Steam generators are heat exchangers that turn water into steam—often using the heat from a nuclear reactor core—which then generates electricity. They can be quite large in size, ranging up to 70 feet tall and 800 tons in weight, especially among those found in industrial plants, and are commonly used inside pressurized water reactors (PWRs) as a means of removing the heat created by nuclear fuel reactions. Pressurized water reactors are one of the most common types of nuclear power reactors and are widely used in US power stations and marine applications.
Inside a pressurized water reactor, water serves as the primary coolant as it courses through the nuclear reactor core. Water is pressurized to prevent boiling, and obtains its heat from the reactor core. The path the water follows through the reactor core is often referred to as the primary loop—a pressurized water reactor primary loop can consist of 3,00 to 16, 000 individual tubes. After coursing through the core, the water continues through the steam generator, where the water is depressurized enough to enable boiling. As the water boils, the steam produced flows through a secondary loop, where it eventually is used to generate electricity. The steam is then re-condensed by cold water that is introduced via a third loop, and then it is re-circulated through the steam generator.
Typical steam generators in the USA can have one of two configurations. In the first configuration, pressure is maintained on the primary side of the system at 15.5 MPa, with the inlet water temperature at 315 degrees C. The outlet water temperature is maintained at 275 degrees C. Once the water reaches the secondary side of the system, that is, when the water leaves the primary tubes and enters into the steam generator, pressure is kept at 6.2 MPa and the inlet water temperature is kept at 220 degrees C. The outlet water temperature (or saturated steam temperature), is 275 degrees C.
The material used to create the tubing can vary, but often high-performance alloys, such as stainless steels, can be used. Other alloys, such as Alloy 400 and Alloy 600TT (thermally treated), are also applicable in steam generator tubing.