The sodium-cooled, Scalable Liquid Metal cooled small Modular (SLIMM) reactor, developed at the University of New Mexico, can generate 10 to 100 MWth without refueling for many years. At 100 MWth, the operation life estimate is ∼ 5.8 full power year (FPY) and as much as 33.3 FPY when operating at 20 MWth. Natural circulation of liquid sodium cools the reactor core during nominal operation and after shutdown, with the aid of an in-vessel chimney (2-8 m tall) and a helically coiled tubes Na/Na heat exchanger (HEX) that serves as the in-vessel heat sink. The liquid sodium enters the core at 610 K, and exits at ≤ 820 K. At these temperatures, sodium corrosion of the HT-9 steel for the UN fuel rods cladding, the core structure, and the reactor vessel is not a concern. Developed is a natural circulation model to simulate the performance of the SLIMM reactor during nominal, steady-state operation. The model solves the coupled momentum and energy balance equations for the in-vessel circulating liquid sodium and the Na/Na heat exchanger. It calculates the total flow rate and exit temperature of the circulating liquid sodium through the core as functions of the reactor thermal power, the chimney height, and the HEX design (total height, number and dimensions of the concentric helical coils, and the coils tube diameter and pitches).