Part of Sellafield's Thorp plant has been closed down after a failure was found in one of the pipes.
The problem was discovered in the plant's Feed Clarification Cell, which holds dissolver solution while tests are carried out.
Safety regulators have been informed and investigations are underway to find out what has happened and what needs to be done before operations can resume.
A spokesman said there was no risk to employees or the local community.
'Safe and stable'
The discovery was made after a camera inspection of the cell, which is a stainless steel totally secure environment, and specifically designed to withstand such pipe failures.
Barry Snelson, managing director of British Nuclear Group, Sellafield said: "I have asked for the front end of the plant's reprocessing operations, including shearing, to be closed down.
"Safety monitoring has confirmed no abnormal activity in air and there has been no impact on our workforce or the environment.
"Let me reassure people that plant is in a safe and stable state."