A spokeswoman said that a reactor at the Dungeness A station in Kent had been shut down but emphasised that it had merely been a "precautionary measure".
The shutdown was carried out after the discovery that part of the refuelling machinery on the 1960s reactor had been damaged but she stressed that there had been no safety risk.
The announcement of the failure on 5 September comes days after the company was fined pounds 250,000 for an incident in which part of a crane fell into a Magnox reactor at Wylfa nuclear power station in north Wales.
The spokeswoman stressed that there was no similarity between the latest incident and the Wylfa accident.
The Dungeness A reactor would remain closed for "days, possibly weeks" until the exact cause of the incident and the extent of the damage could be established, she added.
The 400-ton fuelling machines are situated above the reactor and are designed to remove spent fuel and replace it with fresh fuel.
An inspection of the equipment discovered damage to the tube connecting the fuelling machine to the reactor.
A spokesman for the company said that although the closedown had not been made public at the time, the National Grid would have been informed.
A leaflet would be issued to local people assuring them there was no risk, which the Nuclear Electric spokeswoman stressed was standard practice.
Magnox reactors, named after the magnesium oxide used to fuel them, were the first generation of nuclear reactors and were introduced in the 1960s.