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In FCIs, the fragmentation process is a key factor to determine the ratio of heat transferred to power. So, in this paper, the fragmentation process of melt droplets triggered by boiling effect is investigated. The dynamic boiling process is modeled by writing a momentum equation for vapor film dynamics, an energy equation for each region of the droplet, coolant vapor and liquid and linking each region by the appropriate boundary conditions. The integral approach is used in each region for the energy equations where the differential equation is integrated over the region and a temperature profile is assumed. By using these developed models, the fragmented mass of the droplet triggered by boiling effect can be calculated. The result shows that the fragmentation rate is large than that given by hydrodynamic model. The parameter, which is related to the droplet condition and the acceleration exerted on the droplet surface, is a function of pressure in the vapor film. 1. INTRODUCATION Fuel Coolant Interactions (FCIs) are the important phenomena during the Core Disruptive Accidents (CDAs) in LMFBRs or Core Melt Accidents (CMAs) in LWRs. The fragmentation process is a key factor to determine the ratio of heat transferred to power in FCIs, which estimated possible damage degree. But at present the fragmentation mechanism is not yet clear, the corresponding physics and mathematics model has not been set up completely, and this greatly restricts the practical application of research results. The fragmentation mechanisms of melt droplets under different conditions have been studied for many years. Corradini (1988) reviewed the mechanisms and models and categorized the fragmentation mechanisms into two classes; hydrodynamic and thermal effects. The hydrodynamic fragmentation occurs when the melt droplets are subjected to external surface forces sufficient to overcome the droplet surface tension. Several kinds of fragmentation mechanisms have been proposed for the hydrodynamic effects: Rayleigh-Taylor instability, Kelvin-Helmholtz instability and boundary layer stripping. The thermal fragmentation mechanisms include boiling effects, internal pressurization and surface solidification effects. In thermal mechanism, the internal forces break the droplet into parts (fragments). When the melt droplets are released into coolant, due to the evaporation of the coolant, a vapor film is formed to surround the droplet, as shown in Nelson’s experiment (1985). The behavior of the vapor film is related to the temperatures of the droplet and the coolant, ambient pressure and properties of the coolant and the droplet. The objective of this study is to develop a fragmentation model induced by the boiling effect, which is supposed to be one of the thermal fragmentation mechanisms. Copyright © 2007 by JSME 1 The Japan Society of Mechanical Engineers NII-Electronic Library Service 2. DEVELOPMENT OF MODEL 2.1 Acceleration Mechanism When a droplet is immersed into coolant liquid, a vapor film is formed to surround the droplet. At the beginning of the immersion, because the temperature of the droplet is over the minimum film boiling temperature (MFBT), the vapor film is stable. When the surface temperature becomes lower than MFBT, the nucleate boiling heat transfer is featured at this stage. In this transition region, coolant periodically contacts the droplet surface. The transition can be very violent and continue until the nucleate boiling regime is attained at point of DNB. The liquid-liquid contact induces the high pressure in the vapor film. Due to the change of heat transfer from film boiling to nucleate boiling, the vapor film becomes unstable and a pressure pulse is generated at the vapor film collapse, which is exerted on the droplet surface and may induce the breakup of the droplet. 2.2 Vapor Film Collapse In this study, the vapor film collapse is modeled, which is similar to the method developed by Corradini (1981, 1983). The vapor film collapse model due to heat transfer change is simulated by adding a heat transfer flux term in the boundary condition of energy. The generation of the pressure during the vapor film collapse is related to the heat transfer change, the temperatures and the properties of the droplet and the coolant, and the system pressure. The analytical model is shown in Fig.1. The dynamic vapor film collapse process is modeled by writing a momentum equation for vapor film dynamics and an energy equation for each region, as shown in Fig.1: droplet, coolant vapor and coolant liquid, and linking each region by the appropriate boundary conditions. The equilibrium is assumed on the interface between the vapor and coolant liquid. The integral approach is used in each region for the energy equations where the differential equation is integrated over the region and a temperature profile is assumed. Fig.1 The analytical model for vapor film collapse The momentum equation for vapor film growth is modeled by the Rayleigh equation that neglects the effects of viscosity in a spherically symmetric system. δ δ ρ R P P R U dt dU

ICONE15-10103 STUDY ON THE FRAGMENTATION OF MELT DROPLET TRIGGERED BY BOILING EFFECT