(Editor’s Note: Innovator and I Deliver awards at Xcel Energy reward performance when teams and employees deliver greater-than-expected results. These award opportunities are designed to help fuel collaboration, innovation, continuous improvement and an enhanced customer experience. Xtra is running a series of articles on select winners.)
A larger “gas tank” has been created for Minnesota’s Prairie Island Nuclear Generating Station, which will spell a whopping $70 million of savings over the remaining life of the plant.
A team of engineers in Nuclear developed a new fuel-assembly configuration that allows for longer periods between refueling cycles. The reduction in fuel results in $4 to $5 million in savings for each unit per operating cycle – and the savings will be sustainable going forward for all future cycles.
For their efforts, the team members recently received one of Xcel Energy’s prestigious Innovator awards.
“I extend a well-deserved congratulation to Steve McSorley, Sean Martin and Darius Ahrar for their out-of-the-box thinking and efforts to make an innovative idea real,” said Tim O’Connor, senior vice president of Nuclear Operations and CNO.
“Theirs is a gift that will keep on giving,” he added. “Now, Prairie Island can operate on 24-month fuel cycles versus the previous 18-month cycle. This will lead to the elimination of two entire refueling outages between now and end of the licensed life of the plant.”
The effort began with the team asking: “Can we change the configuration to allow for more fuel in each fresh fuel assembly,” said McSorley, senior engineer.
It’s not an easy process to describe, but here goes.
The new approach involves the combining of two different poisons in one fuel-bundle assembly. Poisons in nuclear fuel refer to compounds that help control and manage the reaction taking place to create heat (to then produce steam, spin a turbine and create electricity).
The poisons control the power of the reaction by “eating” or absorbing neutrons, instead of them all being part of the chain reaction, McSorley explained. They are required because with nuclear power, all the fuel is put in at the start, and the heat it generates is then doled out over a period of time.
“Uranium wants new neutrons to create heat,” he said. “The process is like a bowling ball that hits one large bowling pin, breaking it apart into lots of small pieces and creating heat.”
One poison, called gadolinia or GAD, is fixed in place with uranium 235 on a certain number of fuel rods in each assembly, said Martin, Engineer. Over time, the GAD burns out – allowing for the remaining uranium to keep the reaction going, but also increasing possible safety issues due to more limited amounts of poison at work in the process.
That became the team’s key starting point: “Can we use two different poison types to increase energy, while keeping the core as safe or safer than with just one poison type?”
The team decided to try using another product called Integral Fuel Burnable Absorber or IFBA. This poison entails a boron coating that is sprayed on the uranium pellets held in fuel rods. The idea became known as the IFBA/GAD combo, Martin said, and allowed for the replacement of a lot of GAD with IFBA.
IFBA is not as effective as GAD at slowing the nuclear reaction. So as the IFBA fades during an operating cycle, GAD can take over and create safer and longer operating conditions, he said. It provides two limiting points (points approaching safety limits) as the cycle winds down and the remaining uranium is burned up, rather than one large limiting point.
“In essence, less GAD allows us to control the peaks of the reaction in the core,” McSorley said. “And with two humps or limiting points – one each for GAD and IFBA, instead of just one for GAD – we could then extend the operating cycle to 24 months.”
The overall effort to change the composition of the fuel assemblies has been a long one, said Ahrar, supervisor of Nuclear Analysis and Design. It took years for Westinghouse, the manufacturer of the fuel assemblies, to sort through the proposed changes and agree to create the new assemblies.
“It took a significant amount of effort to help Westinghouse understand the benefit of the change,” Ahrar said. “But it is now a big success in terms of operations, finances and technology – a win, win, win.
“It increases our safety margins and reduces costs,” he said. “It allows us to better control how the fuel is behaving and how the core is burned – and keeps us well away from approaching safety limits and margins.”
“It’s a big cost savings,” McSorley added. “We can run our cycles longer between planned outages, without adding more fuel assemblies or creating more waste.”
Prairie Island’s recent refueling of Unit One became the company’s first use of the new process. Unit Two will follow next year.
On one last positive note, Westinghouse is now marketing the new idea as its IFBA/GAD product line. And Xcel Energy is receiving a portion of sales for its employees’ impressive engineering efforts.
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