Nuclear power in Illinois: Behind the scenes at Byron Generating Station

This story has been update to clarify Byron Generating Station's administrative radiation limit is 2 rems, while federal standards require a limit of 5 rems. It has also been updated to reflect that the plant's backup generators each have the capacity to power essential functions, but not the entire facility.

About 90 miles west of Chicago, surrounded by agricultural fields and winding country roads, distinctive white clouds of water vapor waft into the sky from the two cooling towers of the Byron Generating Station.

When visitors and employees turn into the nuclear power plant's main drive, a giant banner greets them. It reads: "Safety Is A Skill." The phrase is repeated throughout the station's expansive campus, which sits on approximately 1,782 acres.

On a chilly Tuesday morning last week, a Daily Herald reporter and photographer were given a rare opportunity to tour the plant and study the operations of one of six nuclear plants in the state, where nuclear reigns king among other power sources.

The facility is tightly run. Federal background checks are necessary, security checkpoints are common, radiation monitors are required and visitors are never left alone. A minimum of five people operate the control room 24/7. It takes at least two years of training to step foot in the control room as an operator, and operators must take a full week of additional training every five to six weeks.

Employees are limited to 2 rems of radiation per year, though that maximum is rarely reached. The limit is stricter than federal standards, which is 5 rems. In comparison, a chest X-ray emits about 60 millirems, while a CT scan typically clocks in at about 800 millirems.

Nearly every piece of the equipment at the plant has a double that can step in in the face of potential maintenance issues or equipment failures. Even if something were to fail, the system is generally designed to shut down the reactor for repair rather than run without a backup.

"The thing to keep in mind is that a lot of our plant has duplicate equipment, so we have multiple different runs of equipment that support the same function," Connor Friedrichsen, a Byron design engineer, said. "But we don't want to keep running on that single point of vulnerability essentially. Out of precaution, the systems are designed to take the reactor down until we repair the system that's having an issue. We take nuclear safety very seriously."

Though the plant is powered off the grid, it has four 7,600-horsepower diesel generators that were built with the plant to provide backup power.

They are kept in pristine condition, are tested for four hours each month and can be pushed to full power within 10 seconds. Just one of those generators could provide enough electricity to power essential functions like cooling and emergency systems.

Illinois generates more electricity from nuclear energy than any other state, accounting for one-eighth of the nation's total nuclear power generation. In 2022, nuclear plants produced 52% of the state's net electricity generation, according to the U.S. Energy Information Administration. In contrast, coal represented 21% while renewable energy accounted for 14%.

Because Illinois gets a much larger percentage of its electricity from nuclear power than other states do, the state relies on it more to hit its climate goals - namely reaching 100% clean energy by 2050.

As part of the massive climate legislation package that lawmakers passed in 2021, the state agreed to pay up to $694 million over five years to keep a handful of nuclear plants open, including Byron.

Chicago-based corporation Exelon, which also owns ComEd, reversed its plans to retire both the Byron and Dresden generating facilities following the legislation.

Byron's two reactors make enough electricity to power 1.7 million homes. That electricity is dispatched everywhere from locally, out to the Chicago area, and even to neighboring states, including Wisconsin.

Because nuclear energy is so plentiful in Illinois, it can often be dispatched to other states, said Paul Dempsey, the communications manager for Constellation, a subsidiary that separated from Exelon in 2022 and now owns and operates Byron Generation Station along with 11 other nuclear plants across the country.

The Byron reactors run at full capacity more than 90% of the time. That number is primarily informed by scheduled refueling outages, which happen every 18 months for each reactor. At Byron, unplanned outages are few and far between, with the latest occurring in 2016.

Refueling, which is typically scheduled for the spring or fall when demand for power is low, lasts about 16 to 21 days. The plant is currently wrapping up a scheduled refueling for Unit 2 that began Oct. 9.

The process involves unloading the reactor core's 193 rod assemblies. The assemblies each carry 264 rods, which hold the uranium pellets, each the size of a large pencil eraser.

Once the core is empty and the spent fuel is transferred into a cooled fuel pool, the reactor is reloaded. One-third of the assemblies that go into the core are brand new, while the remaining two-thirds have already been used.

Fuel assemblies are reused three times. Once they are fully used, they spend five years decaying in the fuel pool. From there, they are loaded into dry storage casks made of concrete, which are kept on-site and air cooled indefinitely.

Whereas the plant employs about 638 workers year round, the outages bring an extra 1,000 workers to the facility.

That's because aside from refueling, outages are an opportunity for employees to perform tests, inspections and maintenance on equipment that can't be done while the unit is at full power.

During an outage, the plant is a busy hive of activity. Once the reactor has been closed back up and the visiting carpenters, pipe fitters and other specialty workers return home - often to other states - the facility settles down significantly.

"We like it quiet," one operator remarked.

The vast facility includes the two reactor buildings, an auxiliary building that houses the control room, the turbine building, and two service buildings. The cooling towers, which reach 495 feet high, are situated on the east side of the property.

The water vapor the towers emit represents the latter end of the nuclear power cycle.

The cycle begins when uranium atoms in the reactor core split, releasing neutrons and heat. Because the reactor core is immersed in water, as that chain reaction happens, the heat generated is used to create steam.

But Byron's system keeps the water surrounding the nuclear fuel under pressure to prevent it from actually boiling. Instead, it cycles the hot water from the reactor core to a steam generator. There, the water heats a second, separate supply of water, which boils to make steam.

It takes about 15 seconds for one drop of water to cycle from the reactor core, through the stream generators, and back.

The steam then spins a set of turbines, which drives a generator that produces electricity. The mechanism is similar to the turbine used to generate wind power - the only difference being that steam causes the turbine to spin, not wind.

After the steam is used, it gets condensed to water so it can be recycled and reused. The cooling towers are part of the cooling system used to condense the hot steam from the turbine back into water.

Byron's pressurized system differs from some other nuclear reactors, which heat the water surrounding the nuclear fuel directly into steam in the reactor vessel. Of the country's 93 operating reactors, 31 are boiling water reactors and 62 are pressurized water reactors.

* Jenny Whidden, jwhidden@dailyherald.com, is a climate change and environment writer working with the Daily Herald through a partnership with Report For America supported by The Nature Conservancy. To help support her work with a tax-deductible donation, see dailyherald.com/rfa.