The plant is testing the siren system that alerts communities of an emergency.
The tests were scheduled for 10 a.m., 10:45 a.m. and 11:30 a.m.
The sirens will go off several times for about three minutes each time.
The alarms are located in Dana Point, San Clemente, San Juan Capistrano and Camp Pendleton.
Officials asked residents not to call 911, because it's just a test.