A leak at the Sellafield nuclear reprocessing plant in Cumbria was not spotted for three months, an investigation has revealed.
The Thorpe plant handles spent nuclear fuel
More than 20 tonnes of uranium and 160kg of plutonium spewed onto a floor when a pipe fractured at the Thorp reprocessing complex in January.
The British Nuclear Group, which carried out the inquiry, stressed that the material leaked into a sealed cell.
The discovery was made after a camera inspection of the cell in April.
It was classified as a level 3 accident by the International Nuclear Event Scale (INES) because of the acid released in the incident.
INES measurements listed the 1986 Chernobyl disaster as a level 7 incident and Three Mile Island in the United States in 1979 as level 5.
The leak occurred when a pipe - just a few centimetres wide - fractured, sending nitric acid onto the floor of the concrete-lined cell.
The cells, which are 60 metres long and 20 metres high, are not accessible to staff and no-one was exposed to radioactive material.
According to the British Nuclear Group's findings, the pipe failed because of metal fatigue, which may have started to occur as early as August 2004.
The report recommended that improvements be made to the maintenance and testing procedures at Thorp, which remains closed since the leak.
Detailed reviews into engineering and operating practices throughout the plant should also be conducted, it concluded.
Barry Snelson, Managing Director at Sellafield, said: "I will personally be ensuring that recommendations are implemented not just in Thorp, but across Sellafield.
"I am disappointed that plant indicators were not acted upon as quickly as they should have been and I shall be taking action to ensure that any complacency with respect to acting upon plant information is addressed."
Sellafield staff are confident that Thorp can be returned to service, he added.