What are some of the NuScale reactor’s key inherent safety features?
The NuScale design shuts down and self-cools indefinitely with no operator action, no AC or DC power, and no additional water. It replaces many engineered backup systems (such as motors, pumps and valves) with features that operate automatically by relying on natural phenomena such as gravity, convection and conduction.
The reactor operates using the principles of gravity and natural circulation, so pumps aren’t needed to circulate water through the reactor. Instead, the system relies on a natural convection process. Water at the bottom of the reactor vessel heats up as it passes over the core and pulls heat from the fuel. The hot water then rises within the reactor vessel. Once the heated water reaches the top, it passes over hundreds of tubes in the steam generator, heat is transferred through the tube walls and water inside the generator tubes turns to steam.
When water at the top of the reactor vessel has transferred its heat through the generator tubes, it becomes cooler and denser. Gravity pulls the cooler water back down to the bottom of the vessel where it is again drawn over the core, beginning the cycle again.
The plant does not rely on AC or DC power for safety, which simplifies the electrical systems. In normal operation, the plant relies upon electric power to maintain motors, pumps, valves and control rods in the operating position. In the case of an accident that disrupts the plant electricity, the valves and control rods move to their safe configuration by gravity or pressure differentials, and core heat exchange is achieved by convection, conduction and gravity.