A configuration error during the restart of Reactor 3 at the Cattenom nuclear power station has been classified as a level 1 safety event by France’s Nuclear Safety Authority.

The incident occurred on 11 August 2025, when two valves in the primary water treatment circuit were inadvertently closed during maintenance restart operations. This error led to water being diverted into a collection tank at levels above prescribed thresholds, the Grand-Ducal Fire and Rescue Corps (CGDIS) confirmed.

The primary circuit, which ensures the reactor remains cooled, is a closed system subject to continuous monitoring. EDF (French multinational electric utility company) stressed that reactor safety was never compromised, and cooling was maintained throughout.

Normal operations were restored at 4.25pm, following corrective action on the valves. However, EDF acknowledged that the required cause-identification procedure was not immediately applied due to the late detection of the error.

On 14 August, EDF formally declared the event as a level 1 anomaly on the International Nuclear Event Scale (INES), which ranges from 1 to 7. Level 1 represents a “minor anomaly” with no consequences for the public or the environment.

EDF stated that all necessary corrective measures have since been implemented to prevent a recurrence.