Non-essential staff at the Sellafield nuclear site in Cumbria have been asked to stay at home due to elevated radiation levels.
The cause of the higher reading is being investigated.
A statement this morning says: "As a result of a conservative and prudent decision, the Sellafield site is operating normally but with reduced manning levels today.
"This follows the detection of elevated levels of radioactivity at one of the on-site radiation monitors at the north end of the site.
"Essential workers only are being asked to report for work.
"Levels of radioactivity detected 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."