Investigators are continuing to examine the cause of a leak of highly radioactive material at part of the Sellafield nuclear reprocessing plant.
The leak is thought to have began last August
The Thermal Oxide Reprocessing Plant (Thorp) stopped production in April when the leak was discovered.
The Nuclear Installations Inspectorate is compiling a report before a decision on any prosecutions is taken.
Investigations are focusing in part on how long the leak had lain undetected and reliability of monitoring systems.
A clean-up operation is continuing and similar pipework elsewhere in the plant has been checked.
Sellafield's managing director, Barry Snelson, admitted to the BBC that the plant may remain closed for months.
Safety regulators have claimed that the discharge could result in criminal charges.
The accident happened when a narrow pipe fractured, spewing nitric acid onto the floor of a concrete-lined cell in the Thorp reprocessing complex.
The acid contained 20 tonnes of uranium and 160kg of plutonium.
It is thought the pipe may have fractured in August, but the leak was not discovered until eight months later due to a combination of a faulty gauge and human error.
No staff were contaminated.
Last week, Sellafield was told to improve the way it discharged low level radioactive waste water into the Irish Sea.
Environment Agency inspectors issued an enforcement notice after finding its filtering system needed to be improved.
Operators British Nuclear Group said no discharge limits had been breached and it was "committed" to improvements.
File On 4: BBC Radio 4, Tuesday 14 June, 2005 at 2000 BST, and repeated on Sunday 19 June, 2005 at 1700 BST.