|Fig. 1: Image of Isar Nuclear Power Plant wet Cooling towers, located in Germany, that uses the naturally cool Isar river as a coolant source. (Source: Wikimedia Commons)|
Nuclear Power has the capacity to contribute to the decarbonization of US Energy.  With the vast majority of US Nuclear Power plants operating using direct cooling, the US has a vested interest in this continuing to be a viable methodology for cooling its nuclear power plants. However, recent research suggests that climate change and the resulting increases in ambient temperature are threatening the viability of using naturally occurring lakes and rivers to cool power plants without threatening local ecosystems with thermal pollution. 
Nuclear power plants use water in two ways.  First, water is used to convert thermal energy from the core to turn the steam turbines. Water passes by the core, absorbs the thermal energy creating steam, which in turn spins the turbines. These turbines are responsible for generating the electricity that is carried onto the regional power grid. Secondly, water is used to remove surplus heat from the steam circuit in a condenser. The latter use, water is often referred to as the coolant liquid. Most importantly, the bigger the difference between the coolant and the internal heat source, the more efficient the overall system is; resulting in a larger electrical output.
The cooling process can be designed in three different ways; direct cooling, recirculating or indirect cooling, and dry cooling. Dry cooling is less often used and does not require water, so it is exempt from discussion. First, recirculating or indirect cooling utilizes a closed system of water passed by the condenser to absorb thermal energy. This heated water is then transferred to an auxiliary unit where it energy is lost through evaporation. Only approximately 5% of the water needs to be continuously replaced, using significantly less than a direct cooling system. Second, a direct or once-through system utilizes large amounts of running water by directing it through the condenser a single time and then returning it to the original body of water. Once-through systems, therefore, are strategically built near large, cool bodies of water to take advantage of natural resources.
As ambient temperature rises, the production of electricity at nuclear power plants may decrease as a result of both efficiency losses in direct cooling systems and their inability to operate at full capacity. 
First, an increase in external temperature lowers the efficiency of power plants in generating electricity. As aforementioned, an advantage of using a direct cooling system is that it utilizes the naturally cool water of lakes and rivers. For example, as observed in Fig. 1 Isar Nuclear Power Plant utilizes its proximity to the Isar river as a natural coolant (see Figure 1). However, as ambient temperatures rise, so does the water sources. Simply put, when the coolant liquid is warmer, it is less effective at condensing the steam that is used internally because it cannot absorb as much of the thermal energy. Thus, the system operates less efficiently. Previous quantitative research suggests that for every 1˚C increase in outdoor temperature, electrical output decreases by between 0.37 - 0.72%.
Secondly, an increase in temperatures decreases the amount of electricity nuclear power plants are able to produce due to an inability to operate at full capacity. First, a nuclear power plants operation is greatly limited by the condensers maximum pressure. The condensers maximum pressure is determined by the design of the plant and mechanical limitations of the condenser itself. As the ambient temperature increases, the condensers pressure increases proportionally. As the outdoor temperature increases, the condenser will eventually reach its maximum pressure, depending on design, and the plant will be obligated to reduce fuel use, and thus, output. Second, most countries have enacted regulations to limit thermal pollution in lakes and rivers to mitigate impacts on local ecosystems. As follows, if the temperature of the body of water used as coolant increases, then the water will be able to absorb less heat before surpassing the allowable temperature limit for return to water. In these cases, the plant must reduce production to keep the temperature below the limit.
It must be noted that all thermal power plants have exposure to these two impacts, however, because water requirement per electric output is higher in nuclear power plants compared to other thermal power sources, nuclear plants are more vulnerable. Recent research has started into possible ways of coping with the increased scarcity of usable water sources; for example, the feasibility of using temporary cooling towers to avoid shutdowns during the summer months. However, it is clear that these alternatives are not ideal as they involve increased upfront and operating costs. Thus, more effort should be invested in researching alternative cooling methodologies in light of global warming trends.
© Mady Duboc. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 M. G. Morgan et al., "US Nuclear Power: The Vanishing Low-Carbon Wedge," Proc. Natl. Acad. Sci (USA) 115, 7184 (2018).
 K. Linnerud, T. Mideksa, and G. S. Eskeland, "The Impact of Climate Change on Nuclear Power Supply," Energy J. 32, 149 (2011).
 K. Averyt et al., "Fresh Water Use by U.S. Power Plants," Union of Concerned Scientists, November 2011.