Full investigation is under way
Thorp was still shut down at the time for routine maintenance, as part of which work was taking place to replace a stand-by pump in the chemical plants.
A Sellafield spokesman said: “This is a routine operation which has been undertaken on many occasions using the same methodology without complication.
“There has been no release of radioactivity and extensive checks undertaken have confirmed that all plant parameters are normal. Any incident that compromises the health and safety of the workforce very seriously.”
The company said a full investigation is under way.