The sustainable development of nuclear energy depends in particular on its capacity to demonstrate important improvements in terms of safety and acceptance by the general public. In the case of Sodium Fast Reactors (SFR), some Fast Transients Over Power (FTOP) can be responsible of core melting. To make them acceptable, the safety approach generally relies on the demonstration that their occurrence can be practically eliminated. The CADOR (Core with Adding DOppleR effect) approach is quite different and proposes to preclude any excessive power excursion by designing a core with a large inherent Doppler reactivity feedback. The design of such a core is based on a physical analysis (1D Model) of main moderators available and of interest in maximizing the Doppler Effect. Calculations are also made with a full core and a 3D model to validate the first conclusions and the influence of the main parameters on the value of the Doppler Effect.To design the CADOR core, it is also necessary to minimize the fuel temperature in order to maximize the integral of the Doppler feedback coefficient between the nominal and the fuel melting conditions. The design of the core and the verification of its associated safety criteria are also depending on the precision of the Doppler Constant and of the fuel temperature evaluations. To estimate the importance of the fine prediction of these two parameters, three main transients are studied FTOP caused by bubble gas, FTOP caused by a break of the core support structure and an ULOF. Sensitivity studies present the consequences on the core or system design in order to keep the same safety objectives.