A report into a radioactive leak at the Sellafield nuclear reprocessing plant in Cumbria criticised management after "serious" breaches in regulations.
The leak occurred at the Thorp complex at Sellafield
The plant's Thorp facility was shut down in April 2005 after 83,000 litres of acid containing uranium and plutonium escaped from a broken pipe.
No-one was injured in the leak and no radiation escaped from the plant.
The Health and Safety Executive (HSE) said significant failings at the plant included staff ignoring alarms.
Operator British Nuclear Group was fined £500,000 last year after it pleaded guilty to breaching aspects of the Nuclear Installations Act 1965.
In a 28-page report, the HSE made a total of 55 recommendations and actions for company improvements.
The report said a number of failures in management meant the leak remained undetected for eight months. It highlighted a lack of a "questioning attitude" or "challenge culture" at the company.
The review said: "An underlying cause was the culture within the plant that condoned the ignoring of alarms, the non-compliance with some key operating instructions, and safety-related equipment, which was not kept in effective working order for some time, so this became the norm."
The first indication of a leak was on August 24, 2004 when 50 grams of uranium was detected following a sample test.
But the full extent of the leak was finally uncovered on April 14 and Thorp was shut down four days later and remains closed.
A spokesman for British Nuclear Group said the company had implemented a large number of improvements to its operating regime.
He added: "The incident was regrettable and clearly should not have occurred in the first place.
"The company appreciates that mistakes were made which led to the leak and enhancements to workforce training, operating instructions and responses to alarms have been made."