Staff at Sellafield are investigating a leak from a tank in the older part of the nuclear site.
Following maintenance work staff discovered that water levels in one of the sump tanks were falling.
The concrete tank, which is 3sq ft, contains some nuclear material and water, which is used to provide protection against radiation.
A Sellafield Ltd spokesman said: “An engineering team is currently investigating an issue in a concrete sump tank in the legacy ponds area of the site.
“We have detected that the liquid levels in the tank have fallen.
“The discovery was made following recent maintenance work.
“As a precaution we have increased the local monitoring and detection systems around the tank.
“Across the site, operations continue as normal.
“We have informed regulators, stakeholders, and site personnel.”
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