The Sellafield nuclear site in Cumbria has been given the all-clear after it was shut to all but essential staff following reports of the detection of elevated levels of radioactivity.
Its operator, Sellafield Ltd, had said the site would be run at "reduced manning levels today" after radioactivity readings reached levels above those that occur naturally.
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.
In a statement it 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 on site.
"Our in-air monitors are extremely sensitive and pick up on any abnormality. Overnight the monitoring system initially indicated elevated levels of activity. Following investigation and analysis, we can now confirm these levels to be naturally occurring background radon.
"The No 1 priority for us is, at all times, safe secure stewardship of the Sellafield site, which is the most complex and challenging nuclear site in Europe.
"As such, we act in a safety-conscious manner, and take cautious, conservative decisions, such as the one taken overnight to ask non-safety-essential staff to stay at home this morning, rather than come to the site.
"All of our plants and storage facilities were quickly confirmed as operating normally, and we were always confident that the issue posed no risk to the workforce or public because the levels being detected, whilst above background radiation levels, were still low.
"This view was reinforced by the fact that none of our other installed monitors were picking up any kind of increased levels. However, we take such issues so seriously that we investigated fully to confirm that everything was OK.
The Department of Energy and Climate Change has stressed there is currently no risk to the public or workforce.
"Standard weekend working operations will continue, with day staff due back in on Monday as normal.
"We would like to thank our staff and the wider West Cumbrian community for their calm and mature response to events this morning."
Prospect, 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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