A simultaneous drought-related shutdown of all operating nuclear power plants in Southeast Europe would remove nearly 5.9 GW of dependable low-carbon generation from the regional electricity system. While an individual outage could be managed for a limited period, a combined event involving nuclear shutdowns, reduced hydropower availability, thermal plant cooling constraints, extreme summer demand and lower cross-border availability would create a major regional electricity-security challenge.
The stress scenario includes the Paks nuclear power plant in Hungary, Kozloduy in Bulgaria, Cernavodă in Romania and Krško in Slovenia. Together, these facilities provide approximately 5,918 MW of net operating capacity, including 1,916 MW at Paks, 2,006 MW at Kozloduy, around 1,300 MW at Cernavodă and 696 MW at Krško. Although such a scenario is not a forecast, it highlights the risks created by multiple energy assets depending on interconnected regional climate and infrastructure conditions.
The vulnerability is linked primarily to water availability. Paks, Kozloduy and Cernavodă all rely on the Danube River system for cooling, while Krško depends on the Sava River. A prolonged regional drought combined with extreme temperatures could therefore affect several nuclear facilities simultaneously, even if the severity and timing of restrictions differed between locations.
Current European adequacy assessments do not identify an immediate systemic supply problem under normal assumptions. However, extreme events involving the simultaneous loss of nuclear, hydro and thermal capacity expose a different challenge. As ENTSO-E has highlighted, declining dispatchable generation cannot be replaced by intermittent renewable capacity alone without significant investment in storage, demand response, grid infrastructure and firm flexibility resources.
A regional deficit beyond nuclear capacity
Under normal operating conditions, the four nuclear plants would provide more than 5 GW of continuous generation. A complete shutdown would create an electricity deficit of approximately 0.9 TWh after one week, rising to around 1.8 TWh after two weeks and more than 3.8 TWh after one month.
In theory, gas-fired power plants could replace lost nuclear output. However, the practical challenge would be whether Southeast Europe has sufficient available gas capacity, fuel supply flexibility and transmission capability to deliver replacement electricity exactly when and where it is required.
Replacing the lost nuclear production with coal and lignite would increase emissions by approximately 4 million tonnes of CO₂ over one month, while gas-based replacement would still add more than 1 million tonnes of CO₂. These impacts would increase further if drought conditions also reduced hydropower generation.
The nuclear outage would therefore represent only the first layer of the crisis. The same drought conditions affecting nuclear cooling would likely reduce reservoir inflows and river-based generation, while high temperatures could limit thermal plant performance and increase electricity demand.
During severe summer conditions, the combined impact of reduced nuclear, hydro and thermal availability could create a regional supply gap of 10-15 GW during critical evening hours. This figure, rather than the nuclear outage alone, represents the true system-security challenge.
Hungary faces the most immediate pressure
Hungary would experience the strongest direct impact due to its dependence on the Paks nuclear power plant, which normally provides a significant share of national electricity generation. Losing nearly 1.9 GW would leave the country more dependent on gas generation, lignite production, storage and electricity imports.
Although Hungary’s expanding solar fleet would reduce daytime pressure, it would not solve the evening supply challenge when photovoltaic output declines and demand remains high. Batteries could support short-term balancing, but current storage capacity cannot replace continuous nuclear generation during extended periods of low renewable output.
Imports would also become less reliable as neighbouring markets faced the same weather conditions. Physical interconnection capacity does not guarantee electricity availability if surrounding countries are also experiencing supply stress.
Romania loses nuclear and hydro flexibility simultaneously
Romania would face a double challenge if Cernavodă units were unavailable during a severe drought. The country would lose approximately 1.3 GW of nuclear generation while likely experiencing weaker hydropower production across its reservoir fleet.
Romania benefits from a more diversified electricity mix than many regional markets, combining nuclear, hydro, gas, coal, wind and solar. However, the main risk is correlation: nuclear and hydro could decline together, solar production would disappear during evening peaks and wind output could remain limited during high-pressure summer conditions.
The country could rapidly shift from electricity exporter to importer, increasing pressure on domestic transmission networks and regional interconnections.
Bulgaria loses its export role
A Kozloduy shutdown would remove approximately 2 GW of generation capacity and significantly reduce Bulgaria’s ability to export electricity to neighbouring markets.
The country could increase output from the Maritsa East lignite complex, but ageing infrastructure, coal supply challenges, cooling restrictions and carbon costs would limit the ability to compensate fully.
Bulgaria’s role as a regional electricity supplier would weaken precisely when neighbouring countries require additional imports, increasing competition across Southeast European markets.
Smaller systems face disproportionate risks
The shutdown of Slovenia’s Krško nuclear plant would remove around 696 MW, affecting both Slovenia and Croatia due to the plant’s joint ownership structure. Each country would lose approximately half of its allocated output while facing additional pressure from weaker hydrology and higher summer electricity demand.
Serbia, despite having no nuclear generation, would also be highly exposed. The country depends on regional electricity exchanges and could face reduced hydro output from the Đerdap complex, pressure on lignite plants and limited import options as neighbouring markets tighten.
For Albania, Montenegro and Bosnia and Herzegovina, drought conditions would reduce the value of hydropower flexibility. Reservoir management would become critical, requiring operators to preserve water for the highest-value periods rather than maximise short-term generation.
Imports alone cannot solve a regional crisis
Electricity imports are effective during isolated outages but become far less reliable when multiple countries experience the same weather event. During a regional drought, Hungary, Romania, Bulgaria and Serbia could all become import-dependent simultaneously.
The result would likely be the creation of several scarcity zones, with congestion between markets limiting the ability of electricity to move freely. Regional prices could rise sharply, with prolonged periods above EUR 250-500/MWh and extreme scarcity hours exceeding EUR 1,000/MWh.
A month-long nuclear deficit combined with additional hydropower losses could create billions of euros in additional electricity procurement costs, even before accounting for industrial losses and emergency measures.
Flexibility becomes the central investment priority
The first response to such an event would need to focus on operational coordination rather than emergency construction. Transmission system operators would require coordinated dispatch procedures, demand-response activation, strategic reserve utilisation and flexible industrial consumption.
Large industrial consumers could provide significant support through controlled demand reductions. Sectors such as metals, cement, electrolysis, cold storage and pumping facilities can shift or reduce consumption when properly contracted.
A coordinated reduction of 2-3 GW during evening peaks could significantly reduce the risk of uncontrolled outages.
Storage and grid investment are essential
Battery storage cannot replace weeks of lost nuclear production, but it can reduce daily system stress by managing solar peaks, evening ramps and short-term congestion.
A regional target of 8-12 GW of battery power and 30-50 GWh of storage capacity would significantly improve flexibility. Such deployment would require billions of euros in investment but would provide value through balancing services, capacity availability and grid support rather than relying only on electricity arbitrage.
Pumped-storage hydropower would provide additional long-duration flexibility. Developing several gigawatts of new storage capacity would strengthen resilience against multi-day events.
However, generation flexibility is ineffective without transmission capacity. Southeast Europe requires reinforcement of internal networks and cross-border connections, particularly along Hungary-Serbia-Romania, Romania-Bulgaria, Bulgaria-Serbia and Balkan-Italy corridors.
Nuclear plants must adapt to climate risks
The stress scenario does not reduce the importance of nuclear energy. Instead, it demonstrates that nuclear assets must become more resilient to changing climate conditions.
Potential measures include improved cooling systems, upgraded water intake infrastructure, additional heat sinks, hybrid cooling technologies and improved river-temperature forecasting.
The cost of such upgrades may be significant, but it should be compared with the economic impact of prolonged shutdowns at plants that provide a large share of national electricity supply.
A new regional resilience model
The main risk for Southeast Europe is not the failure of one reactor. Electricity systems are designed to withstand individual outages. The real vulnerability is the simultaneous reduction of several supposedly independent resources: nuclear availability, hydropower production, thermal capacity, import capability and evening renewable output.
A severe drought could transform national electricity strategies into regional competition for limited supply. Avoiding that outcome will require coordinated investment in storage, demand response, transmission infrastructure, flexible generation and climate adaptation of existing assets.
The next stage of Southeast Europe’s energy transition will therefore depend not only on adding renewable capacity, but on building the flexibility required to maintain reliability under increasingly extreme operating conditions.
