Development of multi-physics capabilities in coupling computational fluid dynamics and thermodynamics for molten salt reactor applications

Abstract

Existing knowledge gaps in Molten Salt Reactors (MSR), such as understanding fuel salt chemistry and fission product retention, require rigorous investigations to support reactor safety. The objective of this work was to develop a novel computational toolset tailored for multi-physics simulations of MSR studies that could fill the aforementioned knowledge gaps. This toolset studied the intricate dependence between thermal-hydraulics and salt chemistry by coupling the computational fluid dynamics code OpenFOAM with the computational thermodynamics code Thermochimica . This coupling facilitates the simulation of scenarios relevant to MSR reactor safety, which is especially important given that MSRs have a low technical readiness level relative to other reactor technologies. Two demonstration problems exemplify the applications and outcomes of this project, which was in support of the SAMOSAFER Co-ordinated Research Project of the European Commission. The initial problem revolves around the first step of the molten salt clean-up fluorination process, which vaporizes molten fuel components (i.e., UF4) into its volatile form (i.e., UF6). Simulations revealed that the fluorination time is strongly dependent on the molten salt system, salt viscosity, and temperature. Compared with experimental results, the simulations displayed a strong correlation in vaporization rates under steady-state conditions, giving credence to the validity of the local equilibrium hypothesis. The second demonstration problem centred on the molten salt fast reactor. Here, normal operating conditions were examined, focusing on fission product retention and release, such as Cs, La, Xe in promising molten fluoride systems (e.g., LiF – ThF4 – UF4 77.5-20-2.5 mol%). Simulations demonstrated that most fission products are retained by the salt and the evaporation rates of several compounds, such as, LiF, ThF4, UF4, CsF, Cs2, Cs2F2, LaF3, F, F2 posed to be almost insignificant when compared to other known volatile/gaseous fission products, such as xenon. This investigation provided an understanding of how fast the UF4/UF3 molar ratio reaches optimal design limits, which plays a pivotal role in controlling corrosion. In conclusion, the outcomes underscore advancements in computational capabilities, promising to elucidate further the intricacies of designing and testing multifaceted scenarios in the realm of MSRs.