Revealed: huge Sellafield leak went undetected for 9 months

Full scale disclosed of worst nuclear accident for decade. Catalogue of human error led to massive radioactive discharge. Accident may force ministers to shut troubled plant for good

Tens of thousands of litres of highly radioactive liquid leaked unnoticed for up to nine months from a ruptured pipe in the controversial Thorp reprocessing plant at Sellafield in what the IoS can reveal was Britain's worst nuclear accident for 13 years.

Tens of thousands of litres of highly radioactive liquid leaked unnoticed for up to nine months from a ruptured pipe in the controversial Thorp reprocessing plant at Sellafield in what the IoS can reveal was Britain's worst nuclear accident for 13 years.

The leak, detected last month, was the result of a catalogue of human and engineering errors which resulted in a pool of nuclear liquor, half the volume of an Olympic swimming pool, being accidentally discharged. The magnitude of the incident throws the future of the troubled reprocessing plant into doubt this weekend as copies of an internal investigation circulate among senior ministers and officials.

British Nuclear Group, the company that runs the plant, last night admitted that workers failed to respond to "indicators" warning a badly designed pipe had sprung a leak as long ago as last August. The pool of nuclear liquor, 83,000 litres, was eventually discovered on 19 April. The company has ordered a review to check for other potential leaks caused by metal fatigue and an urgent drive against staff "complacency".

But ministers privately concede that Thorp, now owned by a quango, may never re-open as a result of the incident, classified as "serious" by the International Atomic Energy Authority. In a statement released yesterday the Nuclear Decommissioning Agency, the quango that inherited Thorp on 1 April, said it needed time to assess the report's findings before "discussing their implications" with the company and the Government, adding that "safety is the NDA's absolute priority".

The nuclear clean-up agency is thought to be fighting a battle with Downing Street to close the plant for good in a move that would cost taxpayers billions of pounds.

The leak comes just as ministers and nuclear firms are preparing to seek public support for a new generation of nuclear power stations to help meet climate change targets. It explains why Tony Blair and Alan Johnson, the new Secretary of State for Trade, have been so reluctant to start making the nuclear case.

The company has stressed the leak was contained and that the incident did not pose a threat to the public.

The company may yet face a criminal prosecution. A spokesman for the Nuclear Installations Inspectorate (NII) said: "I can confirm we will be seeking to find out what monitors were in place, whether they were working and, if so, why they were not acted on."

Four inspectors have been on the Cumbrian site since the incident happened. In addition to human error, they are concentrating on why engineers failed to modify pipes leading to moveable tanks. Metal fatigue in the pipework was the principal cause of the leak.

It is thought that the investigation will continue for a number of weeks before a decision is made on further action against British Nuclear Group.

A spokeswoman for the Department of Trade and Industry said that Mr Johnson would wait until the completion of the NII probe before deciding on the plant's future.

"It is essential that BNG acts urgently to implement the recommendations to improve operating practice and retrieve the escaped liquid. We are going to wait for advice before taking a decision on the way forward."