When an energy company with over 5 million customers in eight states pledges to go carbon free, that action will have significant impact on society, on the economy and on the environment.
“We have a goal to reduce carbon emissions 80% by 2030 and a vision to provide customers with 100% carbon free energy by 2050,” said Pamela Prochaska, director of nuclear policy and strategy for Xcel Energy. “Nuclear power plays a key role in helping us reach this goal and vision by providing reliable, affordable, carbon free energy for our customers.”
For a company with 3.5 million electric customers and 2 million natural gas customers, reaching those goals will be a challenge, but Xcel said it is moving in that direction already.
“Our nuclear plants provide about 30% of the electricity for our customers in the Upper Midwest,” Prochaska said, “and about half of the carbon-free electricity.”
There are many factors that will help determine whether Xcel Energy can reach these goals, Prochaska said, including new technologies that might make the transformation easier.
“We don’t know exactly how we are going to get to 100% carbon free,” she said, “because there may be other technologies or technology advancements that we don’t know about yet, but we will continue to monitor that and see where technology can take us. ”
One key to those goals will be continued use of nuclear power through the Prairie Island nuclear plant above U.S. Lock & Dam No. 3 near Red Wing. The 520-acre site has two nuclear reactors, with one beginning operation in 1973 and the second in 1974. They were authorized initially under 40-year licenses which were renewed in 2011.
“We worked with the Nuclear Regulatory Commission which is our federal regulator and the state of Minnesota,” Prochaska said. “We received license renewals for 20 additional years, so our units were extended to 2033 and 2034.”
“Safely operating our nuclear power plants and protecting the health and safety of the public is number one,” Prochaska said. “Having grown up in Red Wing and then raising my family here, I take that mission personally.”
How nuclear power works
The use of nuclear power to generate electricity reduces the company’s carbon footprint because the nuclear fuel is carbon free.
“The fuel is a ceramic material,” Prochaska said. “The fuel pellets contain uranium 235 atoms and have a lot of energy. Each pellet is equivalent to a ton of coal or 150 gallons of oil.”
The fuel pellets are placed vertically inside a device called a fuel assembly. Each assembly is about 14 feet tall and 14 inches square, and 121 of those assemblies are located in the reactor where the Uranium 235 atoms create the fission that produces the necessary heat.
“A neutron is absorbed by that uranium atom,” Prochaska said, “and that causes it to become unstable. It wants to get rid of its energy, so it splits and creates fragments and additional neutrons, and in that whole process is creating heat.” The plant is able to control that chain reaction with control rods.
There are three separate water loops within the system. In the first, water circulates around the fuel assemblies and picks up the heat before it flows into a steam generator, essentially a large heat exchanger, with 3300 tubes.The primary water is on the inside of the steam generator tubes physically separated from the secondary system.
“In the secondary system, water is pumped around the outside of the steam generator tubes,” Prochaska said. “It comes into contact with the heated tubes, turns to steam and exits at the top of the steam generator. It goes to a series of turbines and as the steam expands across the turbine blades, they spin at 1,800 rotations per minute. A concept similar to blowing on a pinwheel gets the turbine moving.”
The spinning turbines turn the generator shaft which is where “we are turning that mechanical energy into electricity, and we have it exit on our 345 KV system here and leave our Prairie Island substation,” Prochaska explained.
After the steam in the second water loop passes through the turbines, it goes into condenser tubes where it cools and interfaces with the third water loop which is water from the Mississippi River and also physically separate from the secondary system. The river water is pulled into the system through large condenser tubes where it helps cool the steam and return it to condensate which is much easier to pump back into the steam generator for the next round.
The river water is screened to remove all fish and fish larvae, according to Prochaska. The river water is never mixed with the water from the other two loops, but only helps cool the steam inside the condenser tubes.
Once the river water has run through the condenser tubes, it can be moved outside, cooled, and recycled through the system again, or it can simply be released through a canal back into the Mississippi River.
“All of that comes with permits and regulations through the Minnesota Pollution Control Agency to make sure that we don’t adversely impact the river,” Prochaska said.
The two units at Prairie Island are capable of producing 1,100 megawatts of electricity which can power a million and a half homes, according to Prochaska. Running the nuclear plants is equivalent to removing 2 million cars from the roads in Minnesota. To do that requires a labor force of 800 including the operators, contractors, and security personnel. That number increases every two years when Xcel Energy hires an additional 1,000 workers while they shut down and conduct a refueling process.
The refueling typically takes about 30 days, and about half of the used fuel assemblies are removed and replaced with new assemblies. The spent assemblies are stored temporarily in a large pool of water, then placed inside dry cask containers which are sealed and bolted and placed on a large concrete storage pad.
The federal government initially planned to build a permanent storage facility at Yucca Mountain in Nevada, but no action has been taken on that, leaving facilities like Prairie Island holding used fuel casks indefinitely.
“Our storage here was intended to be temporary,” Prochaska said. “We can and we do safely monitor and manage it. We know how to store it. We don’t have any technical issues with storing the fuel.”
The federal government has a legal obligation to store the spent fuel from all the operating and shut down nuclear plants and per contract was supposed to pick up the spent fuel beginning in 1998. Xcel Energy customers have already paid for it with a fee that was collected.
The Prairie Island plant is capable of running around the clock, but as more renewable sources such and wind and solar enter the system, Xcel Energy can be flexible about working with them.
“We have the ability to integrate with renewables,” Prochaska said. “If we see that there is going to be a high wind penetration tomorrow, we can and do back down the nuclear units. It’s called flexible power operations.”
The recent efforts toward becoming carbon free are a source of pride for Prochaska.
“We were the first major utility to announce a carbon-free energy vision by 2050,” she said. “When I am at industry and community events, there is a lot of curiosity and recognition for what the company is trying to do. It’s the right thing to do. It’s the right thing for the environment, and it’s the right thing for our customers.”