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system, discharging the cooling water –to the lake after it has been heated to a temperature higher than when it was withdrawn.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.