A cooling tower is a large heat exchanger. Heat is carried up the stack and is visible as water vapor.

The water used to cool steam back into water returns to its source slightly warmer after use. Cooling towers provide an energy efficient and environmentally friendly way of removing heat from this circulating water before it is returned to its source. So while they are so closely identified with nuclear power plants, not all nuclear stations use cooling towers, and many non-nuclear plants also use them. In fact, of the more than 250 cooling towers across the U.S., fewer than 100 are at nuclear plants.

It is important to note that since cooling towers are part of a nonradioactive system, no radioactive material is released from them.