CFD Simulations of Helium Flow Through Partial Heating Pebble Bed

Abstract The modular high temperature gas-cooled reactor (HTGR) is an advanced technology of inherent safety, high electricity generation efficiency, and potential process heat applications in the chemical industry. The packed pebble bed in HTGR consists of a large number of randomly distributed pebbles and is cooled down by helium flow. The reactor thermal hydraulic design and safety analysis requires in-depth understanding of helium flow and heat transfer, and reliable correlations in the pebble bed. Previous research mostly focuses on the helium flow in non-heating pebble bed (in hot tests before criticality) or full heating pebble bed (full of fuel pebbles releasing fission or decay heat). Understandings on partial heating pebble bed are insufficient. Current paper uses the Computational Fluid Dynamics (CFD) to study the helium flow in partial heating pebble bed with mixed fuel and graphite pebbles, which is typical in the initial core and running-in phase of pebble bed HTGR. Various flow conditions in the pebble bed were simulated, and the pressure drops were compared with those predicted by Kerntechnischer Ausschuss (KTA) equation. The Reynolds number and porosity used in KTA equation were discussed in calculating the pressure drops. It is found that the pressure drop of uniform heating model and nonuniform heating model are basically the same. The pressure drop predicted by KTA equation is larger than the calculated result. As for the using of KTA equation, the effect of Reynolds number is not significant while the porosity is of great importance. This work lays a certain foundation on the development of a new pressure drop correlation, which could support the design and commissioning of the pebble bed HTGR.