The discharge of water contained tritium, a radioactive form of hydrogen. It might have remained secret had an employee at the Atomic Weapons Establishment (AWE) at Aldermaston in Berkshire not tipped off the Environment Agency, West Berkshire magistrates were told.
The successful prosecution was the first by the agency under the 1993 Radioactive Substances Act. The AWE was also ordered to pay pounds 4,200 costs.
Garrett Burne, prosecuting for the Environment Agency, said Hunting BRAE - the company that has operated AWE since 1993 - began discharging groundwater contaminated with tritium into the stream without the regulator's knowledge. "It started as a minor thing but soon became a major source of discharge of tritium liquid waste - and the company knew it was unlawful," said Mr Burne. The discharges began in April 1997, he said.
Eventually the illegal discharges accounted for 70 per cent of all waste tritium emitted from the Aldermaston site.
Tritium is a byproduct of the production of nuclear weapons, and from the operation of some nuclear reactors. Its radiation is too weak to pose any hazard unless "tritiated" water is inhaled or absorbed through the skin.
Mr Burne said natural levels of tritium were between five and 20 Becquerels per litre of water, but the company had been pumping in about 100 Bqs of tritium per litre, a figure which he said "the company seems to have set itself", but which has "no scientific basis as far as the Environmental Agency is concerned." However, two-thirds of the time the discharges exceeded the 100 Bqs figure.
John Croft, AWEs director of environment, health and safety, last night insisted that the discharges had posed no threat to health. "Under European guidelines you could sell bottled water with 100 Bqs per litre of tritium," he said. The problem arose because AWE had tried to store groundwater emerging from contaminated ground to prevent flooding, he said.
"It was a technical breach," he said. "The fines are quite low. But it is the damage to the company's reputation that is more important."
Hunting BRAE, which took over the running of AWE Aldermaston in 1993, admitted one charge of illegally discharging ground water laced with tritium into the Aldermaston Stream between April 1997 and January 1999. It also admitted a further charge of failing to report the discharge, and a third of making false or misleading statements about the rates of tritium discharge.
The magistrates heard that the company was fined pounds 8,000 in August this year, after two of its workers were contaminated with plutonium.