Water use consists of two distinct processes: withdrawal and consumption. According to the U.S. Geological Survey, thermoelectric power generation is among the smallest consumptive uses of freshwater by any economic sector, at 3.3 percent of total U.S. consumption—about one-half of residential consumption, at 6.7 percent. Thermoelectric power plants withdraw more water than any other economic sector, but they return 98 percent of the water they withdraw back to its natural sources. Thermoelectric power plants—fueled by nuclear energy, natural gas, oil, or coal—generate 90 percent of the electricity in the United States.
To put residential and thermoelectric power water consumption in perspective, a typical U.S. nuclear power plant supplies 740,000 homes with all of the electricity they use while consuming 13 gallons of water per day per household in a once-through cooling system, or 23 gallons of water per day per household in a wet cooling tower system. By comparison, the average U.S. household of three people consumes about 94 gallons of water per day for indoor and outdoor residential uses.
A once-through cooling system returns 99 percent of the water withdrawn back to the water body, at a somewhat higher temperature, as allowed by the plant’s water permit. Wet cooling towers withdraw less water than a once-through system, and discharge water at a temperature only slightly above, or at, that of the water body. But cooling towers consume 70 percent of the water they withdraw. In effect, cooling towers consume twice as much water as a once-through system.
Once-through cooling systems may require plant operators to reduce electricity production to a small degree to observe the discharge water temperature limit during very hot days, while cooling towers do not have to reduce power for this reason. Most proposed new nuclear plants will employ cooling towers, where discharge water temperature will not be a consideration in electricity production.
Nuclear plants strive to be responsible stewards of the environment. Cooling system intake structures are designed to minimize aquatic life mortality, which is usually only one to two percent of the fish population in the water body. Scientific studies demonstrate that these cooling systems do not have any negative impact on the abundance of fish in the water body over time. Though cooling towers consume more water than once-through systems, cooling towers can consume as little as one to two percent of the annual flow of the rivers where they are located.