Purpose: The aim of the paper is to investigate the buckling effects due to the seismic sloshing phenomena interesting for a next generation heavy liquid metal cooled reactor as for example the eXperimental Accelerator Driven System (XADS). Design/methodology/approach: In this study the structural buckling behaviour of a reactor pressure vessel, retaining a rather large amount of liquid and many internal structures, is coupled to the fluid-structure interaction because during a postulated earthquake (e.g. Design Basis Earthquake) the primary coolant surrounding the internals may be accelerated with a resulting significant fluid-structure hydrodynamic interaction (known as "sloshing"). Finite element numerical approach is applied because neither linear nor second-order potential theory is directly applicable when steep waves are present and local bulge appear with a marked decrease in strength of structure. Findings: The numerical results are presented and discussed highlighting the importance of the fluid-structure interaction effects in terms of stress intensity and impulsive pressure on the structural dynamic capability. These results allowed to determine the components mostly affected by the loading condition, in order to upgrade the geometrical design, if any, for the considered nuclear power plant (NPP). Research limitations/implications: The presented research results may be considered preliminary; thus it may be useful for a design upgrading of the reactor vessel and for achieving a first evaluation of the real components capacity to bear dynamic loads in particular in the event of a severe earthquake. Originality/value: From the point of view of the practical implication, it is worth to stress that the safety of liquid retaining nuclear structures subjected to a seismic loading is of great importance in regard to the hydrodynamic forces caused by sloshing and impulsive liquid motion determined by the liquid filling levels oscillatory phenomenon.