Numerical heat transfer analysis of solidification of nuclear material droplets during molten fuel coolant interaction

Abstract

In the case of a severe accident in Sodium cooled Fast nuclear Reactor (SFR), when molten nuclear fuel comes in direct contact with sodium coolant, fragmentation and freezing can happen. This interaction is termed as Molten Fuel Coolant Interaction (MFCI). Analysis of MFCI is crucial in the safety analysis of a reactor because it influences post-accident heat removal rate from core debris settled on core catcher. In this work, a drop of molten nuclear material such as UO2, U-Pu-Zr and stainless steel, surrounded by a pool of quiescent coolant sodium is considered and its temperature evolution assessed. Temperature distribution within the droplet is obtained by solving transient heat conduction including phase change using a commercial CFD code Fluent. The radius of the drop and the cooling condition defined by heat transfer coefficient are varied to assess their role in solidification. In case of oxide fuel, analysis has also been done with water as coolant for the sake of comparison with water cooled reactors. Grid optimization has been done. Results are validated with analytical results. Liquid fraction and temperature contours within the droplet are obtained at different instants of time. The important output parameters are solidification time and time for the droplet surface to reach thermal equilibrium with the coolant. The results indicate that solidification of UO2 droplet is conduction controlled whereas in metallic fuel and SS, freezing is controlled by convective cooling at the surface. For a 4 mm diameter droplet, it is found that the solidification time is 1 s for UO2 while it is 0.1 s for metallic fuel in nucleate boiling regime of sodium. From the heat transfer analysis, a correlation has been developed between Stefan number and dimensionless solidification time.