"Levels of radioactivity detected [by on-site monitors] 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," the company said in a release.
Only essential workers were asked to report for work Friday at the plant in England.
"The site is at normal status and employees and operational plants are continuing to operate as investigations continue," Sellafield said. "All our facilities have positively confirmed there are no abnormal conditions and are operating normally."
A Department of Energy and Climate Change spokeswoman confirmed the elevated radioactivity but said it was "well below levels of concern," the Guardian reported.
Gary Smith, the National Union of General and Municipal Workers' national secretary for energy, said most of Sellafield's employees were told not to report. The site employs more than 10,000 people.