically in the central column is investigated. A significant increase of the computed maximum core temperature after a loss-of-forced-cooling incident is found.