All non-essential staff have been told to stay away from the British nuclear reprocessing site, Sellafield, after detectors recorded an "elevated level" of radiation.
A statement from Sellafield said on Friday that levels of radiation at the Cumbria facility were above that which naturally occured, but "well below that which would call for any actions to be taken by workforce" on or off site.
A statement said: "As a result of a conservative and prudent decision, the Sellafield site is operating normally but with reduced manning levels," it said in a statement.
"The site is at normal status and employees and operational plants are continuing to operate as investigations continue. All our facilities have positively confirmed there are no abnormal conditions and are operating normally."
The UK's decommissioning agency said it was too early to determine whether the radiation came from a leak. It said the workforce and general public were not at risk.
Sellafield is the UK’s largest and most hazardous nuclear site, and a store site for tonnes of highly radioactive waste.
It processes spent fuel and no longer produces power. It is undergoing a decommissioning and dismantling programme, run by the British company Amec, French group Areva and the US firm URS.
A report by the the Public Accounts Committee last year stated that the cost of cleaning the site had reached $111 billion and that costs would continue to rise.
The British energy ministry said it was in "constant contact" with Sellafield and there was "no reason to believe" the incident is more serious than its managers say.