How does natural circulation move coolant through some nuclear reactors?

In natural-circulation nuclear reactors, coolant movement is driven by temperature-induced density differences rather than mechanical pumps.1

This process relies on the principles of thermal convection. As the coolant absorbs heat from the fuel elements within the reactor core, it undergoes thermal expansion, becoming less dense than the cooler fluid elsewhere in the system. This density gradient forces the heated, lighter fluid to rise, while the cooler, denser fluid from the heat exchanger is fed into the bottom of the reactor to replace it. This continuous cycle effectively removes heat from the core through natural buoyancy.21

While many reactors rely on pumps for forced circulation, natural circulation is a key feature in specific marine reactor designs and is increasingly utilized in passive safety systems for modern reactors. These passive designs enhance safety by ensuring that cooling can be maintained even during a station blackout or equipment failure, often by utilizing gravity to circulate water from internal reservoirs.31