Naturally-occurring radiation was to blame for higher-than-normal levels detected at the Sellafield nuclear plant in Cumbria.
A perimeter alarm was triggered at the north of the site, leading to buildings being checked by safety staff, but it was later discovered the readings were due to "naturally-occurring background radon".
Workers were told to stay away this morning while the company carried out an investigation, but have been allowed to return this afternoon, as the plant has been given the all-clear.
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