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Abstract
Pebble-bed fuelled, molten salt-cooled nuclear reactors (PB-FHRs) involve interactions between a fluid coolant phase, solid particle fuel phase and neutron interactions enabling fission of nuclear fuel for heat production. As these three aspects are highly interdependent, this work presents a three-way coupled methodology for simulation of PB-FHRs to provide a robust analysis tool for detailed analysis of such reactor designs.
Computational fluid dynamics (CFD) techniques are implemented for simulation of the coolant fluid’s flow and temperature fields. Discrete element methods (DEM) are used to model individual pebble fuel elements present within the core, focusing on particle-particle interactions and structures formed within packed beds of mono-sized spheres. Modelling of the interaction between these two phases is realised through a two-way coupled methodology and open-source codes. Additional to the interaction of the fluid and pebble phases, consideration of the neutronic interactions resulting in nuclear heating of the fuel pebbles is included. The Monte Carlo neutron transport method is employed for determination of the fission rate distribution for PB-FHR geometries. Inclusion of the neutronics results within the CFD and DEM solvers, and vice versa, results in three-way coupled multiphysics simulations for PB-FHRs.
This work has been built up over successively more complex simulations, starting with CFD-DEM replication of an isothermal pebble re-circulation experiment and culminating in the three-way coupled simulation of a true PB-FHR design. Additional analysis focusing on the neutronic behaviour of the PB-FHR geometry for various geometric and material property configurations is also presented. It was found that the multiplication factor of PB-FHRs with a free surface of pebbles within the core region are highly influenced on the shape of this free surface.
The three-way coupling methodology was shown to provide detailed representations of PB-FHR geometries in a timely manner without the need for simplification of the geometry or material properties. This allowed for detailed analysis of coolant, fuel pebble and neutronic properties throughout the core region. The results obtained demonstrate the suitability for use as an accurate design tool.