The flow dynamics in the upper core structure (UCS) during the expansion phase of a liquid-metal fast breeder reactor core disruptive accident were investigated experimentally and numerically. A simulant material experiment was designed to verify some of the thermal-hydraulic models in SIMMER-II. The experiments showed the large effect of the heat transfer in the UCS and the relatively small effect of friction. The reduction of the work potential of the expanding fuel by the presence of the UCS is shown as a function of the initial pressure and the temperature difference between the core and the UCS, both for simulant materials and UO2 fuel. It is described how the experimental data can be extrapolated to prototypical conditions, which phenomena modeled in the code predictions of SIMMER-II are different for simulant and prototypical transients, and how the experimental results compare to effects of prototypical phenomena that could not be modeled in the experiment.