A nuclear power station operator was today found guilty of allowing radioactive waste to seep from a decontamination unit for 14 years, the Environment Agency said.
Waste leaked into the ground from a sump at Bradwell nuclear power station near Maldon, Essex, between 1990 and 2004.
Magnox Electric Ltd had denied breaching legislation governing the disposal of radioactive waste but was found guilty following a five-week trial at Chelmsford Crown Court.
During the case, it emerged the company had failed to inspect a holding tank that had been leaking liquid radioactive waste since 1990.
Mark Harris, prosecuting on behalf of the EA, said the fault at the power station, which was no longer running, was discovered in February 2004.
"The leaks were caused by a combination of poor original design of the sump... and, in the period when this company was running it... there was no routine inspection or maintenance of the sump until after the leak was discovered," he said.
The EA said the situation came to light after "voluntary disclosures" by Magnox.
A spokesman said the leak had not put any local people or the environment at risk.
Phil Heaton, of the EA, said: "Our prosecution of the company sends a clear message to the nuclear industry that we require the highest standards of operation at all such sites."
Magnox Electric Ltd will be sentenced at the court on 17 February.