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) developed in the framework of the H2020 SAMOFAR project is a circulating fuel nuclear reactor in which a mixture of molten thorium and uranium fluorides acts as fuel and coolant simultaneously. The dual role of molten salt, as nuclear fuel and coolant, in MSFR, and MSRs in general, results in a complex, highly coupled system that poses a challenge in modelling and simulation. Among these features is the presence of gas bubbles in the molten fuel which necessitates the use of two-phase models to accurately simulate reactor behaviour.
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.