Nuclear reactors have to be maintained in a critical state so as to keep the chain fission process stationary and under control. Nuclear stability considerations dictate that the geometry of the core be closely controlled at all times: therefore any modification of it must be predictable, compatible with the requirements of the interfacing reactor systems and safely manageable by (intrinsic and engineered) control mechanisms. This study deals with the evaluation of the deformation of core (and restraint system) geometry due to dynamic perturbations. This deformation may determine, at large or small extent, an assembly compaction, that is generally characterized by a radial inward displacement and, eventually, results in an insertion of reactivity. To the aim it is of meaningful importance to set up and develop an overall and reliable methodological approach to be used in designing the core system (all structures and components characterizing the core region) and evaluating its performance under operation and accident condition. In particular a LMR reactor configuration similar to the Advanced Lead-cooled Fast Reactor European Demonstrator - ALFRED (300 MWth) has been considered. The assessment of the dynamic behaviour of a LMR core is particularly needed for seismic design purposes: these solicitations could deform the core system and fuel assembly. A preliminary finite element model, in which all the core sub-assemblies were represented as masses distributed on the supporting plate, was carried out in order to investigate the dynamic response of the structures once confidence was established by sensitivity analyses of size and type of the adopted elements. The preliminary results indicate that the core region is undergoing local deformations (of about 3 cm) that could influence the normal reactor operation. Although any deformation influences the normal reactor operation, it is expected that the reactivity specifically related to this deformation will not pose concerns to the safe manageability of the associated abnormal operation.