Sodium-cooled fast reactors (SFRs) have emerged as a significant advancement in nuclear engineering, carrying crucial implications for the power industry. In the event of a severe core meltdown accident, the reactor vessel's integrity can be compromised due to energetic interactions between the coolant and the molten core, leading to vapour explosions. Understanding and predicting the physical processes during melt-coolant interaction are paramount for ensuring safety. The first part of the thesis presents the importance and role of SFRs in the power industry, outlining their key technological characteristics. It discusses fundamental approaches to nuclear safety and explores the course of a hypothetical severe core melt accident, along with potential mitigation strategies, with a specific focus on the interaction between the reactor coolant and the molten core. The second part focuses on film boiling around a melt particle in a subcooled coolant flow. A numerical model is developed using computational fluid dynamics, employing a single-fluid formulation of a two-phase system with interface surface tracking. The main contribution of the master thesis is the development and justification of an original phase change model ELMEC (Enhanced Lee Model for Extreme Conditions). Additionally, the study thoroughly investigates the performance of the Lee evaporation and condensation model for film boiling simulations in a subcooled flow. Both models are validated using analytical solution of the Stefan problem and experimental data.
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