A non-radioactive leak triggered an alert at the San Onofre Nuclear Generating Station on Tuesday afternoon.
Southern California Edison declared the alert after identifying an ammonia leak in the steam system used to drive the plant's turbines.
The approximately 30 gallons of leaked ammonia was being collected in a basin underneath the tank that was designed for that purpose.
Because of the strong ammonia smell, there was a precautionary evacuation of some plant workers whose jobs are located close to where the leak was found, spokesman Scott Andresen said.
No injuries were reported.
Orange County activated its Emergency Operations Center, but told cities there was no threat to citizens or surrounding communities.
The sirens first sounded at about 3 p.m. and the alert was lifted shortly before 6:30 p.m.
The chemical leaked from the Unit 3 steam system outside the plant's dome. The ammonia is used to clean resin in the turbines.
The alert was required because fumes could prevent access to certain areas of the plant. The control room remained staffed and fully operational, and the plant was operating at full power, officials said.