Experimental investigations of film boiling heat transfer on a sphere for evaluating thermal-hydraulic behavior of corium particle during fuel-coolant interaction

Abstract

Fuel-coolant interaction (FCI) is a thermal-hydraulic phenomenon under a severe accident, which occurs in a pressurized water reactor with a pre-flooded cavity. In this thesis, we performed experiments for film boiling on a sphere to describe the heat transfer of a fragmented corium particle that determines the dominant heat transfer in this phenomenon. We changed the major parameters such as the temperature of sphere, the liquid velocity, and the degree of subcooling for water in these experiments. Based on the experimental works, we suggested the models for natural convection and forced convection of film boiling on a sphere. For the natural convection film boiling, we improved the conventional correlation by considering the heat loss through the support rod. For the forced convection film boiling, we included the radiation effect for the heat balance at the vapor-liquid interface, which was neglected by the previous researches. With the experimental observations, we found out that the fraction of the wall surface with the thin vapor film depends on the Reynolds number, the wall superheat, and the subcooling condition rather than a constant value. From the two models developed in each regime, we proposed a unified heat transfer model for the film boiling on a sphere including mixed convection regime. The unified model was developed with a physical term (Gr/$Re^2$), which represents the degree of the natural or the forced convection. The unified current model (11.66%) produced more accurate results than the Liu-Thefanous model (21.99%) did in terms of the normalized root mean square deviation. We cooperately developed the transient calculation tool named COCOA (COrium COolability Analysis tool) to apply the developed model for the evaluation of the corium falling stage of FCI in APR1400. In COCOA, we set up the calcaultion procedure for the pressure and energy change induced by the heat transfer from the corium jet and particles during the corium falling stage of FCI. COCOA was validated against large-scale corium experiments and compared with COMETA developed by JRC Ispra. It turns out that COCOA produced better results than COMETA did. We evaluated the results of a hypothetical accident with the design parameters of APR1400 and 20, 100, and 200 tons of the corium. The maximum pressure during the corium falling stage of FCI in the containment was calculated as 2.6 bar, which is far from the design limit (~ 5 bar). When the corium is deposited at the bottom of the reactor cavity, its average temperature is 2126 K for the initially saturated water and 1648 K for the initially subcooled water.