The move to reduce staff at the plant in northern England was a "conservative" decision and only essential workers were being asked to report for their shifts, but the plant was otherwise operating "normally".
The levels of radioactivity were not high enough to necessitate any action outside the site in Cumbria, the operators Sellafield Ltd said.
"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 operator said.