The operator of the Sellafield nuclear plant in Cumbria has been rapped over a leak of radioactive water.
The water leaked during routine tests at Sellafield
The incident happened in February 2006, during routine tests, which required levels of contaminated water in storage pools to be raised temporarily.
But levels remained high for too long and radioactive water leaked from a gap in a pool wall.
Although water was contained within the plant the Environment Agency criticised failings by British Nuclear Group.
The company said a full investigation was carried out and measures taken to prevent a similar incident.
The Environment Agency said the volume of water lost was minute, but that the incident showed a "disappointing" lack of controls at the reprocessing facility.
The agency issued an enforcement notice, which demands action be taken to prevent a similar occurrence. Failure to comply with the notice is an offence.
Andy Mayall, leader of the agency's Sellafield Team said: "Although there was no environmental harm and the response and investigation were effective, we are extremely disappointed about the control, maintenance and other failings that led up to this event.
"We want to see lessons learnt from this event lead to improvements in the management of storage ponds across the site."
A British Nuclear Group spokesman said: "The pond was overfilled as a result of some statutory test work that was being carried out on the pond.
"Although the water didn't actually overflow from the top of the pond, it did escape from an expansion joint in the pond wall. We acknowledge that this should not have happened.
"We can confirm that improvement measures have been taken to prevent a re-occurrence."