Pore Scale Thermal Hydraulics Investigations of Molten Salt Cooled Pebble Bed High Temperature Reactor with BCC and FCC Configurations

Abstract

The present paper systematically investigated pore scale thermal hydraulics characteristics of molten salt cooled high temperature pebble bed reactor. By using computational fluid dynamics (CFD) methods and employing simplified body center cubic (BCC) and face center cubic (FCC) model, pressure drop and local mean Nusselt number are calculated. The simulation result shows that the high Prandtl number molten salt in packed bed has unique fluid-dynamics and thermodynamic properties. There are divergences between CFD results and empirical correlations' predictions of pressure drop and local Nusselt numbers. Local pebble surface temperature distributions in several default conditions are investigated. Thermal removal capacities of molten salt are confirmed in the case of nominal condition; the pebble surface temperature under the condition of local power distortion shows the tolerance of pebble in extreme neutron dose exposure. The numerical experiments of local pebble insufficient cooling indicate that in the molten salt cooled pebble bed reactor, the pebble surface temperature is not very sensitive to loss of partial coolant. The methods and results of this paper would be useful for optimum designs and safety analysis of molten salt cooled pebble bed reactors.

Pronghorn Theory Manual

“…Mesh generation for touching solid bodies is difficult, since the meshes at pebble contact points must be made to share nodes, which often results in highly distorted meshes that can impact convergence [5]. The "near miss" method, where the pebble diameter is set to 99.5% of its actual value to avoid the difficulty of mesh contact, is often used to avoid this issue [5][6][7][8][9][10]; this method is shown in Fig. 1.…”

“…Surface fuel temperature profiles shown in [7] are very complex, and reveal that temperatures are highest at stagnation points and in the recirculation flows that form behind the pebbles [8], and lowest at contact points aligned with the flow direction. The surface temperature on a single pebble varies by about 100-200°C for an ordered packing in flibe at nominal conditions, with the majority of the surface varying between 50-100°C [10].…”

“…However, care must be taken here, because the pebble packing arrangement in a bed is random, and CFD simulations performed for BCC and Face Centered Cubic (FCC) lattices actually show substantial variation from one another [8,17]. Due to the tighter packing, enhanced turbulent mixing, and lack of larger pore spaces to support heat-transport-degrading vortices, FCC lattices have higher velocities (about 35% higher peak velocities) and better convective cooling (about 50% higher Nusselt number) than BCC lattices, which may be nonconservative in predicting fuel temperatures [10,14]. More tortuous fluid paths in FCC lattices do result in higher pressure drops, which is conservative [10].…”

“…Due to the tighter packing, enhanced turbulent mixing, and lack of larger pore spaces to support heat-transport-degrading vortices, FCC lattices have higher velocities (about 35% higher peak velocities) and better convective cooling (about 50% higher Nusselt number) than BCC lattices, which may be nonconservative in predicting fuel temperatures [10,14]. More tortuous fluid paths in FCC lattices do result in higher pressure drops, which is conservative [10]. However, both BCC and FCC lattices differ considerably from randomly-generated beds.…”

“…While the volume averaging approach was used to obtain Eq. (2.215), most porous media codes interpret T s as a volume-averaged solid surface temperature, rather than as a volume average over all regions of the solid [10,36,49,50,56]. This assumption permits a relatively straightforward solution of the solid temperature profile.…”