Abstract: The fuel coolant interaction (FCI) may lead to a steam explosion during a severe accident in a nuclear power plant, which poses a threat to the core components, pressure vessel, and even the reactor containment integrity. However, the fuel coolant interaction is a complex multiphase flow process involving coupled heat and mass transfer and energy conversion phenomena, some physical mechanisms remain to be studied. Jet fragmentation is the key physical phenomenon in the pre-mixing phase during FCI progress, and it greatly enhances the possibility of steam explosion and has a profound influence on the explosion intensity after steam explosion. Thus prediction models of jet fragmentation are very important in the nuclear safety analysis. Based on the molten jet fragmentation mechanism test and theoretical analysis, this paper proposes a jet fragmentation prediction model, and further completes the jet fragmentation prediction and validation by some typical reactor prototype materials FCI tests. In this paper, an experimental apparatus named test for interaction of melt with coolant (TIMELCO) was built to study the molten jet fragmentation behavior of molten tin and stainless steel jets falling into the water. Through the high-speed photography observation, the superheat of the jet and the subcooling of coolant directly change the boiling state of the melt jet, thus changing the jet fragmentation progress. In addition, debris median diameter and breakup length were obtained to gain a deeper understanding of the molten jet fragmentation process. Based on key test phenomena and experimental results, the mechanisms of jet fragmentation under different film boiling conditions were proposed. The model of film boiling on a jet surface was established under different film boiling conditions which consist of laminar flow film boiling and turbulent flow film boiling. Then the model of unstable wave growth on the jet surface was established considering the double interfaces with jet-steam interface and steam-water interface based on the Kelvin-Helmholtz instability theory. Further considering the unstable wave fracture and melt separation, a complete hydrodynamic fragmentation model of melt jet was established. The hydrodynamic fragmentation model was used to predict the jet fragmentation behavior under laminar flow film boiling and turbulent flow conditions. The predicted results show that the predicted debris median diameter and breakup length are in agreement with the experimental results. Furthermore, this prediction model was validated by some typical prototype material jet fragmentation tests, such as KROTOS and FARO. The validation results show the applicability of the model in predicting the molten fuel jet fragmentation.