The concept of in-vessel retention (IVR) is the stabilization of the reactor pressure vessel (RPV) by cooling the outside vessel wall with water in a severe accident. This study offers detailed information on complex thermal hydrodynamic phenomena, including boiling and two-phase flows when external reactor vessel cooling through flooding of the RPV occurs. The thermal hydrodynamic phenomena in an external reactor vessel are simulated using a computational fluid dynamics (CFD) technique with a phase change heat/mass transfer approach for boiling and the volume of fluid (VOF) approach for two-phase flows. The simulation results give a better understanding of the heat transfer from molten corium to the coolant outside the vessel and of the natural circulation mechanism of cooling water, boiling, and two-phase flows in this system. Also, this study provides quantitative data, such as heat flux, natural circulation flow rates, and heat transfer coefficients at various cooling water temperatures.

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