The Farahat sodium natural convection film boiling experiment revisited

With the renewal of interest for sodium-cooled fast reactors, looking at what is known from the past, it appears - in the frame of severe accident studies in general and FCI in particular, that very few is known about sodium film boiling around hot fuel droplets which is a condition allowing premixing. The past Farahat experiment (Reynolds et al., 1976), performed in 1971, in which hot solid spheres were transferred into sodium has been revisited. Looking in the detailed results, it appears that a phenomena has been ignored, i.e. the existence of two film boiling regimes as observed in similar experiments with water (Honda et al., 1995; Bradfield, 1966). These two film boiling regimes have been analyzed since the transition between these regimes could lead to the onset of spontaneous explosions if the melt is still liquid at this transition. A simple model has been built for the estimation of this transition point. As this model is able to describe the transition observed in Sn-H2O experiments (Sher, 2012), it has been used for UO2-Na systems.