In Britain's worst nuclear accident in 13 years, about 83,000 litres of highly radioactive liquid leaked unnoticed for up to nine months from a ruptured pipe in the controversial Thorp reprocessing plant at Sellafield, a media report said. The leak, detected last month, was the outcome 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.The report said 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 April 19.