A nonlinear dynamic model for a passively cooled small modular reactor (SMR) is developed. The nuclear steam supply system (NSSS) model includes representations for reactor core, steam generator, pressurizer, hot leg riser and downcomers. The reactor core is modeled with the combination of: (1) neutronics, and (2) thermal-hydraulics. For the helical-coil once-through steam generator, a single tube depiction with time-varying boundaries and three regions, i.e., subcooled, boiling, and superheated, is adopted. The pressurizer model is developed based upon the conservation of fluid mass, volume, and energy. Hot leg riser and downcomers are treated as first-order lags. Finally, the NSSS model is incorporated with a turbine model which permits observing the mechanical power. The overall nonlinear system is simulated for typical perturbations, e.g., control rod withdrawal and increase in steam demand. A detailed analysis of the results show that the steady-state values for full power are in good agreement with design data and the model is capable of predicting the dynamics of the SMR. (C) 2016 Elsevier Ltd. All rights reserved.