Magnox Electric Limited was convicted of breaking legislation governing the disposal of radioactive waste while operating the Bradwell power station near Maldon.
A judge also ordered the company to pay £150,000 costs following the trial at Chelmsford Crown Court.
Magnox had denied allowing waste to seep out in breach of legislation but had admitted failing to maintain a sump at the power station.
Jurors were told that "radioactive water" had leaked from a decontamination unit at Bradwell between 1990 and 2004.
Judge Peter Fenn said Magnox had missed an opportunity to check a faulty sump at the unit and missed a warning signal.
But he accepted that the "environmental impact" had been "limited".
The court heard that Magnox had been fined £100,000 plus costs eight years ago after being convicted of failing to maintain equipment, breaching laws governing waste disposal and failing to inform industry watchdogs of an unlawful discharge at Bradwell and at the Hinkley Point power station near Bridgwater, Somerset.