A Multiphysics Model for Analysis of Inert Gas Bubbles in Molten Salt Fast Reactor – Part 1: Numerical Modelling

The Molten Salt Fast Reactor (MSFR) is the reference circulating fuel reactor under the framework of Generation IV reactors and is currently being developed under the HORIZON2020 SAMOFAR project. Prediction of dynamic behaviour of Molten Salt Fast Reactor poses multiple challenges arising from the peculiar characteristics of the MSFR which are substantially different from other reactors including the Molten Salt Reactor Experiment (MSRE). However, among the models developed in the past for molten salt reactors, most are focussed on thermal reactors such as the MSRE. In the MSFR, the fast neutron spectrum, and consequently the absence of graphite, leads to broad changes and makes the models developed for thermal systems unfit for use with MSFR. Prediction of MSFR requires a model with a tight coupling between neutronics and thermal-hydraulics. The two-phase mixture of fuel salt and inert gas bubbles present in the MSFR further complicates the modelling the MSFR. In this paper, we present the model equations to describe the thermal-hydraulics and neutronics of MSFR and to predict the void reactivity feedback associated with the inert gas bubbles. The model equations are based on coupling two-phase computational fluid dynamics (CFD) models with neutronic equations for circulating fuel. In MSFR, the flow of fuel salt and gas bubbles mixture can be classified as a dispersed bubbly flow for which the thermalhydraulics has been modelled using an Euler-Euler two-fluid approach. Furthermore, for small volume fraction of the dispersed phase, i.e. for small bubble fraction, the two-fluid model has been further simplified by combining the momentum and continuity equation of the two phases and adding a gas-phase transport equation to track the void fraction. The prompt neutron flux has been modelled adopting the one-group diffusion approximation and the DNP transport equation has been implemented for delayed neutron precursor (DNP) concentration. Reactivity insertion for small changes in the void fraction has been computed through a first order perturbation theory approach.