The Sellafield nuclear reprocessing plant in Cumbria has been given the all-clear after an investigation into higher-than-normal radiation readings at a perimeter fence on Thursday night.
The monitoring system picked up "elevated" levels of radioactivity overnight, leading to workers and contractors being told to stay away yesterday. But following an analysis, Sellafield said the levels were "naturally occurring background radon".
The company is still trying to establish exactly why the alarm was sounded even though it has now established the cause.
A statement said: "Sellafield Ltd can confirm that the radioactivity detected by one of our in-air monitors overnight is not attributable to any issue or problem with any of our operations.
"Standard weekend work will continue, with day staff due back in on Monday as normal."
The Prospect union, which represents 5,000 nuclear specialists at Sellafield, said the elevated radiation readings were within acceptable limits and were not a danger to human health or to the plant.