After being discovered, the drip was quickly contained, said the firm
Sellafield managers have been told to make improvements after a radioactive leak went undetected for more than a year.
In January condensation was found to have been dripping from a pipe at the nuclear reprocessing plant in Cumbria.
The Environment Agency has issued an enforcement notice requiring improved equipment and maintenance routines.
Sellafield Ltd said no-one was exposed to the "low-level radiation", and that improvements were already being made.
For 14 months the condensate leaked from a line connected to an overhead ventilation duct which serves the Magnox reprocessing plant.
It was spotted on 23 January - the day Gordon Brown visited the site to announce possible locations for new nuclear power stations.
It dripped on to ground made up of a concrete slab and gravelled surface, within the "secure boundary" of the facility.
Stuart Page, nuclear regulator from the Environment Agency said: "While this incident has not caused any harm to members of the public, or affected areas outside of the Sellafield site, the failings leading to the incident are extremely disappointing."
The Agency regulates the disposal of radioactive waste in partnership with the Nuclear Installations Inspectorate.
A spokesman for Sellafield Ltd said areas for improvement highlighted in the enforcement notice from the Environment Agency "had been identified as recommendations from our own internal investigation and are already being implemented".
The leak, initially classed as a "level 1" accident, was later upgraded to level 2, but only because of the amount of liquid which leaked in the months it lay undiscovered, according to the firm.
International Nuclear Event Scale (INES) measurements listed the 1986 Chernobyl disaster as a level 7 incident and Three Mile Island in the United States in 1979 as level 5.