The Scalable Liquid Metal cooled small Modular (SLIMM) reactor design concept has recently been developed at the University of New Mexico's Institute for Space and Nuclear Power Studies. It generates 10100 MWth for 66 and 5.9 full power years, respectively, without refueling. Natural circulation of in vessel liquid sodium cools the reactor core during nominal steady-state operation and after shutdown, with the aid of an in-vessel chimney (2-8 m tall) and an annular Na/Na heat exchanger (HEX) of concentric helically coiled tubes. A natural circulation thermal-hydraulics model of the SLIMM reactor is developed to investigate the effects of thermal power, in-vessel chimney height, and HEX design on the circulation rate and core exit temperature of in-vessel liquid sodium and the pressure losses during steady-state operation. The HEX design parameters investigated are the number, pitch and tube diameter of the helical coils, and the temperature pinch between the in-vessel sodium flow in the downcomer and the secondary sodium flow through the HEX coils. Increasing the reactor thermal power or the in-vessel chimney height increases the flow rate of the in-vessel liquid sodium by natural circulation, but decreases its exit temperature from the reactor core. Conversely, increasing the number or the tube diameter of the HEX helical coils decreases the flow rate of in-vessel liquid and increases its exit temperature from the reactor core, and decreases the HEX total height. Depending on the steady-state reactor thermal power and HEX design, the in-vessel sodium flow through the reactor core contributes similar to 64-69% of the total pressure losses, while the sodium flow in the HEX contributes similar to 32-29%, respectively (C) 2016 Elsevier Ltd. All rights reserved.