Heat Transfer Performance Analysis of Sodium-to-Air Heat Exchangers in the Decay Heat Removal System of STELLA-2 Using a 1-D System Code

Sodium Integral Effect Test Loop for Safety Simulation and Assessment (STELLA-2) is a comprehensive sodium thermal-hydraulic test facility developed by the Korea Atomic Energy Research Institute (KAERI) to evaluate the performance of safety systems in a sodium-cooled fast reactor (SFR) and the interaction between decay heat removal systems and the primary heat transfer system. In this study, the heat transfer performance of the FHX (finned-tube-type sodium-to-air heat exchanger) and AHX (helical-type sodium-to-air heat exchanger) within the decay heat removal system of STELLA-2, was analyzed using the GAMMA+ 1-D system code. Each heat exchanger was modeled by nodalizing its components and reflecting the heat transfer characteristics according to its structure in the GAMMA+ input file. Initially, a simulation based on design values provided by KAERI showed discrepancy in heat removal rate (Q) by 1.6% on the tube side and 8.0% on the shell side for the FHX, and 5.7% on the tube side and 2.7% on the shell side for the AHX, confirming that the GAMMA+ modeling adequately aligns with the design conditions. Based on this verification analyses were conducted using experimental data under natural and forced circulation conditions to evaluate the actual performance of the heat exchangers. The results revealed larger differences compared to the design conditions, with GAMMA+ overpredicting heat transfer performance. This difference is mainly due to the limitations of empirical correlations used within GAMMA+. Future studies will focus on enhancing GAMMA+ by improving its input file to provide more accurate predictions of heat exchanger performance. In addition, transient performance evaluations will be conducted to further validate GAMMA+, aiming to enhance the reliability of SFR operational safety evaluations.