Workers stopped the leak by 5 p.m., about two hours after it was detected in a storage tank in the water purification system of Unit 3 at San Onofre Nuclear Generating Station, said Todd Adler, the plant's engineering manager.
The emergency alert was required because fumes could prevent access to certain areas of the plant, Adler told reporters at a media information center in Irvine, Calif. The alert was canceled at 6:07 p.m. and evacuated workers were allowed to return.
The leak was in the non-nuclear section of the plant, which is operated by Southern California Edison. No radioactive material was released, no injuries were reported and there was no danger to the public, the company said.
Approximately 30 gallons of leaked ammonia were collected in a basin underneath the tank that was designed for that purpose, Edison spokeswoman Lauren Bartlett said.
Electricity production at the plant was not affected, and other units remained fully operational, Adler said.
The plant is located about 45 miles north of San Diego, just south of San Clemente, and is jointly owned by Edison, San Diego Gas and Electric and the city of Riverside.
While not dangerous for the public or plant workers, an emergency alert at the power plant is an unusual occurrence.
"This is not normal," Edison spokesman Chris Abel said. "The last time we had one (alert) declared was May 1999, because of a suspected pipe bomb on the freeway."