The nuclear reactor and steam generator, also known as the Nuclear Steam Supply System (NSSS), is a self-contained assembly of reactor core and steam generator tube bundles within a single pressure vessel. Throughout the design, every effort was made to employ existing off-the-shelf technologies to minimize, and in many cases eliminate, the need for additional research and development. The primary coolant (water) is moved by natural circulation, eliminating the need for primary coolant pumps and external power. The NSSS and the passive safety heat removal systems are housed within the compact steel containment.

The NuScale reactor operates using the principles of natural circulation. No pumps are needed to circulate water through the reactor Instead, the system uses a convection process. Water is heated as it passes over the fuel or core.

Water in the reactor system and the Steam Generator system are kept separate. As the hot water in the reactor system passes over the hundreds of tubes in the Steam Generators, heat is transferred through the tube walls. Water inside the tubes turns to steam. The steam turns turbines (7) which are attached by a single shaft to the electrical generator.