Nuclear Reactors Don’t Need to Be So Thirsty

Nuclear Reactors Don’t Need to Be So Thirsty

A major selling point for solar and wind power installations is that they use very little water. But what about nuclear? Can nuclear be similarly thrifty with this limited resource?

The subject can be confusing.

Today’s nuclear plants use water in two distinct roles. And the first tranche of new, small modular reactors is likely to do the same.

Fuel in the reactor vessel is immersed in water. That water “moderates” the flow of neutrons, the sub-atomic particles that are released in fission. It slows the neutrons down to a speed that is more likely to cause another fission.

The water also carries off the heat that is generated by fission, so it can be turned to steam and put to work to spin a turbine, creating mechanical energy that turns a generator and makes electricity.

One problem for steam-electric plants (including coal plants) is that when this steam exits the turbine, depleted of much of its energy, it needs to be reheated, to gain the energy needed to spin the turbine again. But it’s hard to heat steam, so the engineers have to find a way to condense it back into water.

Cooling the steam and turning it back into liquid water creates the need for more cooling water.

In this schematic diagram of a boiling water reactor, from the Nuclear Regulatory Commission, the reactor core, on the left, is immersed in water. That water leaves the reactor as steam (the red line). The steam gives up its energy to spin the turbine (the vertical bars on the green shaft) which turns the electrical generator and makes electricity. Then the steam is condensed back into water in the condenser (the mechanism below the turbine, with the blue at the bottom representing steam turned back to water.) That water is pumped back through the pale green line back to the reactor, for re-boiling. The bright green line is the path of the water that provides the cooling to turn the steam back into water. It may come directly from a river or lake, or may go to a cooling tower.

In some future reactors, water requirements will be smaller. Gas-graphite reactors, for example, heat up an inert gas, use the hot gas to spin the turbine, then route the gas back to the core for reheating. But the material is a gas in the entire process; it does not have to be condensed back to a liquid for reheating.

To understand how reactors in service today use water, and how that compares to plants that burn fossil fuels for energy, it is worth looking into each type in more detail.

Traditional Nuclear Reactors

There are two main nuclear designs in wide use today. The first is the boiling water reactor, which boils the water in the core and uses the steam generated from that process to spin a turbine. The turbine turns a generator, which makes electricity. The second design is the pressurized water reactor, which heats water in the core but keeps it under pressure to prevent boiling. The water is pumped through a heat exchanger, which is a cluster of small tubes encased in a shell. The water that was heated in the core runs inside the tubes; outside, but within the shell, clean water is boiled into steam. The steam then spins a turbine and makes electricity. In both designs, the water in the reactor is always part of a closed loop. Wherever the steam is created, once it runs through the turbine, it goes through another heat exchanger, called a “condenser,” where it is condensed back into water. Then it goes back to the core or to the heat exchanger to be reheated into steam once more.

But it also takes water to condense the steam. In the condenser, cooling water (often from a river, lake, or the ocean) runs inside small pipes, inside a big box. Outside the pipes, steam flows through and is condensed back into water. Older plants, especially those near oceans, typically use a “once through” cooling system. Seawater is strained through a filter, pumped into the condenser through the tubes, then pumped back out into the ocean or river. This system “uses” the water but does not consume it. But there is a drawback; it isn’t friendly to aquatic life. Some fish are caught on the screens and killed; some fish larvae get pulled through the condenser and are killed by the heat.

Other plants send the heated water from the condenser to a cooling tower, where some of it is given off as steam. When you see a nuclear plant with a tower puffing out a white cloud, that cloud is the steam from the cooling tower. Plants with cooling towers have far lower water requirements because they are sending a relatively small volume of water to the condenser, over and over, and letting off a small volume of water as steam. Put differently, in a once-through system, a huge volume of water is heated by just a few degrees. In a plant with a cooling tower, a smaller amount of water is heated much hotter, hot enough to turn to water vapor.

There is nothing special about nuclear plants using water to condense steam. Coal plants do pretty much the same thing. So do plants that burn gas if they are “combined cycle” plants. This is the design used for most plants that burn gas, because they extract more energy value from a given quantity of gas. They run the gas through something resembling a jet engine, to spin a gas turbine. This makes torque to turn a generator. The plants then use the exhaust from that jet engine to boil more water into steam, which turns a steam turbine. This step allows the plants to get more work out of each cubic foot of gas. But the steam has to be condensed back to water so it can be re-heated into steam.

As to how much water all these processes “cost,” a United Nations Economic Commission for Europe study from 2019 takes a stab at some answers. The study put nuclear energy from at an average of about 2.4 liters per kilowatt-hour, counting all types of cooling systems. (A kilowatt-hour is enough to run an efficient kitchen refrigerator for a day.) An ordinary coal plant was about the same, but a coal plant that captured its carbon dioxide was far higher. And natural gas in a combined cycle system was a little lower.

A further study by the U.S. Department of Energy’s National Renewable Energy Laboratory concurred that plants with cooling towers consume far more water than once-through cooling systems, which “withdraw” about 10 times more water per megawatt-hour produced but release the water back to its source.

In the jigsaw puzzle of picking a site for a power plant, considerations include grid access, proximity to rail or barge transport for coal, or availability of pipelines for natural gas. Water is sometimes not a concern. In other locations, it’s a critical consideration. Hence the idea for a reactor that will be cooled by air, at a project in Idaho that will be built for a coalition of public power entities, the Utah Associated Municipal Power Systems (UAMPS), with a cluster of NuScale reactors. The decision about what kind of plant to build is unlikely to be based on water usage; more important are the needs of the electricity system. If nuclear is a good fit, others can go the route that was taken in Idaho.

Future Nuclear

UAMPS’ Idaho project may be a harbinger of the future, at least in dry locations. It will use an air-cooled condenser. The steam leaving the turbine runs inside metal tubes, and fans blow air on the outside of the tubes. This cools the water somewhat the way a car radiator does.

Using an air-cooled condenser makes it much easier to pick a spot to build a reactor; no river or lake is needed. But there’s a downside; the fans need electricity. The penalty is in the range of 5% to 7% of electricity production, depending on climate. NuScale is betting that in Idaho, and in some other locations in the future, the electricity to cool the condenser will be less valuable than the water for a water-cooled condenser would be.

One of the drawbacks, though, is that the amount of electricity needed for cooling goes up when the air is hot. In the summer, when the grid most needs the power, production available for the grid is reduced.

An air-cooled condenser is unusual, but some coal plants do use them. Some of those plants are on rivers that have very low flows in summer, so they use air cooling during hot-weather months, and water the rest of the time. The Russians built some small reactors with air cooling in the arctic, where liquid water wasn’t always available. And some nuclear designs don’t use water to move heat within the reactor at all. In fact, they may use inert gas, liquid metal, or liquid salt.

An advanced reactor that used molten salt or molten metal to move heat from the core to make steam would likewise need water to condense the steam back to water for reheating. But these might need less water per kilowatt-hour than the current generation of reactors because they operate at higher temperatures, and higher temperature steam can make more kilowatt-hours per BTU of heat.

Gas-graphite reactors, which use an inert gas like helium to move energy from the fuel to the turbine, can re-heat the inert gas without condensing it at all.

Some plants will use less water to cool the condenser because they will make beneficial use of the heat that is captured by cooling the steam back to water. That heat, today thought of as waste, could be used for water desalination, hydrogen production, district heating, or industrial heat.

Reactors can also run on water that is unsuitable for other purposes. Palo Verde, in Arizona, which is the largest nuclear plant in the United States (and will continue to be so until Vogtle 4 is finished in Georgia) uses water recycled from a sewage plant.

Water will be a concern going forward because rainfall is becoming more variable than the early nuclear planners had foreseen. Likewise, the temperature of surface water, which is needed for the condenser, is now higher sometimes than the planners expected. But nuclear technology is capable of operation with very low water consumption, and advanced nuclear technologies will make saving water easier.

And nuclear will likely have another connection to water: making it. Desalination is now most commonly done with electricity, and sometimes from direct use of natural gas, using the waste heat of a gas-fired power plant. Reactors are a very good way to make heat and electricity, and do so without adding to the carbon burden of the atmosphere, which is what is causing water problems to begin with.