Don't go running and screaming 'the sky is falling' because...this is a drill....I repeat....this is a drill. Actually, they will announce over the PA that 'this is a drill.' Remember to tune into local TV and Radio stations in the event it is not a drill, as the Emergency Alert System would be activated. Here's the press release from the DOE:

RICHLAND, Wash. – Emergency sirens will be activated for an emergency drill on the U.S. Department of Energy's Hanford Site Thursday, September 19, 2019.  Sirens will be activated between 12:30 p.m. and 1:30 p.m. in the Hanford Site’s 300 Area, near the Columbia River and approximately one-mile north of Richland, as well as the Site’s 400 Area, four miles from the Columbia River and six miles northwest of Richland.
 
The public near these areas may hear the sirens, but no response is required.
 
Public address announcements stating "this is a drill message" will be made prior to and following the siren tone. In a real emergency, county officials would activate the Emergency Alert System to broadcast information to the public via local radio and television stations. Sirens located along the Columbia River would also be activated if it were necessary to warn people on the Columbia River.
 
Periodic emergency drills and siren tests are conducted at Hanford facilities as part of an ongoing program to test emergency equipment and train workers on emergency response procedures.