Only “essential” staff are being asked to come to work at the site in West Cumbria, England.
Sellafield is keen to stress that it believes there is no risk to the workforce or the public.
In a statement this morning it said the site was operating at “reduced manning levels… following the detection of elevated levels of radioactivity at one of the on-site radiation monitors”.
It added that these levels are “above naturally occurring radiation but well below that which would call for any actions to be taken by the workforce on or off the site”.
It said the site was at “normal status” and continuing to operate “as investigations continue”.
This afternoon Sellafield issued a further statement saying it could "confirm that the radioactivity detected by one of our in-air monitors overnight is not attributable to any issue or problem with any of our operations on site".
"Our in-air monitors are extremely sensitive and pick up on any abnormality. Overnight the monitoring system initially indicated elevated levels of activity. Following investigation and analysis, we can now confirm these levels to be naturally occurring background radon."