Cooling options and water use

A nuclear plant is a steam engine with an unusual fire. The reactor makes heat, the turbine turns some of it into electricity, and everything else has to go somewhere. For a typical water-cooled reactor that "everything else" is about two thirds of the heat. The cooling system is how a plant throws it away, and the method chosen decides most of the plant's water footprint.

For data center cooling, which stacks on top of this when a reactor serves a campus, see the data center cooling lesson and the data center water lesson.

Waste heat first

Hotter reactors convert heat more efficiently, so they reject a bit less per megawatt-hour. But no design escapes the problem. A helium-cooled Xe-100 or a sodium-cooled Natrium uses gas or metal to carry heat out of the core, and then still needs a heat sink on the power side. "Gas-cooled" describes the core, not the plant's water use.

The cooling methods

The difference between withdrawal and consumption matters more than any other point in this lesson. A once-through plant pulls enormous volumes but sends most of it back, warmer. A cooling tower pulls much less but evaporates a large share of what it takes. In a river basin where every acre-foot is already claimed, consumption is what hurts other users.

The numbers

The standard reference is NREL's 2011 review by Jordan Macknick and colleagues, which compiled published estimates for each technology and cooling type. Medians, in gallons per megawatt-hour of electricity:

Plant and cooling	Withdrawal (gal/MWh)	Consumption (gal/MWh)
Nuclear, cooling tower	1,101	672
Nuclear, once-through	44,350	269
Nuclear, pond	7,050	610
Coal (generic), cooling tower	1,005	687
Gas combined cycle, cooling tower	253	198
Gas combined cycle, dry cooling	2	2
Utility-scale solar PV	26	26
Wind	0	0

Gas combined cycle plants use less water than nuclear because about two thirds of their power comes from a gas turbine that needs no steam cooling. The NREL data predates any SMR, so treat the nuclear rows as a guide for water-cooled SMRs, not as measured SMR values. Designs that run hotter should land somewhat lower per MWh. No one has measured an operating U.S. SMR yet.

A real example: Kemmerer Unit 1

The NRC's final environmental impact statement for TerraPower's Natrium plant in Wyoming (NUREG-2268, October 2025) says the plant would use a mechanical draft cooling tower. Raw water would come from the Naughton Power Plant's raw water settling basin, which draws from the Hams Fork River, through a new pipeline of about 6 miles. Average demand is listed at 3,689 gallons per minute, maximum 5,270.

Dry cooling and its penalty

Dry cooling rejects heat to air through big finned coils, the way a car radiator does. Water use drops to almost nothing. The cost is efficiency: air is a worse heat sink than evaporating water, especially on hot afternoons, so the turbine can't extract as much work. NREL's review cites national-average annual output penalties for switching from wet to dry cooling of 6.8 percent for nuclear, 1.7 percent for gas combined cycle and 6.9 percent for other fossil steam plants. Dry systems also cost more to build and take more land.

Hybrid systems split the difference: dry most of the year, wet sections on the hottest days. They are a common compromise for arid sites. When a developer says a design "can use" dry cooling, ask whether this site will, and what output figure the economics assume.

Water rights in the West

In Idaho and most western states, a new plant can't simply pump water. It needs a water right, usually by buying and transferring an existing one, under prior appropriation rules where older rights are served first in shortages. The prior appropriation lesson explains the system, and new claims from data centers and energy covers how large new users fit in. A 5,000 acre-foot consumptive need is a large transfer in any basin. Intake structures on rivers and lakes also fall under Clean Water Act section 316(b), which regulates how cooling water intakes affect fish.