The Sellafield nuclear reprocessing plant in Cumbria has been partially closed after reports of 'elevated' radiation levels.
A spokesman said the decision had been taken to operate the site at reduced manning levels, commensurate with safe operations.
He added that readings are "above the normal rating but below anything to cause us to take any action".
The company stressed there was no risk to the public or workforce.
Part of the site is being cleared to allow detailed investigations, the spokesman said.
Non-essential staff have been told to stay at home.
Other workers, including transport and utilities personnel, are working as normal.
The plant on the west Cumbria coast lies 34 miles from the Isle of Man.
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