In this study, reactor cavity cooling systems (RCCSs) based on atmospheric radiation and atmospheric natural circulation were compared. The RCCS based on atmospheric radiation is only affected by temperature changes in the atmosphere. A typhoon or tornado has no effect because this RCCS has no air ducts to generate a chimney effect. Thus, it can be designed to maintain heat transfer surfaces at temperatures below 100 °C to prevent low-temperature ignition of combustibles and wind-driven fire (i.e., there is a large safety margin for temperature). In contrast, an RCCS based on atmospheric natural circulation is affected by temperature, wind speed, and humidity changes in the atmosphere. It is also affected by the decrease in the heat transfer coefficient from turbulent to laminar flow. It is designed to maintain heat transfer surfaces at over 250–300 °C to drive the chimney effect, which can lead to low-temperature ignition of combustibles and wind-driven fire. Based on the above comparison, a new RCCS based on atmospheric radiation is suggested because of the excellent degree of passive safety features, and the amount of heat removal by heat transfer surfaces can be controlled. This RCCS can be adopted by all types of small modular reactors, including high-temperature gas-cooled reactors and very-high-temperature reactors.
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