The plant has been shut down for nearly a year because of issues with unusual wear on its steam generator tubes, which carry radioactive water. The NRC is weighing a proposal by plant operator Southern California Edison to restart one of the two units and run it at partial power, which the company says would alleviate the conditions that led to the wear.
The commission had previously said it might make a decision as soon as March. But it is now projecting a late April or early May date.
NRC spokesman Victor Dricks said the timeline was extended because additional on-site inspections were required. The schedule remains tentative, he said.
Edison said in a statement: "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."
It would take several weeks to restart the plant if the NRC gives the go ahead, meaning that the unit could potentially be back online by summer.
Southern California got through last summer without blackouts in the absence of San Onofre's 2,200 megawatts of power, but some of the measures taken to replace the plant were temporary. Energy officials have been working on a backup plan for this summer, should the plant remain out of service.
The commission has a public meeting on San Onofre scheduled for Feb. 12 from 6 p.m. to 9 p.m. at Capo Beach Church in Capistrano Beach to give an update on the status of its review.
This story was reported by Times Staff Writer Abby Sewell.