Staff at the facility – which is about 22 miles from Jersey – were working to load new fuel rods into the number two reactor earlier this month when a valve on the 'safety injection system', which ensures that the core can be cooled in the event of an emergency, failed to close.
Under French nuclear regulations, if the device becomes inoperable, fuel-loading operations must stop within one hour.
However, staff at the plant did not notice the problem for a number of hours, it has been reported.
EDF, which runs the plant, have since reported the incident to the French nuclear regulator, the Autorité de Sûreté Nucléaire, and have recommended that the incident be classed as level one – an 'anomaly' – the second-lowest category on the seven-tier nuclear incidents scale.
In a statement, EDF said that the accident 'had no impact on the safety of the installations, but that the correct action to be taken in the case of such an event was not taken'.
Earlier this year, emergency communication procedures between Jersey and French authorities were put into action following an explosion at the plant.
It was later revealed that a faulty ventilation fan had overheated and that the incident was not related to a nuclear reactor.