NRC spokesman Victor Dricks said radioactive gas "could have" escaped the San Onofre Nuclear Generating Station on the northern San Diego coast after the Tuesday shutdown.
Southern California Edison spokesman Gil Alexander said the amount would have been "extremely small" and possibly not detectable by monitors.
The company and federal regulators said the release would not have posed a safety risk for the public.
"It would have been very, very small, low level, which would not pose a danger to anyone," said Victor Dricks, NRC spokesman.
The leak occurred in equipment installed in the plant in fall 2010. The leak happened in one of thousands of tubes which carry radioactive water from the Unit 3 reactor.
Dricks said the company has found damage to other tubes, which he called "unusual."
Edison spokesman Alexander could not confirm any additional damage, pending an inspection of the equipment.
He said the leak at the plant doesn't meet standards for an emergency classification.
"U.S. nuclear power plants classify emergencies in four ways from the least severe to the most severe," said Alexander during an interview on KPBS-FM's Midday. "This doesn't even rate the lowest rating."
Carlsbad resident Ace Hoffman said despite recent upgrades, the aging plant needs to be mothballed.
"This plant in the middle of this population after this many years is a catastrophe waiting to happen," said Hoffman. "But what it does show is that the plant is continuing to degrade. And they don't know where it's going to fail next. And even if they do a whole lot of maintenance, they don't know where it's going to fail next."
Hoffman is a member of the group, Nuclear Free California, which wants to shut down the two nuclear power plants operating in the state.
The group, Friends of the Earth, said the radioactive release from the San Onofre nuclear plant is "a sharp reminder that a Fukushima could happen here."
The San Onofre plant is owned by Southern California Edison, San Diego Gas & Electric and the city of Riverside.
Southern California Edison serves nearly 14 million residents with electricity in Central and Southern California.
Dricks with the NRC said the federal regulatory agency is evaluating the plant's response to the leak.
Alexander said San Onofre personnel will evaluate the cause of the leak and the steps required to repair it before resuming operations.
Once the problem is resolved, Alexander said it is expected to take several days for the Unit 3 reactor to be restarted.
Unit 2 is currently offline for a planned outage intended for maintenance, refueling and technology upgrades.
But Southern California Edison said it has ample reserve power to meet customer needs while Unit 3 is offline due to the leak.