What is meant by passive cooling and natural circulation?
Natural circulation is a process that does not require active mechanical equipment, such as a pump, to move water through the reactor and keep it cooled. In the NuScale system, water is heated as it passes over the nuclear fuel. As it is heated the water becomes lighter and rises within the reactor vessel cylinder. As the water reaches the top of the cylinder, it is drawn down over the steam generator tubes where it passes its heat to a second enclosed system to produce the steam that makes electricity. When it transfers heat, the water cools, becoming more dense or heavy. Gravity pulls the heavy water down the outside of the cylinder where it is then drawn back inside the cylinder by the heat of the fuel. The overall result is a natural circular flow of water during both normal operations and when the system is shut down.
