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

Fuel
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.

Nuclear Fuel: What do we do with that stuff?

All nuclear power reactors use uranium fuel to generate electricity. This fuel consists of small ceramic pellets stacked into metal fuel rods, which are bound together into fuel assemblies that are eight to 12 feet in length.

Pellet

Once fuel has been used in a reactor, it needs to be stored. Currently, all used nuclear fuel is stored at the plant site in either used fuel pools or in dry storage.  The Nuclear Energy Institute estimates that all the used nuclear fuel produced by the U.S. nuclear energy industry in nearly 50 years—if stacked end to end—would only cover an area the size of a football field to a depth of less than 10 yards.

Jason Smith, a lead engineer specializing in nuclear fuel management for Duke Energy, says that safe used fuel storage is a priority at all nuclear power plants. “Currently, every site is responsible for storing its used fuel. There’s a tremendous amount of work and planning that goes into managing used fuel. Whether it is stored in fuel pools or dry cask storage, it poses no risk to the public or our workers,” Smith said.

Diagram of a typical dry cask storage system.

Diagram of a typical dry cask storage system.

On average, nuclear fuel spends about four years in a reactor. Once removed from the reactor, the fuel is stored in the used fuel pool for a minimum of five years, which allows the fuel to cool and lowers radioactivity levels. The used fuel remains in the pool until it meets certain requirements necessary for it to be transferred to dry cask storage.

Dry cask canisters are concrete and stainless steel. To transfer fuel from the pool to dry storage, workers carefully place the canisters into the fuel pool and load the used fuel. Once the fuel is loaded into the canister, the water is removed and it is filled with helium and welded shut. At this point, the canister is transported into a storage module for long-term storage.

The fuel will continue to give off heat and radiation, but it is safely contained within the dry storage cask. “Once the fuel is placed in the concrete storage module, the exposure is very low and poses no threat,” Jason Smith said. “Of course, we continuously monitor it.” At the Robinson Nuclear Plant, the used fuel on site dates back to the beginning of operations in 1971.

Containers used to store and carry used fuel are extremely robust and provide additional layers of protection beyond that provided by the fuel rods themselves. The Nuclear Regulatory Commission requires thorough tests and analyses prior to certifying used fuel containers.

Facilities such as Sandia National Laboratories have tested containers under extreme circumstances to ensure they will protect the public in the unlikely event of an accident during transport. Tests have proven that containers can withstand high-speed crashes, extremely hot and long-lasting fires, puncture, and submersion in water.

Dry casks being monitored by plant workers.

Dry casks being monitored by plant workers.

The approved containers are massive, weighing 25 to 40 tons for truck shipments and 75 to 125 tons for rail shipments. Multiple layers of steel and other materials confine the radioactivity. Typically, for every ton of fuel, there is more than three tons of protective shielding.

There is still a need for a sustainable, long-term solution for storing used nuclear fuel. However, dry cask storage remains a viable, safe and secure storage option for the foreseeable future.

Want to learn more about the disposal of nuclear fuel, click on the links below:

NEI – Transporting Used Nuclear Fuel

 NEI – Safely Managing Used Nuclear Fuel