In the present paper a comparative analysis of the thermalhydraulic performance of helical coil and bayonet tube steam generators (SG) is presented. The tool chosen for the analysis is RELAP5-3D/4.0.3. The reference conditions are the ones of the primary and secondary fluids of the SMART Small Modular Reactor. The analysis has been carried out by considering different operating conditions. First, the performance of the SGs in nominal conditions has been compared; subsequently, assuming a power load control at constant average primary temperature, the thermal hydraulic response of the components at different operating conditions has been studied and the helical coil SG results to be the most compact configuration. It’s behavior is characterized by higher frictional pressure losses and lower inner heat transfer coefficient if compared with the bayonet tube SG. To analyze the performance of the components out of the nominal condition a region of the map of operation is studied by varying the inlet temperatures of the fluids. A common behavior is found in the considered region. Differences of approximately 12% for the outlet temperatures and 6% for the power have been found
