During the course of a severe accident in a nuclear power plant with core meltdown,the molten fuel(corium)can interact with water and under certain circumstances lead to a dangerous explosive Fuel Coolant Interaction(FCI).The progression and consequences of FCI are highly dependent on the characteristics of the premixing stage,which consists in the coarse fragmentation of a corium jet and the produced droplets and their dispersion in the water pool.FCI is generally modeled using CFD codes with sub-grid models to compute jet and droplet fragmentation.However,the fragmentation mechanisms remain unclear even in isothermal cases without phase changes.Nowadays,it is possible to perform DNS(Direct Numerical Simulations)of isothermal liquid metal droplet breakup in a water flow.This allows us to study the mechanisms responsible for breakup.In our work,we use the open source code "Gerris" that uses the VOF-PLIC method to capture accurately the dropletwater interface.An adaptive mesh refinement is used to increase the spatial resolution around the droplet interface and in zones where the gradient of vorticity and/or the interface curvature are high.After a strict validation stage,different refinement criteria were chosen to capture the highly deformed droplets,the daughter small droplets and also the complex dynamics in and outside the droplet.New breakup regimes have been identified thanks to simulations carried at different Weber numbers,where droplet-"vortex ring" interaction seems to be prevailing in their transition.The conversion of small VOF droplets to Lagrangian particles ensures mass conservation,save of computational resources and an accurate prediction of the droplet size distribution.Results will be compared to experimental results of Fields metal liquid droplet falling in a water pool.These experiments are performed on a newest drop-on-demand device at our laboratory(LEMTA,France).