The Sellafield nuclear reprocessing plant in Cumbria is partially closed after “elevated levels of radioactivity” were detected.
A perimeter alarm has been triggered at the north of the site and each building is being checked, a spokesman said.
No evidence of radioactive release or accident had been found but “there can be no guessing on nuclear sites”, he added.
The company stressed there was no risk to the public or workforce.
In a statement, The Department of Energy and Climate Change said: “We are in constant contact with Sellafield but we have no reason to believe that it is any more serious than they’ve said.”
Part of the plant is being cleared to allow detailed investigations and the “relevant experts” are on site, a Sellafield spokesman said.
That the alarm had detected naturally occurring levels of radiation could not be ruled out, he added.
David Moore, chairman of the West Cumbria Cumbria Stakeholders’ Group, an independent watchdog funded by the Nuclear Decommissioning Authority, said the company’s actions did not imply “anything has gone wrong on the site”.
“They’ve never been scared to raise the levels for safety,” he said. “I think they’ve taken the right decision here.”
Day personnel, agency staff and contractors have been told to stay at home.
Other workers, including transport and utilities personnel, are working as normal “in support of plant continuity requirements”.