EDF Energy, which own the nuclear power station in Romney Marsh, Kent, said that no radiation was released and members of the public were not in danger.
The incident on June 29 is understood to have taken place in a building away from the reactor, when a piece of rubber became trapped in a fuel coupling.
A subsequent review found that the type of foam injected into the coupling as a fire precaution was not permitted for such use under safety rules.
"The coupling did not fail, there was no plant damage, no staff were injured and there was no release of radioactivity," British Energy, which was bought by EDF last year, said in a statement.
"There was no impact on the safety of our workforce or the public at large and there was no damage to the plant."
The incident was provisionally classified as a level two event on the seven-level International Nuclear Event Scale (INES), meaning that it may have highlighted significant failures in safety provisions but had no off-site impact.
There are typically just one or two similar incidents across all of Britain's nuclear power stations in a year.
The Chernobyl diaster that killed 56 people in Ukraine in 1986 is the only recorded level seven event.