About 40,000 gallons of radioactive water leaked from a pipe at a nuclear power station which closed at the end of last year.
The plant had been producing electricity for 40 years
The leak, at Sizewell A power station in Suffolk, happened on Sunday afternoon after a pipe broke.
A spokesman for Sizewell said the water was contained within the plant and no one was injured.
Health and safety inspectors are investigating at the plant, which had produced electricity for 40 years.
Its reactors are now being decommissioned.
A spokesman said the escape of the water was quickly detected and "stopped rapidly".
The Nuclear Installations Inspectorate confirmed it was investigating, adding that no water had left the site and there had been no contact with members of the public or workers.
A spokesman said the water contained traces of a radioactive substance.
Gary Smith, national officer of the GMB union, said: "The incident highlights decades of chronic under-investment in our nuclear industry.
"It also comes at a time when the Government is proposing to cut funding to the Nuclear Decommissioning Authority (NDA)."