Westinghouse said this is the first insertion of EnCore fuel assemblies into a commercial nuclear power plant.
The operation was carried out during a scheduled refuelling outage at Byron-2.
Westinghouse said the two lead test assemblies contain chromium-coated zirconium cladding for enhanced oxidation and corrosion resistance, higher density fuel pellets for improved fuel economics, and uranium silicide pellets.
The EnCore fuel has the potential to increase safety through the use of new materials, while improving the economics of plant operation through longer fuel cycles and improved flexibility, Westinghouse said.
In January 2019, Westinghouse received $93.6m in funding from the US Department of Energy in support of its accident-tolerant fuel programme.