Investigation of film boiling thermal hydraulics under FCI conditions: results of analyses and a numerical study

Film boiling on the surface of a high-temperature melt jet or of a melt particle is one of key phenomena governing the physics of fuel-coolant interactions (FCIs) which may occur during the course of a severe accident in a light water reactor (LWR). A number of experimental and analytical studies have been performed, in the past, to address film boiling heat transfer and the accompanying hydrodynamic aspects. Most of the experiments have, however, been performed for temperature and heat flux conditions, which are significantly lower than the prototypic conditions. For ex-vessel FCIs, liquid subcooling is big and can significantly affect the FCI thermal hydraulics. Presently, there are large uncertainties in predicting natural convection film boiling of subcooled liquids on high-temperature surfaces. In this paper, research conducted at the Division of Nuclear Power Safety, Royal Institute of Technology (RIT/NPS), Stockholm, concerning film boiling thermal hydraulics under FCI condition is presented. Notably, the focuses are placed on the effect of water subcooling, high-temperature steam properties, the radiation heat transfer and mixing zone boiling dynamics on the vapor film characteristics. Numerical investigation is performed using a novel CFD modelling concept named as the local homogeneous slip model (LHSM). Results of the analytical and numerical studies are discussed with regards to boiling dynamics under FCI conditions. (orig.)