Naturally occurring radiation was the cause of an alert at the Sellafield nuclear waste plant which prompted the owner to tell staff to stay at home.
Last week non-essential workers were advised not to come to the West Cumbrian site.
The operator said the radioactivity detected by one of its in-air monitors was not attributable to any issue or problem with operations on site.
Sellafield said its in-air monitors are “extremely sensitive and pick up on any abnormality”.
In a statement it added: “Following investigation and analysis, we can now confirm these [elevated] levels to be naturally occurring background radon.”