An ammonia leak in the San Onofre Nuclear Generating Station prompted the station to evacuate its employees in the area of the leak just before 3 p.m. on Tuesday.
There is no radiation leak, according to officials. Both units are operating normally.
San Diego County officials said the conditions did not threaten public safety.
An alert is the second lowest of four emergency classifications used at nuclear plants. It was prompted by discovery of an ammonia leak in the Unit 3 steam system in a non-nuclear portion of the plant.
The County of San Diego, in response to the alert, activated and staffed the Operational Area Emergency Operations Center with county agency representatives, in order that it may be prepared should the situation at the power plant deteriorate.
It never got to that point. By 6:07 p.m. the alert was terminated and clean up continued. About 25 gallons of ammonia leaked and although it could be hazardous to humans who breathe it in, it did not injure any workers at the plant.
Power officials are now determining what caused the leak.
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