The Sellafield site has been closed to all day workers in response to an operational condition. The firm says the decision has been taken with the safety and security of the workforce as a priority. The site remains at normal status.
The Sellafield nuclear plant in Cumbria has been given the all-clear after an investigation into higher-than-normal radiation readings detected at a perimeter fence.
The monitoring system picked up "elevated" levels of radioactivity overnight, leading to workers being told to stay away.
But following an analysis by safety staff, 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.
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.
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