Sellafield is fined pounds 500,000 after leak.
Around 83,000 litres of acid containing 20 tonnes of uranium and 160kg of plutonium escaped from a broken pipe into a sealed concrete holding site at the site. No one was injured in the leak and no radiation escaped from the plant in west Cumbria.
The company was handed the fine at Carlisle Crown Court after pleading guilty at an earlier hearing to three counts of breaching conditions attached to the Sellafield site licence, granted under the Nuclear Installations Act 1965.
The court heard that the leak should have been detected within days rather than the eight months it took.
Passing sentence, Mr Justice Openshaw said British Nuclear Group Sellafield "did not have a good safety record".
The penalty must serve as a reminder that health and safety is a serious matter and achieving public safety is of paramount importance, he said. The court was told that a change in the handling process had caused the leak.
It was not spotted because a 'floating' bobbin in the machinery had become stuck which produced "wild and unaccountable" gauge readings of radioactive levels over a five-year period.
Richard Matthews, prosecuting, said the first indication of a leak at the Thermal Oxide Reprocessing Plant (Thorp) was on August 24, 2004 when 50 grams of uranium was detected following a sample test. The company failed to react effectively to the sample result, he said.
It was not until November 2004 that the company found a discrepancy in the levels of uranium at the site. The full extent of the leak was finally uncovered on April 14 and Thorp was shut down four days later and remains closed.
Mr Matthews said that following a visit by nuclear safety inspectors, 55 recommendations were made to the company.
He said: "They found leaked detection procedures were flawed, there was an inadequate safety management system and a culture of tolerating alarms directly contributed to the breaches.
He said there was no evidence of any leak from the containment cell but he added the company had "lost its ability to detect leaks as it relied on its last line of defence."