Flamanville: safe shutdown of reactor 1 following a leak in a small-diameter pipe connected to the primary system
On 25th March 2025, EDF declared a significant event to ASNR relating to the application of incident/accident operating procedures on Reactor 1 of the Flamanville nuclear power plant. This procedure, designed to bring the reactor to a safe shutdown state, was initiated following the detection of a leak in a small-diameter pipe connected to the primary system.
The main primary system contains the water that cools the reactor core and transfers energy from the nuclear reaction to the steam generators. The water in the primary system is maintained at a high pressure and temperature.
The general operating rules are a collection of rules approved by ASNR that define the authorized operating range of the facility and the associated reactor operating requirements. In particular, they prescribe a maximum leakage rate from the primary system, above which the operator must apply the instructions on operational management for incidents or accidents to bring the reactor to a safer state.
On 22nd March 2025, while the operator was carrying out final tests prior to criticality search and divergence operations on Reactor 1, following a planned shutdown for maintenance and refueling, fire detectors inside the reactor building were triggered. As per procedure, the operating team immediately called for outside emergency assistance.
They simultaneously continued to analyze the situation. The team was able to determine that the fire detection system was not triggered due to fire, but rather a leak of water from the primary system, in the form of steam. As the leakage rate exceeded the threshold set by the general operating rules, the operating team applied the procedures for operational management of incidents or accidents. They therefore lowered the pressure and temperature of the primary system.
This event had no impact on the environment. Nevertheless, due to the safe shutdown in application of incident/accident operating procedures, it has been classified as level 1 on the INES scale.
The reactor building containment remained intact throughout the event. The water from the leak was collected by dedicated systems in the reactor building.
The operator was subsequently able to carry out investigations, which showed that the leak had originated from a small-diameter pipe (blow-off and vent circuit) connected to an auxiliary pipe in the primary system.
ASNR carried out a reactive inspection on 26th March 2025 to check how the event had been managed and the measures implemented by the operator to deal with the situation. In particular, the inspectors found that the reactor procedures for operational management of incident or accident had been correctly applied.
ASNR will ensure that the operator carries out the repair in proper nuclear safety conditions for the reactor and occupational safety for the workers involved. ASNR will also ensure that the operator carries out expert assessments to determine the precise causes of the leak.