The SCK•CEN is developing a 100MWth multipurpose hybrid research reactor called MYRRHA. Heavy structures like MYRRHA reactor building founded on soft ground may experience significant differences in their seismic response with respect to the same structures founded on stiff rock. Soil-structure interaction problems are complex and involve wave propagation, soil and structural dynamics, soil constitutive behaviour and numerical modelling. It is important for the design to understand the soil structure interaction(SSI) effects of the reactor building founded on multi-layered soft soils housing safety critical structures, systems, and components (SSC) including reactor vessel when subjected to EL-1 (Earthquake Level 1, or Design Basis Earthquake (DBE)) and EL-2 (or Design Extension Condition Earthquake (DEC-E)). The output of the SSI analysis provides with floor response spectra (FRS) as seismic input motions for the SSC, especially, reactor vessel, as one of the most saftey critical structure, which contains heavy liquid metal LBE (Lead Bismuth Eutectic) and its safety related internals.