British Energy said the incident was not connected Dungeness B's reactor
An investigation has begun after an alert at Dungeness nuclear power station in Kent.
British Energy said the incident on 29 June was in the fuel building and was not connected to the reactor at Dungeness B.
It was provisionally rated at level two out of seven on the International Nuclear Event Scale, with zero representing no safety significance.
The Chernobyl disaster is the only accident to have reached level seven.
In a statement on Wednesday, British Energy said the incident caused no release of radioactivity, and no impact on the safety of workers or the public.
The alert was triggered when it emerged a piece of rubber had become trapped during the fuel assembly process.
The statement from British Energy, part of EDF Energy, said: "The coupling did not fail, there was no plant damage, no staff were injured and there was no release of radioactivity."
It added: "In co-ordination with the regulator, we are carrying out a full investigation into the cause of this event and have undertaken to put necessary measures in place to prevent this happening again.
"The company takes such incidents extremely seriously and will take any appropriate action which may be necessary."