Britain's Sellafield nuclear reprocessing plant was given the all-clear after "elevated" levels of radioactivity were detected.
The plant in northern England had told all staff except for essential workers to stay home after its monitoring system picked up higher-than-normal radiation readings overnight.
But operators Sellafield Ltd later said the higher readings were "naturally occurring background radon".
"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," the company said in a statement.
"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."
Sellafield had stressed earlier yesterday that the move to reduce staffing was a "conservative" decision and that the plant was otherwise operating "normally".
Day staff would return on Monday as usual, the operator added.
