The San Onofre plant in coastal San Diego County has been shut down since January 2012 because of unusual wear on its steam generator tubes, which carry radioactive water, the Los Angeles Times reported.
The plant's operator, Southern California Edison, has proposed restarting one of the plant's two units and running it at partial power, which would alleviate the problems that led to the wear, the company said.
The NRC previously said it would make a decision as soon as March. However, now the commission says the decision will likely come in April or early May.
NRC spokesman Victor Dricks said the schedule was extended because more inspections at the plant were required.
In a statement, Edison said: "As we have said from the beginning, there is no timeline on safety. SCE is confident in its submitted response and restart plan and is currently working on our response to technical questions from the NRC."