Sellafield nuclear plant in Cumbria has been told to improve the way it discharges low level radioactive waste water into the Irish Sea.
Sellafield bosses say no discharge limits have been breached
Inspectors from the Environment Agency have issued the unit with an enforcement notice after finding its filtering system needed to be improved.
But it stressed that radiation doses were well within "legal limits."
Operators British Nuclear Group said no discharge limits had been breached and it was "committed" to improvements.
Inspectors also found that the plant needed to cut down on the build-up of solid material in its lagoon - this is the container where waste water is held before it is pumped into the sea.
They also found inconsistencies in the way some discharges were measured and reported.
Environment Agency nuclear regulator, Andy Mayall, said: "In recent years we've seen significant improvements in some areas at Sellafield.
"Radioactive discharges from the site are already low - radiation doses to the public are well within legal limits and any risk to the public is very small.
""However, being in compliance with limits does not mean that the company should not be committed to continuous improvement - it needs to address certain issues to demonstrate this."
A spokesman for Sellafield said the plant was committed to making improvements.
He said: "No authorised discharge limits have actually been breached, however we are committed to making improvements to minimise discharges and improve monitoring of our discharges."
Last month it emerged a leak at the plant's Thorpe reprocessing complex lay undiscovered for three months.
More than 20 tonnes of uranium and 160kg of plutonium spewed onto a floor in a sealed cell when a pipe fractured in January.