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 find out exactly why the alarm was sounded even though it has now established the cause.
A statement said: "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 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."
The Prospect union, which represents 5000 nuclear specialists at Sellafield, said the elevated radiation readings were within acceptable limits and were not a danger to human health.
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 while the relevant specialist team investigates."