The Molten Salt Fast Reactor, currently under development in the framework of the SAMOSAFER H2020-Euratom project, foresees an in-core helium bubbling system for the removal of gaseous and metallic fission products from the fuel salt. The correct assessment of the removal capability of the off-gas system inside the fuel circuit is of paramount importance in the determination of the radioactive source term in the MSFR. This paper deals with the extension of the modelling capabilities of an OpenFOAM multiphysics solver previously developed at Politecnico di Milano and its application to study the transport of the gaseous fission products (GFPs). In particular, the behavior of GFPs and their interaction with the MSFR helium bubbling system are considered to determine the mass transfer of GFPs from the salt mixture to the gaseous phase and then to the off-gas system. xenon-135 is taken as the reference isotope for modelling the production, the transfer between liquid and gaseous phases (i.e. the fuel salt and helium, respectively), and the extraction of the gases. The capabilities of the solver are presented with an analysis carried out on a simplified axisymmetric model of the reactor. The efficiency of the helium bubbling system in the removal of the gaseous fission products is evaluated through a characteristic renewal time of the core calculated from the outcomes of the simulation. The latter is found to be in the order of tenths of seconds in accordance to preliminary assessments performed by reactor designers.