The most severe consequence of a pressurized water reactor in-vessel steam explosion is a molten fuel slug impact against the head of the reactor pressure vessel that could cause a failure of this head and lead to missiles endangering the reactor containment. An investigation is described that attempts to determine the maximum slug impact that a vessel head is capable of withstanding without failing and, consequently, without impairing the containment safety-related function. Preliminary theoretical assessments are presented that suggest that the head might be able to withstand rather strong impacts and that the shape of the fuel slug will have only a moderate influence on the results, provided the upper internal structures are taken into account. A low sensitivity against the slug shape is an essential prerequisite for a reliable safety proof. However, investigations primarily based on computational models are not sufficient; therefore, an investigation concept is proposed that relies on model experiments in which the geometry is scaled down by factors of 10 and 20, respectively. Theoretical and experimental investigations for liquid-structure impact problems in different scales are discussed to assess the degree of similarity that can be obtained. Finally, model experiments are described in some detail simulating the molten fuel slug impact on the vessel head.