Officials with the Nuclear Regulatory Commission say they need more information about the AP1000's ability to stand up to seismic activity.
Westinghouse has contracts to build the next-generation reactors at sites across the Southeast, including two at Plant Vogtle. But construction can't begin until the NRC certifies the design, a decision that until now was expected by the end of the Summer.
Todd Terrell is a spokesman with Plant Vogtle's owner, Southern Company. He says he still believes the company will get a license to build and operate the reactors soon.
"We support the process and we have confidence that the AP1000 will emerge from that process so that we can get our COL by the end of this year."
But Sarah Barczak with the environmental group Southern Alliance for Clean Energy says any delay isn't fair to Southern Company customers already paying a surcharge for construction of the new Vogtle reactors.
"There has to be frustration because people are still dealing with tough economic times and dollars coming out of their pockets in advance for something that may never materialize."
Right now the AP1000 design is in it's 18th revision.