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system, discharging the cooling water –to the lake after it has been heated to a temperature higher than when it was withdrawn.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

Lake Temperatures Near Nuclear Plants

Duke Energy actively monitors water quality year-round.

Duke Energy actively monitors water quality year-round.

Nuclear stations generate electricity by heating water to create steam to turn  turbines, which turn  a generator. As part of electricity production, these stations need a way to cool this steam back to water for reuse. Because of this water need,  nuclear  plants are typically  located near lakes, rivers or the ocean.  However, because these lakes and rivers are used for other purposes, such as drinking water, irrigation and industrial uses, Duke Energy established environmental programs decades ago to protect aquatic life.

The cooling water, drawn through large pipes by pumps, never touches the steam. Cool lake or river water circulates through pipes, cooling, or condensing, steam back to water when the steam falls on the pipes (similar to how a car radiator works). In the process, the cooling water picks up a little heat from the steam and is warmer when it is returned to the lake. However, to ensure water quality of the water used, the U.S. Environmental Protection Agency and state agencies regulate the temperature of the water that is discharged. The state  sets discharge temperature limits for each plant. These limits  are  established using scientific data and research. Compliance with these limits is ensured through regular data collection and reporting by the utility.

Duke Energy teammates Linda Hickok (Water Resources Manager) and Bill Foris (Lead Scientist) put things in perspective through the following Q&A.

Q: Why are lake temperature limits in place for power plants?

A: Temperature limits are in place to protect the aquatic ecosystem of the water body receiving the heated discharge.  The limits are set at levels that allow integration of the heat into the ecosystem without disrupting fish, plant and other aquatic organisms in the lake.

Q:  What lake discharge temperature limits must nuclear plants adhere to? 

A:  Temperature limits are established during a permitting process based on site-specific conditions.  The site permit contains these specific limits.

Q:  Who regulates the limits and how are they regulated? 

A:  Typically, permits are issued by the state as the designated agency to implement federal and state laws and regulations governing wastewater and thermal discharges. The permitting agency reviews monitoring data and conducts inspections to assess permit compliance.

Q:  Why is it important for stations to stay within permitted limits?

A:  To ensure protection of the aquatic life and water quality within the water body (lake, river, etc.).

Q:  How does Duke Energy ensure temperature limits are not exceeded?

A:  In most instances, water temperature of the thermal discharge is measured at least hourly year-round to assure the site-specific limit is met. This data is reported to the state on a regular basis, and is readily accessible by nuclear station personnel for reviewing and making operational decisions .

Q:  How does Duke Energy maintain healthy aquatic life?

A:  Duke Energy actively monitors the water quality, plankton and fishery of reservoirs throughout the year, and submits annual reports to the state summarizing these important environmental attributes.

Q:  Is there any type of special permit requirement for nuclear stations? 

A:  The National Pollutant Discharge Elimination System (NPDES) permit includes requirements for discharge quality (limits on chemicals or heat in the discharge water), monitoring and reporting. NPDES is a program created by the federal Clean Water Act to regulate wastewater and thermal discharges from industrial and municipal facilities.


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.