The Sellafield nuclear reprocessing 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 and contractors 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.
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 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 number one 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 okay.
"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."
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.
National officer Gill Wood said: "Higher than normal radiation readings have been detected at one monitor at a perimeter fence. As a precaution, non-essential staff have been advised to stay at home today while the relevant specialist team investigates.
"The company's decision to partially close some areas at the site is a precautionary and measured decision and a safety measure that is recognised worldwide."