This paper deals with the treatment of methods for a preliminary safety assessment of next generation nuclear power plant (of Liquid Metal cooled Fast Reactor types) structures. As an example, it was considered the European Lead-cooled System (ELSY) international project response under a reference design basis earthquake (Safe Shutdown Earthquake-SSE) excitation, taking into account also the main isolating device effects in order to increase the reactor safety. This study is intended to analyze the structural effects of a reference SSE by means of an appropriate dynamic FEM code, for possible geometries of both not isolated and isolated ELSY containment building foundation cases. A great attention should be focused too on the arisen hydrodynamic forces and coupling effects between fluid and structures (sloshing phenomenon) that may impair the structures resistance and/or operating capabilities. The obtained numerical results were critically analysed with the intent also to contribute to a step of the safety optimization of the mentioned systems. (C) 2010 Elsevier Ltd. All rights reserved.