A Multiphysics Model for Analysis of Inert Gas Bubbles in Molten Salt Fast Reactor – Part 2 : Application and Results

The Molten Salt Fast Reactor (MSFR), the reference circulating fuel reactor under the framework of Generation IV reactors being developed under the HORIZON2020 SAMOFAR project, features a molten fluoride salt that acts as both the fuel and coolant. This peculiar feature of MSFR poses a challenge in reactor design and modelling. The fuel salt velocity significantly influences the delayed neutron precursor (DNP) distribution in the core and subsequently the reactor kinetics. Moreover, the bubbling system envisaged for the on-line removal of fission products results in an additional variable affecting the reactor dynamics of MSFR. An additional application of the bubbling system foreseen in the MSFR design is in reactivity control by exploiting the highly negative void feedback coefficient inherent to MSRs. For the objective of reactivity control, the impact of void distribution on neutronics must be computed and a void reactivity feedback coefficient must be defined. However, under the current state-of-the-art in MSFR modelling, the impact of the voids has been defined mainly by adopting a homogeneous void distribution. This approach results in unrealistic assumptions and less accurate results and better results can be obtained by coupling CFD with neutronics to simulate the two-phase flow of salt/bubble mixture. This paper presents the implementation of two-phase flow model equations in COMSOL Multiphysics and its application to a simplified MSFR core geometry proposed under the EURATOM EVOL project. The model has been validated by comparison of thermal hydraulic and neutronic results for the case of single phase flow with a previous single-phase study available in literature. The two-phase studies highlight the impact of gas bubbles on the thermal hydraulics and neutronics of MSFR and the void feedback coefficient is evaluated based on the average void fraction in the core. The spatial dependence of the bubbling feedback coefficient is analysed based on comparison with Monte Carlo simulations performed using homogeneous bubble distribution in the core. The outcomes of the present analysis serve as a reference point for further investigation of bubbling system as a reactivity control method for MSFR.