LONDON -- The operators of the Sellafield nuclear reprocessing plant say elevated radiation levels that led to some staff being told to stay away are due to "naturally occurring background radon" and not to any problems with the plant.

The company said Friday that the radiation monitors at the plant in northwestern England are extremely sensitive and pick up any abnormality.

"Overnight the monitoring system initially indicated elevated levels" that were later found to be naturally occurring, the company said.

Staff were instructed to return to work Monday as usual.