Multiphysics simulation of the molten salt fast reactor using Griffin and Pronghorn

Molten-salt reactors (MSR) are tightly-coupled multiphysics systems: neutronics and thermal-hydraulics conditions strongly interact during steady-state and transient operations. Multiphysics Object-Oriented Simulation Environment (MOOSE) based tools are ideally suited for the simulations of coupled neutronic-thermal hydraulic transient simulation of MSRs. This paper presents the capability of two specific MOOSE-based applications, Griffin and Pronghorn, to simulate MSR designs. The incompressible flow equations in their Boussinesq approximation are then solved in Pronghorn, while Griffin models the neutronics using a multigroup diffusion model. The set of equations are discretized using the continuous finite element method (CFEM). The multigroup diffusion and the neutron precursors equations are solved as a monolithic system of equations using a Preconditioned Jacobian-Free Newton Krylov (PJFNK) method, while the Newton method is used to solve the pseudo transient incompressible flow equations. The two problems exchange power density and fluid temperature during the pseudo-transient and the simulation is run long enough to let the two physics converge to a steady state. A simplified MSR model has been set up to test the neutron precursors convection system in a realistic core configuration. The steady state analysis shows that the results are in accordance with the values found in the literature for this simplified design, but the design must be further refined to avoid localized undesired effects. A sensitivity analysis performed using the steady-state model shows that the reactor model behaves as expected, and provides valuable information on the importance of some of the design parameters