There is a distinction between water use and water consumption; in many cases much of the water is returned to the source after it is used for cooling, albeit at a higher temperature. A nuclear plant using once-through cooling uses about 95,000–227,000 liters of water per megawatt-hour and consumes about 1,500 liters/MWh, whereas a nuclear plant using closed-cycle wet cooling uses about 3,000–4,200 liters of water per megawatt-hour of electricity produced and consumes about 2,700 liters/MWh. For comparison, a coal-fired power plant using once-through cooling uses about 76,000–189,000 liters of water per megawatt-hour and consumes about 1,100 liters/MWh, and a coal plant using cooling towers uses about 1,900–2,000 liters of water per megawatt-hour of electricity produced and consumes about 1,800 liters/MWh.
After 2020, alternatives such as dry cooling may be able to reduce water use further. Dry cooling is usually accomplished with mechanical-draft air-cooled condensers to which turbine exhaust steam is ducted through a series of large ducts, risers, and manifolds. Dry cooling still has significant disadvantages, including higher costs, higher operating power requirements, and reductions in plant efficiency and capacity during periods of hot weather. Dry cooling has been used for some coal-fired plants,41 but at present, no nuclear plants have been constructed using this technology.
Hybrid cooling, which typically consists of a dry cooling system operating in parallel with a conventional closed-cycle wet cooling system, is an alternative that is finding increased use at some new coal-fired plants. A hybrid cooling system was built in 1988 at the Neckarwestheim Nuclear Plant in Germany. Hybrid cooling is also proposed for use in several evolutionary nuclear plants intended to be built in the United States in the near term, including the new reactor proposed by UniStar for the Calvert Cliffs site in Maryland (Pelton, 2007).
The water use impacts of future nuclear plants will depend on where the plants are sited and what cooling technologies are employed. Water use and consumption will be a consideration in siting new nuclear plants in areas such as the American southwest with growing populations but limited water supplies. In some instances, wet cooling systems can use nonfreshwater sources such as seawater (if located on the coast), brackish water from wells or estuaries, agricultural runoff, produced water from oil and gas drilling operations, or treated municipal waste-water (Veil, 2007).