A CFD-based design optimization of air-cooled passive decay heat removal system

The concept of the APDHR (Air-cooled Passive Decay Heat Removal) system was suggested to preserve the safety of a nuclear reactor during accidents. Until 3 days after the reactor shutdown caused by non-LOCA accident, water in the Passive Condensate Cooling Tank (PCCT) was used to condense the steam in secondary side. Since then, the steam was cooled by natural convection of air passively and indefinitely. The focus on this study is on the system performance during the natural convection period. Both, finned and bare heat exchangers (HXs) were considered for the design optimization of the APDHR. As a result, fin height of 4 cm, fin spacing of 4 cm and fin thickness of 0.2 cm was determined as a reference fin geometry regarding heat removal capacity and economic fin installation. Then, the sensitivity of several design parameters of APDHR, such as pitch, height, wall temperature of the HXs and the interval of spacer grids, was checked and determined in a viewpoint of better heat removal capacity and compact construction. The pitch between HXs was determined as 20 cm with the outer diameter of HXs were 5.08 cm, and the height of the HXs was decided as 10 m through the height sensitivity study. Based on the results, the numbers of HXs and PCCTs were analyzed considering the decay heat of 3 days after the shutdown to suggest the overall design of the APDHR. The heat transfer coefficients were 10.07 W/m2K and 15.76 W/m2K in the case of the bare and the finned HXs, respectively. Therefore, the numbers of HXs and PCCTs required can be reduced by using the finned HXs.