The amount of water a nuclear power plant uses is a complex variable determined by its design, location, and cooling technology. All thermal power plants, including nuclear, coal, and natural gas, rely on water to convert heat into electricity. This requires significant access to a water source like a river, lake, or ocean for continuous operation. Water usage must be separated into two distinct metrics: the total volume taken in and the volume permanently lost to the environment.
The Essential Role of Water in Nuclear Energy
Nuclear power plants use the heat generated from splitting uranium atoms (fission) to produce steam. Water acts as the working fluid in the steam cycle, transferring thermal energy into mechanical energy. This superheated water converts to high-pressure steam, which spins a turbine connected to a generator, creating electricity.
All thermal power generation requires that spent steam be cooled and condensed back into liquid water so the cycle can repeat efficiently. Water is an effective coolant because it takes significant energy to change its temperature and state. The quantity of water used by a nuclear facility is almost entirely dedicated to this process of heat rejection, dumping waste heat to the environment to complete the cycle.
Defining Water Usage: Withdrawal Versus Consumption
Power plant water use is measured by two metrics that describe different impacts on the local watershed. Water withdrawal is the total volume pulled from a source, such as a river or ocean, for use in the plant’s cooling system. This metric is typically very high.
The majority of this withdrawn water is returned to the original source, often within the same hour. Water consumption is the portion permanently lost to the local water cycle, primarily through evaporation into the atmosphere. This consumed water is not available for immediate reuse by other local users.
For a typical nuclear plant, the difference between these two metrics is vast. A facility using a once-through cooling system might withdraw 25,000 to 60,000 gallons per megawatt-hour (MWh) of electricity produced, but consume only about 400 gallons per MWh. Conversely, a plant using a cooling tower system might withdraw only 800 to 2,600 gallons per MWh but consume a higher volume (600 to 800 gallons per MWh) due to the high rate of evaporation in the towers.
How Cooling Technologies Impact Water Requirements
The choice of cooling technology is the most important factor determining a nuclear plant’s water footprint, dictating whether it is a high-withdrawal or a high-consumption user. Three primary cooling architectures are employed across the thermal power sector.
Once-Through Cooling
Once-Through Cooling systems draw a massive volume of water from a source, pass it through the condenser, and discharge the warmed water back. This results in extremely high water withdrawal rates, potentially reaching billions of gallons per day. While most water is returned, the discharge of warmer water (thermal discharge) can have localized ecological effects on aquatic life.
Closed-Loop Cooling
Closed-Loop Cooling systems use prominent cooling towers, circulating the same water repeatedly through the condenser and the tower. In the tower, water is cooled primarily through evaporation, releasing waste heat into the atmosphere as water vapor. These systems drastically reduce the volume of water withdrawn compared to once-through systems, but they significantly increase water consumption due to constant evaporative loss.
Dry Cooling
The third method, Dry Cooling (air-cooled systems), uses ambient air instead of water to condense the steam, similar to a car radiator. This technology virtually eliminates water consumption for cooling, reducing water needs by over 90%. However, dry cooling is less common in nuclear power because it is less efficient, which can slightly reduce the plant’s electricity output, and it is more expensive to operate.
Water Intensity Comparison to Other Power Sources
When assessing the environmental footprint, water consumption is the most relevant metric for comparison. Nuclear power plants equipped with cooling towers typically consume around 672 gallons of water per megawatt-hour (gal/MWh) of electricity generated. This rate is comparable to, or slightly higher than, other thermal power sources with similar cooling systems.
Coal-fired power plants with wet cooling towers show a similar consumption rate, though natural gas combined-cycle plants are more water-efficient. Concentrating Solar Power (CSP) plants using wet cooling are often the most water-intensive, sometimes consuming as much as 897 gal/MWh due to lower operating efficiency.
Renewable technologies like wind and solar photovoltaic (PV) have a negligible operational water footprint because they do not rely on a steam cycle for power generation. These sources consume far less water than any thermal plant, making water-use efficiency a differentiator among energy technologies. Hydroelectric power also has water consumption due to increased evaporation from large reservoir surfaces, but the water itself is not consumed during generation.