All non-essential staff at the Sellafield nuclear site have been told to stay at home after elevated levels of radiation were measured at the plant.
Around 8000 staff, including contractors and day personal, have been told not to come to the reprocessing site in Cumbria for their own safety because of the high radiation levels.
The plant said it is still operating at a normal level and no action is needed by workforce on or off the site.
Part of the site has been cleared while an investigation is carried out into the higher than normal levels of radiation.
A Sellafield spokesperson said: "As a result of a conservative and prudent decision, the Sellafield site is operating normally but with reduced manning levels today.
"This follows the detection of elevated levels of radioactivity at one of the on-site radiation monitors at the north end of the site.
"Essential workers only are being asked to report for work.
"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.
"The site is at normal status and employees and operational plants are continuing to operate as investigations continue. All our facilities have positively confirmed there are no abnormal conditions and are operating normally."
The firm added: "Personnel should consider how they may continue to support the business during this period. In some cases, provided this can be agreed with their line management, this may include home working if appropriate."
There are also reported to be traffic jams around the site since people arriving to work have been immediately sent away.
A report in 2012 by the National Audit Office condemned the 68-year-old site, which had deteriorated so much that its "contents pose significant risks to people and the environment". The report resulted in a clean up at the site which is believed to have cost more than £65bn.