The plant in Cumbria, northeast England was closed to all day workers on Friday following Sellafield Ltd.’s warning that radioactivity readings exceeded naturally occurring levels.
"Levels of radioactivity detected are above naturally occurring radiation but well below that which would call for any actions to be taken by the workforce on or off the site," said plant operator Sellafield Ltd.
However, it stressed in a statement that the warning poses no risk to the public or workforce.
"One of the 20-odd site perimeter monitors that we have is registering above normal levels of radiation. It's not a level that would trigger any kind of activity on or off site. It's below levels that would demand us to do sheltering or anything like that," said Rory O'Neill, director of stakeholder relations.
As the UK’s largest nuclear site, the plant is located around 480km northwest of London, and it’s been a key player in the country’s nuclear industry since the 1940s.
Britain suffered its worst nuclear incident in 1957, when a fire broke out in one of the site's reactors.
The plant is going through a decommissioning programme, which includes dismantling infrastructure and decontaminating nuclear waste.
The cost of decommissioning the plant has been estimated at over 70 billion pounds.
Objections to the site have been raised by the governments of Ireland, Norway and the Isle of Man, which are concerned by the risks of contamination.