The operator of the Sellafield nuclear reprocessing plant in Cumbria has pleaded guilty to breaching safety regulations over a radioactive leak.
The leak occurred at the Thorp complex at Sellafield
British Nuclear Group Sellafield (BNGSL) now faces an unlimited fine after the criminal hearing on Thursday.
Acid containing 20 tonnes of uranium and 160kg (353lb) of plutonium escaped from a ruptured pipe and was found in a sealed cell at the site in April 2005.
The hearing was adjourned for sentence on 7 July at Carlisle Crown Court.
No one was hurt and no radioactive material escaped into the atmosphere after the leak at the Thermal Oxide Reprocessing Plant (Thorp) part of the site.
But the spillage, discovered in April 2005, may have gone unnoticed for eight months.
At Whitehaven Magistrates Court, Sellafield's operator BNGSL pleaded guilty to three counts of breaching conditions attached to the Sellafield site licence, which was granted under the Nuclear Installations Act 1965.
The Health and Safety Executive (HSE) brought the prosecution, arguing the firm failed to ensure safety systems were in good working order and that radioactive material was effectively contained.
Work at the Thorp complex has been halted since the incident.