Out-of-pile experiments were performed on fuel configurations, having a geometry similar to that of reactor fuel elements to study the thermal interaction between molten UO2 and subcooled sodium. The fragmented fuel generated was investigated by means of a scanning electron microscope. The composition of the fuel particles and the test results, especially the pressure pulses measured, lead to the following conclusions concerning the processes taking place. The liquid fuel escaping from the fuel rods has already been coarsely dispersed by the escaping filling gas. Finer fragmentation is mainly caused by two factors. The first are mechanical stresses occurring while the pieces solidify; the shell-shaped particles show that liquid fuel had been expelled from the interior of the pieces during the process of solidification. Second, coarser pieces of fuel were fragmented by small amounts of penetrating liquid sodium that was superheated and subsequently evaporated. The measured pressure pulses are due to rapid evaporation of this entrapped sodium.