The operator of Sellafield will pay a second fine of £500,000 over an 80,000 tonne radioactive leak, Carlisle Crown Court ruled on Monday.
Safety at Sellafield fuel reprocessing plant has led to protests from the Irish government
British Nuclear Group (BNG) who operate the THORP nuclear reprocessing plant in Sellafield has already been hit with a £2m fine over the same leak, which saw 80,000 tonnes of acid contaminated with 20 tonnes of uranium and 160kg of plutonium escape from a broken pipe into secondary containment in April 2005.
Although none of the radioactive liquid made it out of the back-up containment area, the incident raised serious safety concerns because the leak took eight months to detect, the judge said.
An internal investigation into the incident severely criticised the Sellafield plant's safety procedures and accused it of having a "new plant culture" - a careless attitude based on the assumption that nothing can go wrong in a new plant.
The incident fuelled further protests from nuclear-free Ireland which fears radioactive pollution from the plant or potential accidents.
Ireland's environment minister Dick Roche welcomed the ruling, saying the court was right to hold those responsible for safety failures responsible.
"The level of this fine, together with the fines already imposed by the Nuclear Decommissioning Authority earlier this year, goes some way towards reflecting the serious issues which resulted in the leak of this large volume of toxic material. However it gives little comfort that the poor, ongoing safety culture identified can, or will, be tackled by the UK authorities," Dick Roche said.
He went on to say: "We have been here before. ... The Irish Government's concerns are in no way diminished by this episode. This leak provides further evidence, if such were needed, that the UK authorities should make the current shutdown of the THORP plant a permanent feature."
20th October 2006