Modeling and analysis of molten fuel vaporization and expansion for a sodium fast reactor severe accident
Résumé
The safety assessment of Sodium-cooled Fast Reactors (SFR) requires to account for hypothetical severe accidents involving the melting down of the core materials. This paper deals with the modeling of a fuel vaporization transient that might occur in a SFR in case of severe accident. After a nuclear power excursion, some fuel might be molten and vaporized. In this case, the expansion of fuel vapor might generate a mechanical stress on the reactor vessel and structures. Assessing the vessel integrity is of major importance for the reactor design. A fuel vaporization and expansion modeling, which has been simplified using a Dimensional Analysis, is presented. The modeling is implemented in a tool, called DETONa, able to perform fast calculations, of the order of one minute. The vaporized fuel’s thermal exchange with the reactor liquid coolant leading to a possible coolant vaporization is simulated by DETONa. The coolant is assumed to be entrained into the fuel vapor. A droplet entrainment model based on Rayleigh-Taylor instabilities associated to their diameter’s limitation using Weber stability criterion is proposed. The modeling is validated on experimental results and on code-to-code comparisons. Parametric calculations are conducted on a reactor case. The impact of the initial molten fuel mass, its initial temperature, critical Weber number and radiative heat transfer are investigated. The non-adiabatic modeling and the adiabatic modeling yield results different by 40 percent in certain cases. DETONa is shown to be sensitive to the fuel initial temperature, the heat transfer coefficient and the Rayleigh-Taylor wavelength, involving variations that can range to 18 percent.
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