Nuclear Energy Insight July 2009
—Thermoelectric power plants, whether fossil-fueled or nuclear, require cooling water systems. The fuel source—uranium, coal or natural gas—heats water into steam, which drives a turbine generator that produces the electricity.
The exhausted steam from the turbine must be condensed back to water and recycled to the steam generator or boiler to begin the process anew. This condensation occurs by passing it through a heat exchanger—or condenser—where low-temperature cooling water absorbs the heat of the steam and cools it down to water again.
The majority of power plants use one of two types of cooling water systems. In a once-through or open-cycle cooling system, water is withdrawn from a water source, such as a lake, river or reservoir. The water passes through the condenser and is then returned to its original source, with a negligible amount of heat transferred to the aquatic environment.
In a recirculating or closed-cycle system, cooling water is pumped from the condenser to a “wet” cooling tower, where the heat of the water transfers to the ambient air by evaporation. The resulting lower temperature cooling water is then returned to the condenser, and the amount of water that evaporates in the cooling tower is replenished.
Once-through systems withdraw more water than recirculating systems, but consume little of it—on average, only about 1 percent of the water withdrawn is ultimately consumed. Recirculating systems withdraw much less water than once-through systems, but consume about 70 percent to 90 percent of what they withdraw by evaporation in the cooling towers. Cooling towers consume about twice as much water as once-through systems.
Both systems typically withdraw only a very small quantity of water relative to the overall size of the water bodies on which they are located—typically 1 percent to 2 percent of the average river flow. The cooling water at nuclear plants that is returned to lakes and rivers is never made radioactive and is entirely safe.
Of the 104 U.S. nuclear power plants, 60 use a once-through cooling system, 35 reactors use wet cooling towers, and nine use hybrid systems—a combination of once-through and cooling tower systems. Environmental conditions determine which is used at any given time.
The electric power industry is pursuing strategies to use less water, less freshwater, or no freshwater at all for plant cooling.
For instance, the Palo Verde nuclear plant in Arizona, the largest power plant in the United States, is the only nuclear plant in the world to use recycled, partially-treated municipal wastewater for the plant’s cooling towers. Palo Verde is the only U.S. nuclear plant not situated on a large body of water.
Another innovative cooling strategy is used by the Limerick nuclear plant near Philadelphia, which makes use of mine pool water to augment river flow during shortages.
Some companies planning to build new nuclear plants intend to use hybrid cooling systems, or use water treatment technology, such as seawater desalination to conserve freshwater resources, or even dry cooling systems that would use forced air rather than water. —Read more articles in Nuclear Energy Insight and Insight Web Extra.