In advanced nuclear reactors and small modular reactors (SMRs), safety during accidents like loss-of-coolant accident, station blackout, etc. is achieved by passive safety systems. The passive containment air cooling system (PCACS) is one of the safety systems used to maintain containment integrity during accidents. Steel containment is preferably used in advanced nuclear reactors, including SMRs. The steel containment acts as a heat exchanger during accidents, and PCACS uses natural convection air flow to remove decay heat for an indefinite period. It is very important to understand the natural convection air flow around the containment so as to design and evaluate the performance of the PCACS.

With this objective, a numerical investigation was performed for the experimental facility reported in the literature. A computational fluid dynamics study with different viscous models has been performed. A standard k-ε model was found to be in good agreement with the experimental results. The difference between the numerical study and the published experimental data was less than 7% for this model. The natural convection characteristics of air around the containment were investigated based on temperature, velocity, and streamline contours.

The effect of various parameters, such as ambient air temperature, containment surface temperature, gap between chimney and containment, chimney height, and chimney diameter at outlet, on the air thermal hydraulics and performance of PCACS is presented. The heat transfer rate was found to decrease with an increase in the ambient air temperature. The optimum value for the gap between the chimney and the containment was found to be 18 mm for the present work. With an increase in the chimney height and containment surface temperature, the heat transfer rate was found to increase. The optimum value for the chimney outlet diameter was found to be 240 mm; however, the value can be selected between 140 and 280 mm. This study will help in evaluations of the performance of real-size containments for a better understanding of natural convection flow around the containment, ensuring improvements in safety.