Numerical simulation of nuclear pebble bed configurations

Abstract

High Temperature Reactors (HTRs) are being considered all over the world. An HTR uses helium gas as a coolant, while the moderator function is taken up by graphite. The fuel is embedded in the graphite moderator. A particular inherent safety advantage of HTR designs is that the graphite can withstand very high temperatures, that the fuel inside will stay inside the graphite pebble and cannot escape to the surroundings even in the event of loss of cooling. Generally, the core can be designed using a graphite pebble bed. Some experimental and demonstration reactors have been operated using a pebble bed design. The test reactors have shown safe and efficient operation, however questions have been raised about possible occurrence of local hot spots in the pebble bed which may affect the pebble integrity. Analysis of the fuel integrity requires detailed evaluation of local heat transport phenomena in a pebble bed, and since such phenomena cannot easily be modelled experimentally, numerical simulations are a useful tool.As a part of a European project, named Thermal Hydraulics of Innovative Nuclear Systems (THINS), a benchmarking quasi-direct numerical simulation (q-DNS) of a well-defined pebble bed configuration has been performed. This q-DNS will serve as a reference database in order to evaluate the prediction capabilities of different turbulence modelling approaches. A wide range of numerical simulations based on different available turbulence modelling approaches are performed and compared with the reference q-DNS. This paper reports a detailed comparison of LES, Hybrid (RANS/LES) and RANS models with the reference q-DNS. These simulations are performed for a well-defined single face cubic centred pebble configuration. The obtained flow and thermal fields are extensively analyzed to understand the flow physics in such complex flow regime. Furthermore, lessons learned from these simulations are summarized in the form of guidelines for such complex flow configurations. In addition, following these guidelines, a strategy has been developed to perform large eddy simulations of a realistic limited sized random pebble bed.