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system, discharging the cooling water –to the lake after it has been heated to a temperature higher than when it was withdrawn.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 (not sure anyone would know what this is…heat exchangers? to cool steam or hot water that gets re-used by the plant. 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.

Dry Cask Storage: An Alternative for Storing Used Fuel

McGuire Nuclear Station dry cask storage stores spent fuel on site.

McGuire Nuclear Station dry cask storage stores used fuel on site.

One aspect of nuclear energy that makes it unique is the issue of used fuel storage.  Used fuel is nuclear fuel that is no longer useful for sustaining a chain reaction in a reactor.  While the fuel is no longer useable for producing electricity, it continues to give off radiation and heat and must be stored properly.

The United State government has promised electric utilities it will create a long-term storage solution for used fuel, but that has not yet came to fruition. The Nuclear Regulatory Commission has selected Yucca Mountain as a potential disposal site but has been contested.  Until the government makes a decision regarding the long-term storage location of spent fuel, nuclear facilities are either store it onsite or send it to specially equipped landfills. While a central repository for used nuclear fuel is the long-term goal, nuclear facilities are well equipped to safely handle the storage of used fuel on site.

After being removed from the reactor, used fuel spends approximately five to ten years in a large, deep pool of water on site known as a used fuel pool. The water cools the fuel and acts as a radiological barrier. Once the fuel cools down to an appropriate temperature and meets strict radiological and chemical requirements, it is moved to dry cask storage.

The cask is a round, stainless steel canister that holds approximately 24 used nuclear fuel bundles. Dry cask storage means exactly that – dry. The water that is in the cask when it is loaded with fuel is pumped out through siphon ports and backfilled with helium to ensure it is dry. It is protected by a reinforced concrete building called a horizontal storage module.

The fuel is permanently cooled through a system of natural circulation. The horizontal storage module has vent ports located in the front of each module that allow air to flow around the canister and back out again. In addition, nuclear professionals monitor the modules by performing observations and using radiation and temperature monitors.

Duke Energy Nuclear Employees Participate in Earth Day

Earth Day is April 22, 2015. Each April 22, for the past 45 years, citizens around the world have honored a commitment for a clean environment by hosting festivals, rallies and environmental activities, all with support  from a  world-wide community.

Duke Energy and its nuclear employees  also support a clean environment. Did you know that nuclear energy is America’s largest source of clean-air, carbon-free electricity, producing no greenhouse gases or air pollutants — 64 percent of carbon-free electricity is produced by nuclear energy.

In recognition of Earth Day this year, Harris Nuclear Plant near Raleigh, N.C., will be the host of local sixth graders for a program on nuclear, renewables and energy efficiency. This will be the first visit to the nuclear facility by many of these students.

Catawba Nuclear Station and Oconee Nuclear Station employees will be participating in roadside  cleanups.

Employees at Catawba Nuclear Station, located in York, S.C., including members of the Catawba chapters of Women in Nuclear (WIN) and North American Young Generation of Nuclear (NAYGN), will spend the afternoon picking up litter along the road to the plant, as part of adopt-a-highway program.

Employees at Oconee Nuclear Station, located in Seneca, S.C.,  in partnership with the Keep Oconee Beautiful Association (KOBA), will participate in the Adopt-A-Spot program by l doing a litter sweep along the roads near the plant’s entrances. In addition, Oconee Maintenance  Department employees  have “adopted” their own section of local highways as part of their commitment to the community and the environment.

trash

Duke Energy nuclear employees keep local roads clean.