Emergency services are taking part in the largest nuclear alert exercise seen in Cumbria for 15 years.
The aim is to test emergency procedures around Sellafield, near Whitehaven, in the event of an incident at the reprocessing plant.
Tuesday's Operation, Oscar 8, will see about 700 people involved in the exercise in the Whitehaven area and another 350 at Carlisle's Sands Centre.
Plant operator British Nuclear Group (BNG) is co-ordinating the operation.
The exercise, including members of police, fire, ambulance and local authorities, will also involve the sounding of the site siren at Sellafield.
Donald Norrie, chief emergency planner for Cumbria County Council, said: "The alert will give those involved the opportunity to test fallback arrangements that are in place within the county to use an emergency centre other than the one designated in West Cumbria.
"This county has been at the forefront of developing plans for dealing with emergency situations, including an incident at Sellafield."