LAUSR

Abstract Much effort has been devoted by the nuclear community to develop mechanistic models to predict the heat dissipation of fuel rods during a loss of coolant accident (LOCA). However, there is still the challenge to find correlations that accurately predict the rates of heat transfer corresponding to each involved phenomena and gather them all in a single physical model. Looking for improving thermal-hydraulics calculations in LOCA conditions, this work introduces a novel mechanistic model implemented in a new code named NECTAR to calculate heat and mass transfer phenomena and droplets dynamics in a polydispersed flow film boiling. The simulation results are compared with experimental measurements using three different geometries that represent the cladding ballooning at sub-channel scale. NECTAR can predict with great accuracy the heat dissipation by the internal steam-droplets flow with less than 8% mean deviation for all the test cases. The droplets characteristics downstream of the heated tube are also well predicted by NECTAR. In a detailed analysis of each heat transfer process, we found that wall-to-steam convection plays the major role in the internal heat dissipation, followed by the heat removed by the droplets impact onto the wall in spite of the very low volume fraction of droplets. Moreover, droplets impact and steam-to-droplets convection are also the main responsible for the droplets evaporation. Finally, considering the droplets diameter polydispersed distribution into thermal-hydraulics calculations (instead of a single mean diameter) has an effect only for higher volume fractions of droplets.