The owners of Sellafield have admitted that radioactive liquid waste leaked at the nuclear plant today.
Officials said the effluent, mainly water with a very low level of radioactivity, spilled from holding tanks into a concrete room designed to contain overflows.
They say a small amount escaped into an adjacent concrete corridor but that all material was recovered.
It is understood the plant was closed for maintenance when the spillage occurred.
The British Nuclear Installations Inspectorate and the Environment Agency were informed.