[Decoding] Blackout in Spain: What would have happened with the French nuclear fleet?

On April 28, a sudden drop in electricity production in Spain triggered a major blackout and a disconnection from the European grid. The country’s three operating nuclear reactors immediately shut down and were brought to a safe state. Had such an event occurred in France, it would not have had the same impact on the nuclear fleet: the French reactors would have entered “island mode” to continue operating at a very low power level, enabling a faster return to the grid.

At 12:38 p.m. on Monday, April 28, the Iberian Peninsula experienced a massive blackout that almost entirely shut down Spain’s electricity system. Although the precise cause is still unknown, the French Transmission System Operator (RTE) stated: “Spain lost 15 GW of production in a matter of seconds. At the same time, the Iberian Peninsula was disconnected from the European grid.” It took several hours to restore services across the country, initially through interconnections with France and Morocco, and then by restarting thermal and hydroelectric plants domestically.

Spain operates seven nuclear reactors, three of which were running at the time of the incident: Almaraz II, Ascó I & II, and Vandellós II. According to their design specifications, and due to the loss of offsite power, these reactors automatically shut down. Diesel backup generators were activated to power the systems needed to cool the reactors, including those already offline.

The safe shutdown of the Spanish nuclear reactors was confirmed on X by the Director General of the International Atomic Energy Agency (IAEA): “The IAEA has confirmed with the Spanish nuclear regulatory authority CSN that the situation is stable and under control, with no impact on nuclear safety or security. Some nuclear plants are using offsite emergency power and are gradually restoring external power.” The reactors began regaining external power from 4:30 p.m. on Monday, allowing restart procedures to begin—a process that will take several days.

The @IAEAorg confirmed with Spain’s Nuclear Regulator @CSN_es that the situation is stable and under control, with no impact on nuclear safety or security. Some nuclear power plants are using backup off-site power and are gradually restoring external supply. pic.twitter.com/An5GhSprgR

— Rafael Mariano Grossi (@rafaelmgrossi) April 28, 2025

Island Mode Instead of Automatic Shutdown

French nuclear facilities were not affected by the Spanish incident. However, if a similar event were to hit France, the response would be significantly different. In the event of a grid disconnection, French reactors would not shut down but instead switch to island mode. In this mode, a reactor continues operating but is disconnected from the grid.

As explained by Paolo Olita, research engineer at CEA IRESNE, in the Revue Générale Nucléaire (RGN 2023 No. 3): “Island mode involves disconnecting the unit from the electrical grid by opening the circuit breaker linking the turbo-generator to the grid. Electrical power for the primary pumps and other auxiliary systems is then supplied directly by the unit’s own generator, rather than the external grid. The self-powered unit, now isolated from the grid, forms an electrical island.”

A Real Case in 2023 and Regular Testing

This is not a theoretical concept. In November 2023, during Storm Ciaran, the electricity transmission infrastructure in the Manche department (northwestern France) was damaged. This led to the disconnection of both reactors at the Flamanville nuclear plant. Cédric Lewandowski, Director of EDF’s Nuclear and Thermal Fleet, wrote at the time: “The reactors reduced their power output, entered island mode—thus maintaining autonomous power supply to their auxiliary systems—and continued operating safely.”

The benefits of islanding reactors, as is done in France, rather than shutting them down completely, are twofold:

• Avoiding the use of diesel backup generators,

• Enabling a quicker reconnection to the grid once normal conditions are restored.

However, islanding is a delicate operation, as core power must be rapidly reduced—down to around 30% of nominal output. Part of this power is used to supply the turbine, generating approximately 40 MWe, while the remaining steam bypasses the turbine and is sent directly to the condenser. Paolo Olita adds: “Operational feedback over time and constant improvements in the instrumentation and control systems of the reactor fleet have led to a higher success rate for islanding over the years.” According to EDF data, the islanding success rate has reached 90% over the past ten years. This is essential, as RTE requires a success rate of over 60% to begin grid restoration after a widespread network incident (IRG).

EDF tests islanding operations every four to five years, as part of a contractual agreement with RTE. On April 28, the company released a statement describing two planned islanding tests carried out in March at the Gravelines nuclear plant. EDF explained: “These tests verify the plant’s ability to send voltage back to the grid or to remote facilities under specific configurations. In the event of a network failure (blackout), the plant can demonstrate its capacity to progressively re-energize the power grid and secure its installations.”

A Different Technological and Energy Context in Spain

In Spain, the reactors are based on Westinghouse technology, which differs from the technology used in France. Spanish reactors cannot reduce their power output below 40% of nominal capacity, making island mode technically unfeasible. Additionally, nuclear accounts for only 20% of Spain’s electricity mix, compared to approximately 70% in France. As a result, the urgency to quickly bring reactors back online is less critical in Spain than in France.

Although the Golfech nuclear plant in France was impacted by the instability, experts from the French Nuclear Society (Sfen) pointed out that the frequency fluctuation wave from Spain hit the European—and primarily French—grid, where the rotating inertia of French nuclear generators helped absorb the shock and prevented a domino effect.■