Numerical study on melt drop collision and hydraulic fragmentation during FCI of a nuclear reactor severe accident

Melt drop collision is an important phenomenon in the circumstance of pressure wave propagation during fuel–coolant interaction (FCI). The deformation and fragmentation of melt drops can increase the contact area with coolant, and as a result will affect the heat transfer and melt oxidation. In this study, a numerical model was established by considering surface tension and validated with the experimental results that were obtained from water droplet collision in gaseous environment. Then, the head-on collision of two UO₂ melt drops in water pool was investigated at different Weber numbers, and the melt morphology, contact area, and the number and size distribution of children droplets were analyzed. The results show that interfacial waves and wrinkles could be clearly observed on the melt film, and the finger structures presented at the rim of melt film and finally separated into children melt droplets. The increase of fuel–coolant contact area could be divided into the inducements by deformation and fragmentation. The melt deformation only had influence to the area increase at intermediate process but no effect to that at steady state. By contrast, the final fuel–coolant contact area was determined by the intensity of melt fragmentation. The size distribution of melt children droplets was like an off-centered normal distribution, where the maximum number of children droplets existed in the size range of 0.03 < D/D₀ < 0.045, but the droplets in this size range contributed little to the fuel–coolant contact area. With the above analysis, an empirical model to calculate the contact area was developed. The model had a discrepancy of ±10% in the range of We_c = 290–1815 that the model was established in, but the discrepancies were about 13.3% and 19.3% for the cases of We_c at 2196 and 2612, respectively.