In order to improve safety and design of Sodium-cooled Fast nuclear Reactors (SFR), the CEA (French Commission for Atomic Energy and Alternative Energy) is involved in hypothetical severe accident scenario studies. These scenarios may lead to a nuclear power excursion that might induce the melting down of nuclear fuel, followed by its quick vaporization pushing away the surrounding liquid sodium. The mechanical energy released by the fuel vapor expansion leads to the reactor vessel mechanical loading. This paper presents a methodology relying on Non-Equilibrium Thermodynamics (NET) and Dimensional Analysis (DA) which allows to identify the driving physical phenomena that should be considered for the modeling of the vapor expansion. DA allows to simplify the balance equations of the fuel vaporization and expansion multiphase model. NET application to the model yields its entropy production. The driving phenomena are identified from this entropy production. This method is applied and compared to a physical model of COMBUS, an analytical CEA software. A study of the Rayleigh-Taylor Instability phenomenon and of the associated heat and mass transfer highlights the importance of accounting for fuel vapor condensation.