Nuclear Regulatory Commission senior inspector Greg Warnick said the agency is beginning to prepare a detailed plan of what would need to be done to bring San Onofre safely back to service.
The seaside plant between San Diego and Los Angeles hasn't produced electricity since a tiny radiation leak led to the discovery of excessive wear on hundreds of steam generator tubes that carry radioactive water in January 2012.
Workers for Southern California Edison are visiting other plants this week to become familiar with working conditions after a year with San Onofre on the sidelines.
"The equipment isn't used to operating at power, but the people aren't used to it either," Warnick said Monday.
Edison has asked the NRC for permission to restart one of the reactors, Unit 2, and run it at reduced power. A decision isn't expected until at least March.
NRC chair Allison Macfarlane toured the plant Monday and told reporters she's "concerned about the situation."
The trouble at San Onofre centers on steam generators that were installed during a $670 million overhaul in 2009 and 2010. After the plant was shut down, tests found some generator tubes were so badly worn that they could fail and possibly release radiation, a stunning finding inside nearly new equipment.
In effect, what was intended to be an upgrade that would extend the plant's operating life for years could instead cut it short — or even end it.
Macfarlane, who is not directly involved in the restart decision, promised a thorough review of Edison's proposal. She defended the agency against criticism that it was silencing the public in the review.
She said no decision would be made to restart the plant until "we are sure they can operate safely."
As Edison waits for a decision on a restart, the plant faces a host of lingering issues. The future of the heavily damaged Unit 3 reactor is not known — its nuclear fuel has been removed. Environmental groups are challenging various aspects of Edison's plans.
It's also not known if the damaged generators can be repaired and operated at full power.
"I don't know where that's going," said NRC Deputy Regional Administrator Art Howell, who heads the agency team overseeing San Onofre.
Friends of the Earth, a group critical of the nuclear power industry, is among several environmental organizations pushing the NRC to require Edison to seek an amendment to its operating license to restart the plant, a process that would involve court-like hearings and could take up to two years.
The generators, which resemble massive steel fire hydrants, control heat in the reactors and operate like a car radiator. At San Onofre, each one stands 65 feet high, weighs 1.3 million pounds and has 9,727 U-shaped tubes inside, each three-quarters of an inch in diameter.
Cracked and corroded generator tubing has vexed the nation's nuclear industry for years.
Decaying generator tubes helped push San Onofre's Unit 1 reactor into retirement in 1992, even though it was designed to run until 2004. The following year, the Trojan nuclear plant, near Portland, Ore., was shuttered because of microscopic cracks in steam generator tubes, cutting years off its expected lifespan.
San Onofre is owned by SCE, San Diego Gas & Electric and the California city of Riverside. The Unit 1 reactor operated from 1968 to 1992, when it was shut down and dismantled.