The Lead Cooled Fast Reactor (LFR) is one of the six concepts selected by the Generation IV International Forum (GIF) as candidates for the long term evolution of nuclear technology. Due to the significant technological innovations it implies, the European Sustainable Nuclear Energy Technology Platform (SNETP) recognized that LFR complete development requires the realization of a demonstration plant (DEMO) as a fundamental intermediate step. In this paper, a preliminary approach to the simulation of DEMO primary system dynamic behavior is presented. The need of investigating reactor responses to temperature transients has led to a simplified model reckoning with all the main feedbacks following a reactivity change in the core, which have been calculated by means of ERANOS deterministic code ver. 2.1 coupled with JEFF3.1 data library. A lumped-parameter approach has been adopted to treat both neutronics and thermal-hydraulics: indeed, the point-kinetics approximation has been employed and an average-temperature heat-exchange model has been implemented. Due to the latter, the dynamic mechanical behavior of DEMO core — modeled as a cylinder — has been addressed by considering expansions and contractions instantaneous with temperature variations, i.e. neglecting mass inertia effects. The very simple linearized model treated in the present work turns out to be a helpful tool in this early phase of the DEMO pre-design, in which all the system specifications are still considered to be open design parameters, since it allows a relatively quick, qualitative analysis of dynamics and stability aspects that cannot be left aside when refining or even finalizing the system configuration.