Multiphysics modelling of gaseous fission products in the Molten Salt Fast Reactor

The Molten Salt Fast Reactor (MSFR) is a nuclear reactor concept under development in the framework of the H2020-Euratom project SAMOSAFER. Given its peculiar characteristics as a circulating-fuel reactor, new simulation tools need to be developed and tested for its study. In this work, a multiphysics solver previously developed in OpenFOAM is extended with new functionalities for the analysis of this reactor design. The main focus is the modelling of the behavior of Gaseous Fission Products (GFPs) inside the core and their interactions with a helium bubbling system, foreseen to remove both GFPs and metallic fission products through flotation. On the basis of an Euler-Euler solver able to model the presence of the two phases – namely the liquid fuel and the gaseous bubbles – , a multi-component mixture approach is adopted to simulate the behavior of GFP species in the core, with particular focus on their production, consumption, transport, mass exchange and removal to the off-gas system. The new capabilities are tested on both 2D and 3D MSFR cases, considering Xe-135 as reference nuclide, with focus on the efficiency of the bubbling system in the removal of GFPs from the salt. As main figure of merit of the removal capability, a cycle time related to the extraction of the specie from the system is defined and calculated for different conditions of helium injection. This work constitutes a further advancement in the development of the MSFR concept, extending the modelling capabilities of the previous multiphysics solver. Reliable estimation of the evolution of the gaseous fission products in the reactor core and the related effect of the helium bubbling system is a fundamental requirement for the analysis of the radioactive source term and for the prediction of the overall operating conditions of this innovative nuclear system.