The Sodium Test Loop for Safety Simulation and Assessment (STELLA) program for demonstration of decay heat removal performance of the Prototype Generation-IV Sodium-cooled Fast Reactor (PGSFR) is in progress at Korea Atomic Energy Research Institute. As the first phase of the program, the STELLA-1 facility has been constructed, and separate-effect tests for the sodium heat exchangers of the safety-grade passive decay heat removal system (PDHRS) have been conducted. A natural-draft sodium-to-air heat exchanger, one of the key heat exchangers of PDHRS, was tested for the performance demonstration and the design code verification and validation. Twenty-nine cases of experiments were conducted with 13 different test conditions for the selected operating and design conditions of PGSFR. Heat transfer rates were experimentally estimated based on the measured inlet/outlet temperatures and flow rates of both the shell side and the tube side. The experimentally obtained heat transfer rates were compared with the values calculated from the design code, which showed good agreement within a 12.6% error range. Finally, the average Nusselt number was obtained from the experimental results considering the convection mode.
