The United States produces roughly 4 trillion kilowatt hours of electricity annually, 90 percent of which is generated by
thermoelectric power plants.i
Plants fueled by coal, natural gas, oil, nuclear fission, and some renewable energy
technologies boil water to produce steam, which then turns a turbine to generate electricity. After it passes through the
turbine, more water is needed to cool the steam back into water to reuse for generation; this steam-cooling step accounts
for virtually all of the water used in most power plants. Nuclear fission is the most water intensive method of the
principal thermoelectric generation options in terms of the amount of water withdrawn from sources. In 2008, nuclear
power plants withdrew 8 times as much freshwater as natural gas plants per unit of energy produced, and up to 11
percent more than the average coal plant. ii
Nuclear power plants are about 33 percent efficient,
which means that for every three units of thermal
energy generated by the reactor core, one unit of
electrical energy goes out to the grid and two units of
waste heat go out into the environment through
cooling systems. iii Of the 104 nuclear reactors in the
United States, 35 are boiling water reactors (BWR) and
69 are pressurized water reactors (PWR). About 60
percent of these nuclear power systems use
recirculating cooling; the remainder use once-through
Since a large nuclear power plant that utilizes a oncethrough
cooling system may withdraw 800 million to 1
billion gallons of water a day, these plants are usually
built next to rivers, lakes, or oceans.v As the name
implies, once-through cooling uses water a single time
to cool and condense steam produced for electricity generation. Water produced from the condensed steam is reused in
the generation process, but the water used for cooling is discharged back into the lake, river or ocean, with a temperature
increase of up to 30 degrees.vi
The temperature increase in the bodies of water can have serious adverse effects on aquatic life. Warm water holds less
oxygen than cold water, thus discharge from once-through cooling systems can create a “temperature squeeze” that
elevates the metabolic rate for fish.vii Additionally, suction pipes that are used to intake water can draw plankton, eggs
and larvae into the plant’s machinery, while larger organisms can be trapped against the protective screens of the pipes.
Blocked intake screens have led to temporary shut downs and NRC fines at a number of plants.
While once-through cooling systems withdraw 25,000 to 60,000 gallons of water for each megawatt-hour of electricity
produced, recirculating cooling systems, also known as closed-cycle cooling systems, withdraw only 800 to 2,600 gallons
per megawatt-hour and are used when nearby water sources lack sufficient volume to allow once-through cooling. After
water is withdrawn from a source to cool steam, it is then cooled and pumped back into the condenser for reuse.
Though plants with closed cycle cooling systems withdraw far less water than once-through cooling systems, they
consume (through evaporation) about 600-800 gallons per megawatt-hour, roughly half the amount they withdraw.
Other water uses for nuclear power
While cooling systems account for the vast amount of water
withdrawn by nuclear power plants, fuel extraction and refining have
also impacted water sources. Uranium fuel extraction, for example,
requires 45-150 gallons of water per megawatt-hour of electricity
produced and uranium mining has contaminated surface or ground
water sources in at least 14 states.viii Additionally, nuclear power plants
intake water to cool service equipment, such as chillers for air
conditioning units or lubricating oil coolers for the main turbine.
Service water system flow rates can range from 13,500 to 52,000
gallons per minute depending on the season and the power plant.ix
Nuclear power in a warming world
Water cooling systems can also pose significant problems from an economic standpoint. When water is warmed, either
by plant discharge or ambient temperatures, cooling requires even more water and power plants operate less efficiently.
Moreover, if water cannot be cooled, it can neither be recirculated nor returned to the river, lake or ocean without
threatening aquatic life. Therefore, during hot summers or heat waves, the problem compounds: during times of extreme
heat, nuclear power plants operate less efficiently and are dually under the stress of increased electricity demand from air
conditioning use. When cooling systems cannot operate, power plants are forced to shut down or reduce output. The
combination of high electricity demand and reduced output can result in higher energy prices for ratepayers. Droughts
can have a similar effect as heat waves, limiting the amount of water available for cooling.
U.S. Energy Information Administration. Electricity in the United States, 2009. ii Averyt, et al. Freshwater use by U.S. Power Plants: Electricity’s Thirst for a Precious Resource. Union of Concerned Scientists, EW3, 2011. iii Lochbaum, David. Got Water? Union of Concerned Scientists, 2007. iv Union of Concerned Scientists. How it Works: Water for Nuclear, 2010. v Palo Verde nuclear power station buys treated wastewater to use in its recirculating cooling system. It is the only nuclear power station not located near a body of water. vi UCS. 2007. vii UCS,EW3. 2011. viii UCS. EW3. 2011. ix UCS. 2007.