Nuclear power plants, which generate electricity by harnessing the heat from controlled nuclear fission, are a major source of global energy. Like most large-scale thermal power generators, these facilities require significant cooling to manage the heat produced. Assessing the water footprint of nuclear energy involves distinguishing between water withdrawal—the massive volumes temporarily borrowed from a source—and water consumption—the much smaller fraction permanently lost to the environment. This distinction is necessary to accurately assess the impact on local water resources.
The Essential Roles of Water in Nuclear Power
Water serves several essential functions within a nuclear power plant. The primary purpose is to convert the thermal energy released by nuclear fission into mechanical energy. This is achieved by using water to create high-pressure steam, which then spins a turbine connected to an electrical generator. In light water reactors, water also acts as a moderator, slowing down the high-speed neutrons released during fission to sustain a controlled nuclear chain reaction. Finally, water is indispensable for thermal cooling, removing excess heat from the reactor core and condensing the low-pressure steam back into water so the closed-loop steam cycle can repeat.
Understanding Water Withdrawal and Consumption
To accurately quantify water use, a clear distinction must be made between water withdrawal and water consumption. Water withdrawal refers to the total volume taken from a source, such as a river, lake, or ocean, for the plant’s cooling systems. This volume is often extremely high, particularly for plants using once-through cooling designs.
Water consumption is the volume permanently lost to the atmosphere, primarily through evaporation from the cooling system. This consumption rate is the difference between the water withdrawn and the water returned to the source. Nuclear power plants using recirculating cooling towers typically consume between 400 and 720 gallons of water per megawatt-hour (gal/MWh) of electricity produced.
This consumption rate is a more accurate measure of the stress placed on a local water supply than the withdrawal rate. Consumption figures vary based on the plant’s thermal efficiency and local climate conditions. Nuclear power generation accounts for approximately 40% of all thermoelectric water withdrawals in the U.S., but a smaller 28% of the total water consumption, illustrating the withdrawal-consumption difference.
How Cooling System Design Impacts Water Use
The most significant factor determining a nuclear plant’s water footprint is the design of its cooling system. Plants employ two main strategies: once-through cooling and closed-loop cooling, each presenting a distinct trade-off between withdrawal volume and evaporative loss.
Once-Through Cooling
Once-through cooling systems draw a vast volume of water from a source, circulate it once through the condenser, and then return the water to the source at a slightly elevated temperature. This system is characterized by extremely high water withdrawal rates, sometimes exceeding 60,000 gallons per megawatt-hour, but very low water consumption because most of the water is returned. While the consumption is low, the returned water’s increased temperature can cause thermal alteration in the aquatic ecosystem.
Closed-Loop Cooling
Conversely, closed-loop systems utilize large cooling towers or cooling ponds to recirculate the water within the plant. Heat is rejected primarily through evaporation, which necessitates the continuous replacement of evaporated water, known as “make-up” water. These systems have significantly lower withdrawal rates, typically ranging from 800 to 1,100 gal/MWh, but result in a higher water consumption rate due to continuous evaporative loss. The choice of cooling technology is often dictated by the plant’s location and the availability of water, with coastal sites frequently opting for once-through cooling using seawater.
Nuclear Water Use Compared to Other Energy Sources
Comparing water consumption rates for different electricity generation technologies provides essential context for the nuclear footprint. Focusing on consumption (water permanently lost), nuclear power is competitive with or better than some thermal alternatives, but uses substantially more water than non-thermal renewable sources.
For plants using wet cooling towers, nuclear facilities consume approximately 720 gal/MWh. This consumption rate is generally higher than that of a modern natural gas combined-cycle plant (around 200 gal/MWh) and conventional coal-fired power plants (about 480 gal/MWh).
The water footprint of certain renewable technologies can be comparable to nuclear power. Concentrated Solar Power (CSP) plants that use wet cooling systems consume a similar amount, ranging from 500 to 800 gal/MWh. Non-thermal renewables, such as wind and photovoltaic (PV) solar, use minimal water, primarily for cleaning, often consuming only about 20 gal/MWh.