In the absence of a specific cause for last week’s massive power outage, several factors have emerged over the days, creating a perfect storm and leading to “zero electricity.” A key aspect is the negative impact of the drop in the price of electricity from renewable sources, which not only benefits households and businesses but also leads to losses for producers, including nuclear power plants. On that day, two of Spain’s seven reactors – Almaraz I and Cofrentes – received authorization from Red Eléctrica not to operate because electricity was not profitable during the hours when prices would have been zero or negative. Without these two reactors, which also include the fueled Trillo power plant, the system would have had less capacity to stabilize the power grid in the face of the three disruptions identified so far, which ultimately led to a historic power outage.
In addition to the questionable role of renewable energies and the excessive presence of photovoltaics that day, the question arose from the outset as to whether nuclear energy would have been able to avoid the “electrical zero point.” On Monday, the president of the Nuclear Forum, Ignacio Araluce, stated that he did not know, but countered with government President Pedro Sánchez, who noted a day later that nuclear energy was by no means a solution, but “a problem.” Araluce emphasized that while the plants did not pose an obstacle during the blackout, they did not provide sufficient stability to maintain the system, which ultimately led to the disruption.
Three reactors were shut down, two of them for economic reasons. This insufficient stability could also have been due to the fact that two of the seven nuclear reactors in Spain’s five power plants were not operating that day for economic reasons. Red Eléctrica, which, as usual, saw no problems securing supply in such situations, stated: “There was sufficient [combined cycle] generation in the system to resolve any constraints.” The decision not to operate the reactors rested solely with the nuclear power plants, according to the agency.
Like combined-cycle power plants that generate electricity from gas or hydropower, nuclear power contributes to system stability because it has a lower capacity to respond to frequency decreases in the grid, which must be maintained at 50 hertz. This “inertia” means that the system gains valuable seconds to adjust to an anomalous frequency and bring it back to 50 hertz.
On April 28, however, nuclear power fulfilled this role to a lesser extent, as two of Spain’s seven reactors were shut down. Apart from the Trillo power plant, which was operating due to its refueling, Almaraz I and Cofrentes were inactive because they were losing money during their operation. After the notification, Red Eléctrica reviewed its compatibility with security of supply and, as usual, adjusted its water and gas production program “to meet security criteria.”
The energy mix for the following day was already compiled on the eve of the major power outage, taking into account electricity demand forecasts, prioritizing the most cost-effective technologies to avoid the most expensive gas sources.
In this process, overseen by the Iberian Energy Market Operator (OMIE), the electricity companies that own Almaraz and Cofrentes—in both cases, Iberdrola, and Endesa and Naturgy at the Extremadura power plant—determined that on the day of the blackout, electricity prices would be zero or negative for so many hours that the average daily price would be insufficient to cover the required tax payments. That day, there were four hours of zero prices and five hours of negative prices of up to -3 euros/MWh, resulting in a daily average of 18 euros/MWh.
Ignacio Araluce, president of Foro Nuclear, explained that Red Eléctrica was asked for permission to shut down the reactors and that, in this case, they raised no objections for reasons of system stability. In a summary of the events on the day of the blackout, Foro Nuclear noted that the two reactors were shut down “without Red Eléctrica being obliged to operate them due to technical limitations.”
This was not the first time that Red Eléctrica had not raised concerns about the shutdown of nuclear power plants due to economic unprofitability. For similar reasons, in 2024, Cofrentes was shut down for certain periods between March 8 and April 3, and Almaraz I was shut down between March 8 and 24. There were also shutdowns during the Easter period, which the sector was considering in light of the This cannot automatically explain the low electricity prices. Such decisions are made not only because of hours with zero prices, but because the daily average is unprofitable, and the analysis for the day of the massive blackout suggests that this may also be the case in the following days. In this situation, the plant operator, chaired by Beatriz Corredor, can allow the plants to reduce load and operate at lower power.
The events of April 28 raise two aspects. One, purely economic, concerns the demand by nuclear power plant owners to the government to reduce or abolish taxes, including the Enresa tax, which they must also pay to the autonomous communities. The other aspect related to the blackout concerns the uncertainty about what would have happened if the Almaraz I and Cofrentes reactors had been operating that day, and whether they would have provided other plants, hydroelectric power plants, and combined-cycle power plants with the necessary inertia to prevent destabilization of the power grid below 50 Hz.
Regarding the hypothesis that the blackout would have been avoided if these two reactors had not been shut down for economic reasons, Araluce replied: “I have no idea.” He added that the 10% of nuclear power being fed into the system at that time would not have been stopped. “Red Eléctrica told us to stop,” Araluce added, while the operator emphasized that the nuclear power plants “did not require a special authorization to operate,” apart from the technical restrictions necessary to ensure supply.
According to Araluce, the nuclear power plants were “not an obstacle.” In keeping with the Ministry of Ecological Transition’s mantra, which emphasizes that what happened after the blackout is known, but not before, and that this must be investigated, he denies that the nuclear power plants were a “problem.” After the blackout, they did not participate in the grid restoration effort, which was led by interconnectors, hydraulics, and combined-cycle power plants, and the plants did not resume operations until almost two days later.
“The equipment remained idle and took advantage of the opportunity for maintenance,” Araluce explained, rejecting the accusation that the nuclear power plants consumed electricity necessary to run the system. During the blackout, the power plants ran diesel generators to maintain basic processes, and when power was restored, they were connected to the grid, but they consumed only 20 MW per unit—compared to the thousands of megawatts needed for restoration.
