Officials say the reactor was shut down as a precaution after a sensor went off on Tuesday detecting a radiation leak.
"It triggered a sensor telling us we have a leak. Again, it's a very small amount of water," said Gil Alexander of Southern California Edison.
Southern California Edison officials say the leak could be in one of the steam generator tubes that carries water heated by the reactor inside the southern containment dome.
Officials insist it was so small, it did not warrant an emergency declaration. The levels were so low that by regulation, they did not have to shut down the reactor, but they did just in case, officials said.
"There is no emergency declaration, there's no significant release of radiation. There's no danger to our workers," Alexander said.
But that's not enough reassurance for San Clemente Green, a community group that would like to see the nuclear power plant shut down.
"It's another red flag. I don't know how many red flags we'll get before something really tragic happens here," said Gary Headrick, founder of San Clemente Green.
The cause of the leak is not known. Workers must wait until the unit completely cools down before they can go in to find the leak and plan on how to fix it, which could take days.
The plant provides power to 1.4 million homes in Southern California. Officials say residents will not be affected while repairs are under way.