A review on advancements in passive decay heat removal system of advanced nuclear reactors

The Fukushima Daiichi accident highlighted the necessity of inherently safe, fully passive systems capable of sustaining core cooling without external power or operator action. Passive Decay Heat Removal Systems (PDHRS) play a pivotal role in extending post-shutdown grace periods and ensuring long-term reactor safety. This paper presents a comprehensive review of advanced PDHRS concepts and their integration within next-generation reactor designs, including Small Modular Reactors (SMRs) and Generation IV systems. The review examines design principles, operational mechanisms, and thermohydraulic behavior of key passive cooling technologies such as natural-circulation heat exchangers, heat pipes, and phase-change materials (PCMs). Emphasis is placed on experimental and numerical studies addressing performance, scalability, and reliability under steady-state and transient conditions. Literature findings confirm that heat-exchanger-based systems remain the most mature and widely implemented PDHRS solutions, while latent-heat devices—particularly heat pipes and PCMs—show strong potential for compact and modular microreactor applications. The paper concludes by outlining critical research needs related to scaling laws, material compatibility, and long-term stability, offering guidance for the development and deployment of next-generation passive decay heat removal technologies.