An "emergency exercise" is being carried out at the Sellafield site in west Cumbria today.
This is designed to test the readiness of those who work there to cope with an emergency situation.
As a result, people living near the site may hear unexpected noises including sirens, firearms being discharged and small explosions.
These exercises are carried out regularly at nuclear sites.
A spokesperson For Sellafield said:
They adde: "Emergency exercises are held regularly at all nuclear licensed sites to test their readiness in the event of an emergency."
The exercise has no fixed end time but is expected to be completed this afternoon.