The Mysterious “Hot Hole”

If you read our last post, you may have realized just how important water is to electricity production at nuclear power plants. Because of this water need, nuclear plants are typically located near lakes, rivers or the ocean.

Most power plants use one of two types of cooling water systems. A once-through cooling system withdraws water from a water body and circulates it within the plant to condense the steam from the turbine into water through heat absorption. McGuire Nuclear Station, located on Lake Norman, just 10 miles north of Charlotte, N.C. is a an example of a once-through system, discharging the cooling water –to the lake after it has been heated to a temperature higher than when it was withdrawn.

In a wet cooling tower system, this same (condensing) cooling water from the plant moves through  cooling towers and is then cooled by dissipating the steam into the atmosphere. Plants like Catawba Nuclear Station (York, S.C.) and Harris Nuclear Plant (New Hill, N.C.) recirculate their cooling water through these cooling towers. The water is then  pumped back into the plant to be reused.

While the cooling systems are similar, they vary on how water is discharged. Let’s take a look at how water is discharged in a once-through cooling system. To help explain this system, we sat down with a lead engineer at McGuire Nuclear Station to learn more.

Q. How is water discharged in a once-through cooling system?

A. Many plants, like McGuire, use manmade “discharge canals” to cool the water before it reaches the waterbody. The canals enable natural processes to dissipate heat from the water.

Q. How does the discharge canal work?

A. The water from the discharge canal cools the steam used to spin the turbine-generator to make electricity, and if needed, would cool emergency equipment.. For both purposes, the canal water flows through exchangers. The canal water, warmed by several degrees, is returned to the canal network.

Q. What’s the temperature difference between Lake Norman and the discharge canal? Is this pretty standard industrywide?

A. With both McGuire units at full power, the discharge canal water can be 15 to 20 degrees warmer than the lake water. For some, the discharge canal is known as the “hot hole” (see below).

The temperature increase is pretty standard based upon the size or megawatt output of the plant.

Q. How deep is the discharge canal at McGuire? Is this pretty standard industrywide?

A. The McGuire discharge canal is typically 35 to 40 feet deep. The “bottom” of the canal is 720 feet mean sea level – the water depth varies with the lake level. When Lake Norman is at full pond, 760 feet mean sea level elevation, the discharge canal is 40 feet deep. Today, for example (this interview was conducted on May 19), the lake level is 758 feet so the discharge canal is 38 feet deep.

The depth of discharge canals varies  depending on the layout and geography of each plant.

Q. What makes the water look like its churning in the canal?

A. The water churn is due to the high flow rate of the water being pumped through the plant. The flow rate through one unit at McGuire is approximately one million gallons per minute.

Q. Discharge canals have been known to attract certain species of fish. In fact, the “hot hole” makes quite the fishing spot for local anglers.

A. Lakes with warm-water discharges from power plants keep fish feeding actively throughout the winter. The smaller bait or forage fish (shad, alewife), are attracted to the warmer water in the discharge canal during the cold months. This attracts the larger fish like largemouth bass, striped bass and hybrid bass that feed on the smaller fish.

It’s important to note that the warm water quickly dissipates in the lake, causing no harm to the aquatic environment.

Q. Are nuclear plants the only generating sources that have discharge canals? If not, what other plants use discharge canals?

A. All power plants with steam turbines have cooling water and therefore have some sort of discharge canal. Marshall Steam Station, a coal plant located on the north end of Lake Norman, has a discharge canal.

Q. Does the discharge canal have to adhere to any environmental standards?

A. Yes, there are temperature limits for the water in the discharge canal. These are continuously monitored by our team of environmental scientists and biologist and their data are reported to the state.

Water quantity and quality: Power plants routinely monitor water source levels, temperature, and flow; water intake volume and the temperature of discharged water.

Aquatic life: Power plants monitor the species, number and survival rate of fish and shellfish that may possibly be impacted by the plant’s cooling system.

For more information on the wet cooling tower system, read one of our previous posts.

It’s The Cool Thing to Do

The cooling tower; it’s the icon of the nuclear world. It’s also mistakenly thought to be spewing pollution. It’s actually water vapor (or steam).  Let’s take a look at how cooling towers produce the water vapor you see.

Thermoelectric power plants, whether fossil-fueled or nuclear, require cooling water systems. The fuel heats water, turning it into steam, which drives a turbine generator that produces the electricity. The steam, once used to turn the turbine, can be recycled back into water and reused. To reuse this steam, it is cooled and condensed back into water.

View animated image of a Pressurized Water Reactor

Most power plant use one of two types of cooling systems:

  • Once-through system – Water is drawn from a water source such as a lake, river, or pond. This water passes through a condenser where it absorbs the heat from the steam and is returned to where it came from, but at a higher temperature.
  • Closed loop cooling system – Condenser cooling water (in darker blue in the link above)  is circulated to remove the extra heat is has gained. The water is pumped to the top of the cooling towers and is allowed to pour down through the structure. At the same time, a set of fans at the top of each tower pulls air up through the condenser water. This lowers the temperature of the water by about 24 degrees. After it is cooled, the condenser water flows back into the turbine building to begin its work of condensing steam again.  Water from a lake, river or pond is added as needed to compensate for evaporation. 

How Cooling Towers Work

Cooling of the water is achieved through direct contact of the water with air. Water vapor is a byproduct of the cooling process within the cooling tower.  During this phenomenon, which is widely known as evaporative cooling, heat is transferred from water to air. This heat is then rejected to the atmosphere, either through the use of fans or natural convection. Here’s how it works:
Mechanical Draft Cooling Towers

With mechanical draft cooling towers, the incoming condenser cooling water is sprayed throughout the tower’s interior. The spray flows downward to baffles that maximize the time of water contact with the air.  Air enters the interior of cooling tower through properly designed “louvers.” Air is drawn through the baffled area by large fans which reduce the water temperature.  

Diagram of a mechanical draft cooling tower

Natural Draft Cooling Towers

In a natural draft cooling tower, air enters the shell of the tower through louvers at the tower’s base.  Water is typically sprayed around the periphery of the cooling tower and cascades to the bottom. As the air inside the tower is heated, it becomes lighter and starts rising through the tower. This process draws more air through the louvers along the tower’s base, creating a continuous natural air flow. Continuous air circulation is accomplished due to the density difference between the warmer air inside and the cooler air outside.

Natural draft or convection cooling towers have a much bigger footprint compared to mechanical draft cooling towers and also cost much more to build. Due to these reasons, natural draft cooling towers are built less frequently compared to other types.

Diagram of a natural draft cooling tower