Flamanville 3: the EPR completes its commissioning tests and opens a new industrial cycle
After nearly two years of testing, gradual power ramp-up and technical validation, the Flamanville EPR has completed its commissioning programme. This marks a major industrial milestone, now opening its first phase of operation, ahead of a first major outage: VC1.
The Flamanville EPR has reached a decisive milestone. On 27 April 2026, EDF confirmed the full completion of the reactor’s commissioning tests. This announcement marks the end of an industrial sequence lasting nearly two years, which began with fuel loading in spring 2024 and was punctuated by several key milestones.
This step goes beyond a standard commissioning process. With Flamanville 3, France completes the start-up of its first new nuclear reactor in more than twenty-five years and rebuilds, at full scale, industrial expertise that is set to become central again with the EPR2 programme. “The connection of a new reactor to our fleet of 56 units is a major moment. The Flamanville EPR has completed its 34 commissioning test sequences. The reactor is now fully in operation,” explains Étienne Dutheil, Director of the Nuclear Generation Division at EDF.
Two years of achievements
The true starting point of this sequence dates back to 8 May 2024, when EDF completed the loading of fuel into the reactor vessel, following approval from the nuclear safety authority. With the insertion of 241 fuel assemblies, Flamanville 3 entered the active nuclear phase.
Four months later, on 3 September 2024, the EPR reached another essential and technical milestone with its first criticality, the moment when the nuclear chain reaction becomes self-sustaining in the reactor core. This can be seen as the first heartbeat of the installation.
The power ramp-up then continued in successive stages, in line with the commissioning logic of a Generation III+ reactor. Unlike a plant in routine operation, a new reactor must be tested under multiple configurations: load variations, planned shutdowns, dynamic stress on safety systems, validation of control systems and qualification of turbine performance. Each stage represents a verification step.
On 21 December 2024, Flamanville 3 fed electricity into the national grid for the first time. The reactor reached its maximum nominal power nearly one year later, on 14 December 2025. With its 1,650 MWe, the EPR then became the most powerful reactor in the French nuclear fleet. One of the final major technical demonstrations was the successful completion of its full-power islanding test on 7 April 2026. This test consists in simulating a loss of the external grid while maintaining autonomous power supply to the plant’s internal systems.
Towards VC1
“The commissioning of the EPR reflects the commitment and efforts of the entire French nuclear sector. We are all very proud to have a third-generation reactor within the nuclear fleet,” says Étienne Dutheil. EDF is now preparing the reactor’s next major milestone: its first periodic outage (VC1). This regulatory shutdown, expected to last around 350 days, will be the reactor’s first major industrial milestone. This outage will include a series of heavy maintenance operations, in-depth inspections and regulatory checks. Among these requirements is the replacement of the reactor vessel head, in accordance with initial operating requirements, as well as several detailed inspections of major plant components. EDF plans to mobilise 2,500 personnel and carry out nearly 20,000 technical operations during this phase. ■