The Sellafield nuclear reprocessing plant in Cumbria, northwest England, is asking only essential workers to report for work after “elevated levels of radioactivity” were detected.
A perimeter alarm was triggered at the north of the site and each building is being checked, a spokesman told NucNet.
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 Sellafield said that as a result of “a conservative and prudent decision”, the site is operating normally but with reduced manning levels.
Director of stakeholder relations Rory O’Neill said: “One of the 20-odd site perimeter monitors that we have is registering above normal levels of radiation.
“It’s not a level that would trigger any kind of activity on or off site. It’s below levels that would demand us to do sheltering or anything like that.”
The UK’s Office for Nuclear Regulation said the levels are not increasing and it is satisfied that the licensee is “making progress with determining the source of the activity”.