In nuclear power plants, atoms are split during fission reactions to produce an enormous amount of heat. This heat changes pure water into steam, and the steam is used to turn a series of turbines connected to an electric generator. To cool the steam and condense it back into water so it can be used again, excess heat must be absorbed using water from a separate source. In "once-through" cooling systems, water is pulled from a nearby body of water, used to condense the steam, and pumped back into the same body of water at an increased temperature. In many cases, once-through cooling systems can cause the temperature of the entire body of water to increase, which can disrupt aquatic ecosystems.

A diagram of a nuclear reactor with a once-through cooling system is shown below. In the diagram, warmer water is shown in red and cooler water is shown in blue.

Suppose the operators of a nuclear power plant want to modify their once-through cooling system to reduce the plant's impact on nearby aquatic ecosystems without affecting the system's cooling capabilities. A viable solution would most likely reduce the of water entering the