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.

Nuclear Fuels Engineers Are Masters of Interior Design

Nuclear power plants in the United States produce electricity 24/7 and are baseload generating plants. Baseload power refers to generating resources that operate continuously and provide reliable power to customers, only shutting down for scheduled refueling and maintenance.

Nuclear power plants use uranium fuel, in fuel assemblies, to produce heat through fission (splitting of atoms). When uranium atoms split, they release energy in the form of heat.

Uranium Fuel

Source: Nuclear Energy Institute

Source: Nuclear Energy Institute

This heat is used to heat water for creating steam, which turns the turbine-generator to make electricity.

How a Nuclear Plant Works

Source: U.S. Nuclear Regulatory Commission

Source: U.S. Nuclear Regulatory Commission

During power operation, the nuclear fuel assemblies are located in the reactor vessel in a cylindrical arrangement called the reactor core. Just like an automobile, nuclear power plants are refueled on a regular schedule – normally an 18 or 24-month fuel cycle – meaning that every 18 or 24 months, the nuclear generating unit is shut down for refueling. Once shut down, about one-third of the nuclear fuel assemblies (the oldest assemblies) are removed from the reactor core and placed in the used fuel pool for storage. This fuel has typically been used in the reactor for four-and-a-half to six years.

Used Fuel Pool

Source: Nuclear Energy Institute

Source: Nuclear Energy Institute

The remaining fuel in the reactor is rearranged and combined with new fuel assemblies that replace the ones that were permanently removed – this makes up the reactor core for the next operating cycle. The arrangement of the fuel assemblies, known as the core design, is analyzed to provide for maximize energy use from each fuel assembly.

At Duke Energy, our in-house Nuclear Fuels Engineering group is responsible for designing and managing each nuclear reactor core to ensure the fuel is safely used and satisfies the energy requirements of the next operating cycle. This takes a comprehensive understanding of plant operations, careful monitoring and detailed fuel analysis. The effort to model, analyze, establish limits and provide data for operating the cycle is roughly an 18-month effort. However, if changes must be made, the team at Duke Energy has demonstrated its ability to adjust the core design in as little as a few weeks. This can only be accomplished because of the high degree of automation, streamlined processes and expertise of Nuclear Fuels Engineering.

In past years, most nuclear plants operated on 12-month fuel cycles. Over the years, these cycle times were extended due to improvements in fuel design, maintenance and operations. These improvements have allowed for fewer refueling outages and improved on-line reliability, which also results in less labor to refuel the plant and lower costs for our customers.

Below are cross sections of a nuclear fuel assembly and a nuclear reactor core design. In Duke Energy’s boiling water and pressurized water reactors, the fuel assemblies contain from 80 to 264 individual fuel rods. These assemblies are selectively arranged to create core designs of 157, 177, 193 assemblies for the pressurized water reactors and 560 assemblies for the boiling water reactors.

Cross section of typical fuel assembly (Each circle represents a fuel rod; color coding represents rod type)

Cross section of typical fuel assembly (Each circle represents a fuel rod; color coding represents rod type)

Cross section of typical reactor core (Each square represents a fuel assembly; color coding represents regions)

Cross section of typical reactor core (Each square represents a fuel assembly; color coding represents regions)

Although not unique, Duke Energy is one of only a few U.S. utilities licensed by the U.S. Nuclear Regulatory Commission to perform its own nuclear core design analyses. This in-house design has enhanced competitive fuel supply, resulted in more economical fuel designs and provided the ability to respond swiftly to changing needs. It also provides for a number of interesting and challenging career paths for the Nuclear Fuels engineers.

Outage workers boost local economies

OutageEach spring visitors of a special kind arrive in the sleepy little coastal town of Southport, N.C.  These visitors come with great purpose and bring with them a mini-economic boom for this small town and many others like it across the entire country.

Southport, a town of 3,000 residents, is accustomed to visitors. Each year the population swells as tourists flock to the area for summer vacations. Situated at the mouth of the Cape Fear River, Southport is a hub of activity for those headed to Bald Head Island, the nearby island community known for its affluent clientele, or Oak Island that sits across the intracoastal waterway. The tourists give the area a massive economic boost with estimates of well over 100,000 people visiting the area each year. That impact is typically limited to the time frame between Memorial Day and Labor Day, creating a vibrant summertime  village with busy art galleries and restaurants.

During the remainder of the year, the town resembles any other small town in North Carolina with residents going about their normal routine. But because Southport is home to one of Duke Energy’s nuclear power plants, the town enjoys extra benefits, including well-paying jobs and a stable tax base that supports the entire county. Each spring, when the plant shuts down for refueling and maintenance, additional temporary jobs come open and local businesses enjoy the influx of outage workers.

Outage season typically lasts for 30-60 days and brings both challenges and benefits. Although the number of workers fluctuates each year, the arrival of these workers stimulates the local economy, especially for businesses offering services such as lodging, dining and entertainment. Residents notice increased traffic and might have to call ahead for reservations at their local restaurants, but most recognize more upsides than down.

Quantifying the impact of outage season is challenging, but the Nuclear Energy Institute  has taken some of the best data from across the country to build a generic model of how outages  stimulate local economies. In Southport, the local hotels and motels report near capacity occupancy rates, while the restaurants introduce special hours designed to attract outage workers. As one resident puts it, “you can tell it is outage season when you see specials starting at 7 a.m.” When you consider the larger picture, there are about 100 towns just like Southport across the country that benefit when  outage workers arrive.

During a typical outage, workers at the plant will replace approximately one-third of the fuel, conduct inspections and make repairs and upgrades. Some of the work is highly specialized but there are also jobs for welders, pipe-fitters and crane operators.  Unlike the more stereotypical construction worker, nuclear outage workers are a special breed who understand the unique environment they work in, the importance of safety and why utilities work to complete the work as efficiently as possible.  Outage workers have to pass a series of background checks and investigations to earn the chance to work at a nuclear plant.