[en] During the course of a hypothetical severe accident in a light water reactor, molten liquid may be introduced into a volatile coolant, which, under certain conditions, results in explosive interactions. Such fuel-coolant interactions (FCI) are characterised by an initial pre-mixing phase during which the molten liquid, metallic or oxidic in nature, undergoes a breakup (fragmentation) process which significantly increase the area available for melt-coolant contact, and thus energy transfer. Although substantial progress in the understanding of phenomenology of the FCI events has been achieved in recent years, there remain uncertainties in describing the primary and secondary breakup processes. The focus of this work is on the melt jet and drop breakup during the premixing phase of FCI. The objectives are to gain insight into the premixing phase of the FCI phenomena, to determine what fraction of the melt fragments and determine the size distribution. The approach is to perform experiments with various simulant materials, at different scales, different conditions and with variation of controlling parameters affecting jet and drop breakup processes. The analysis approach is to investigate processes at different level of detail and complexity to understand the physics, to rationalise experimental results and to develop and validate models. In the first chapter a brief introduction and review of the status of the FCI phenomena is performed. A review of previous and current experimental projects is performed. The status of the experimental projects and major findings are outlined. The first part of the second chapter deals with experimental investigation of jet breakup. Two series of experiments were performed with low and high temperature jets. The low temperature experiments employed cerrobend-70 as jet liquid. A systematic investigation of thermal hydraulic conditions and melt physical properties on the jet fragmentation and particle debris characteristics was performed. The coolant temperature was found to significantly affect the shape and size of the debris. The maximum fragment size was found to increase with reduction in coolant temperature. No effect of coolant voiding on the fragment size distribution was observed. A series of high temperature melt jet experiments were performed, in the MIRA-20L experimental facility. Three types of oxidic melts, namely; CaO-B₂O₃, MnO-TiO₂ and WO₃-CaO were employed as melt jet liquid. The melt jet fragmentation was classified into two regimes, the hydrodynamic-controlled regime and the solidification-controlled regime. The delineation between those regimes was observed from the size characteristic and morphology of the solidified debris which was formed. The temperature of the coolant was the primary parameter in determining which regime the jet fragmentation would fall into. It was found, at low subcooling, the fragments are relatively large and irregular compared to smaller particles produced at higher subcooling. The melt density was found to have significant effect on the particle size. The mass mean size of the debris changes proportional to the square root of the coolant to melt density ratio. A systematic investigation of the performance of statistical distributions which may be used to describe the size distributions of fragments obtained from molten fuel coolant interaction (MFCI) experiments was performed. The statistical analysis of the debris produced in both experiments showed that the sequential fragmentation theory fits best the particle distribution produced during the jet fragmentation process. In the second part of the second chapter, analysis of the jet breakup experiments are performed. The low temperature jet fragmentation experiments are simulated with a recently developed Multiphase Eulerian Lagrangian Method. The effect of particle diameter and particle drag on the jet dynamics and penetration behavior is investigated. The third part of the second chapter deals with simulation of Kelvin-Helmholtz instabilities. A high order Navier-Stokes solver is employed along with the front tracking Level-Set algorithm, to eliminate numerical diffusion. The effect of surface tension and viscosity on the development of instabilities is investigated. Three regimes are identified, and delineated, based on Weber and Ohnesorge numbers. The third chapter is devoted to breakup of liquid drops in water. The emphasis is directed towards delineating the roles which melt to coolant heat transfer, melt solidification, melt fusion heat and melt mushy zone play in the fragmentation process. Coolant temperature is found to have a significant impact on the droplet fragmentation behaviour for subcooled conditions. The melt superheat greatly affects the characteristic time for solidification, and thus strongly affects the deepness of the fragmentation process. The fusion heat of the eutectic melt contributes significantly to the solidification time scale, and thereby enables the eutectic melt drop to feature deeper fragmentation. The presence of the mushy zone during the phase change of the non-eutectic melts significantly prevents these melt drops from completing the deformation and fragmentation process, especially when the melt superheat is small. An instability analysis on the crust breakup was performed. A modified dimensionless Aeroelastic number Ae_* is obtained as a criteria for breakup of the plain crust. It is found that the modified Aeroelastic number can be employed to evaluate the breakup behaviour of a droplet with a thin solidified layer on its surface