File:Generic PWR OTSG.svg

English: There are two loops in a PWR steam generator, the primary loop and the secondary loop. The primary loop connects the steam generator with the reactor, and the secondary loop connects the steam generator with the turbine. This is a once-through steam generator (OTSG), where secondary water exits the secondary loop as slightly superheated steam. The OTSG is a Babcock & Wilcox design.

The flowpath for the primary loop is typically:

1. Primary coolant flows over the fuel bundle in the reactor and is heated.
2. Primary coolant leaves the reactor vessel through the hot leg.
3. Primary coolant enters the upper bowl of the steam generator (the channel head).
4. Primary coolant flows through the "tube side" of the steam generator, inside the steam generator tubes, which are welded to the steam generator tubesheet.
5. As coolant flows through the tubes, heat is transferred to the secondary system water (which surrounds the outside of the tubes)
6. Coolant exits the tubes into the lower bowl.
7. Primary coolant exits the steam generator cold leg and is returned to the reactor vessel, where it is re-heated by the fuel, completing the primary coolant loop.

The flowpath for the secondary loop is typically:

1. Secondary feedwater enters the "shell" side of the steam generator through the feedring, which is a torus with short, inverted-J shaped nozzles spaced around its major circumference.
2. Secondary water continues down the downcomer, which is the annular space between the outer shell of the steam generator and the shroud covering the tube bundle.
3. Secondary water flows over the tubes in the short gap between the bottom of the shroud and the top of the tubesheet.
4. As the secondary water is heated by the heat transferred through the tube wall thickness, it becomes less dense and tends to rise.
5. Secondary water continues to be heated as it flows over the tubes until it becomes superheated steam.
6. The steam flows slightly down the upper annular space to the steam outlet, where it will be sent to the turbine to turn the electrical generator and generate electricity. The exhaust steam will be condensed and sent back through the feedwater system, completing the secondary loop.

There are also various supports to keep the tubes located and prevent them from contacting each other. These are the tube support plates, which support the straight section of the tubes.