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. The scenario combines nuclear unavailability with reduced hydropower availability, thermal plant cooling constraints, extreme summer demand and lower cross-border availability.
The stress case covers Paks in Hungary, Kozloduy in Bulgaria, Cernavodă in Romania and Krško in Slovenia. Together, the four sites 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. While not a forecast, the scenario is used to illustrate risks tied to interconnected regional climate and infrastructure conditions.
The vulnerability is primarily linked to water availability for cooling. Paks, Kozloduy and Cernavodă 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 multiple nuclear facilities at the same time.
Water-linked cooling constraints and adequacy under normal assumptions
Current European adequacy assessments do not identify an immediate systemic supply problem under normal assumptions. However, extreme events involving simultaneous loss of nuclear, hydro and thermal capacity present a different challenge for system operators. ENTSO-E has highlighted that declining dispatchable generation cannot be replaced by intermittent renewable capacity alone without significant investment in storage, demand response, grid infrastructure and firm flexibility resources.
The stress scenario also links nuclear outages to wider generation impacts through river flows and reservoir inflows. Drought conditions affecting nuclear cooling are expected to reduce reservoir inflows and river-based generation. High temperatures could also limit thermal plant performance while increasing electricity demand during summer.
Nuclear shutdown deficit and replacement limits
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.
Gas-fired power plants could theoretically replace lost nuclear output, but the practical issue is whether Southeast Europe has sufficient available gas capacity, fuel supply flexibility and transmission capability to deliver replacement electricity when and where it is needed. 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. Emissions impacts would increase further if drought also reduces hydropower generation.
The nuclear outage is described as only the first layer of a broader shortfall driven by correlated drought impacts across generation types. During severe summer conditions, reduced nuclear, hydro and thermal availability could create a regional supply gap of 10-15 GW during critical evening hours. This figure is presented as the main system-security challenge rather than the nuclear outage alone.
Country-level pressure points across Hungary, Romania and Bulgaria
Hungary faces the most immediate pressure due to its dependence on Paks. 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 address evening supply needs when photovoltaic output declines while demand stays high.
Batteries could support short-term balancing but current storage capacity is described as unable to replace continuous nuclear generation during extended periods of low renewable output. Imports would also become less reliable if neighbouring markets experience the same weather conditions because physical interconnection capacity does not ensure electricity availability when surrounding systems face supply stress.
Romania faces a double challenge if Cernavodă units are unavailable during a severe drought. The country would lose approximately 1.3 GW of nuclear generation while likely seeing weaker hydropower production across its reservoir fleet. Romania’s mix includes nuclear, hydro, gas, coal, wind and solar, but correlation risks remain: nuclear and hydro could decline together while solar output disappears during evening peaks and wind output could be limited during high-pressure summer conditions.
The potential shift from exporter to importer would increase pressure on domestic transmission networks and regional interconnections as supply tightens within the country.
Bulgaria export reduction and smaller systems’ exposure
Bulgaria would lose export capability if Kozloduy shuts down. The shutdown would remove approximately 2 GW of generation capacity and significantly reduce Bulgaria’s ability to export electricity to neighbouring markets.
Bulgaria could increase output from the Maritsa East lignite complex, but ageing infrastructure, coal supply challenges, cooling restrictions and carbon costs are cited as limits on full compensation. The reduction in export role would weaken precisely when neighbouring countries require additional imports.
Krško-related impacts extend beyond Slovenia due to joint ownership structure with Croatia. A shutdown would remove around 696 MW, with each country losing approximately half of its allocated output while facing additional pressure from weaker hydrology and higher summer electricity demand.
[Serbia], despite having no nuclear generation, is described as highly exposed through reliance on regional electricity exchanges. Reduced hydro output from the Đerdap complex could combine with pressure on lignite plants and limited import options as neighbouring markets tighten.
Drought-driven import dependence and price outcomes across markets
Drought conditions also reduce hydropower flexibility value for Albania, Montenegro and Bosnia and Herzegovina. Reservoir management becomes critical for preserving water for highest-value periods rather than maximising short-term generation.
The report notes that 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.
This could lead to several scarcity zones as congestion between markets limits electricity movement across borders. Regional prices could rise sharply with prolonged periods above EUR 250-500/MWh, while extreme scarcity hours could exceed EUR 1,000/MWh. A month-long nuclear deficit combined with additional hydropower losses could create billions of euros in additional electricity procurement costs before accounting for industrial losses and emergency measures.
Operational coordination and investment priorities for flexibility
The first response described focuses on operational coordination rather than emergency construction. Transmission system operators would require coordinated dispatch procedures alongside demand-response activation, strategic reserve utilisation and flexible industrial consumption.
Larger industrial consumers could support controlled demand reductions across sectors including metals, cement, electrolysis, cold storage and pumping facilities when properly contracted. A coordinated reduction of 2-3 GW during evening peaks is cited as a way to reduce risk of uncontrolled outages.
Batteries cannot replace weeks of lost nuclear production but can reduce daily system stress by managing solar peaks, evening ramps and short-term congestion. A regional target of 8-12 GW battery power and 30-50 GWh storage capacity is presented as improving flexibility through balancing services, capacity availability and grid support rather than relying only on electricity arbitrage.
Pumped storage needs transmission reinforcement alongside climate adaptation for nuclear plants
Pumped-storage hydropower is cited as providing additional long-duration flexibility through development of several gigawatts of new storage capacity to strengthen resilience against multi-day events. Generation flexibility is described as ineffective without transmission capacity because Southeast Europe requires reinforcement of internal networks and cross-border connections along Hungary-Serbia-Romania, Romania-Bulgaria, Bulgaria-Serbia and Balkan-Italy corridors.
Nuclear plants are also expected to adapt to climate risks tied to cooling performance under drought conditions. Potential measures include improved cooling systems, upgraded water intake infrastructure, additional heat sinks, hybrid cooling technologies and improved river-temperature forecasting; these upgrades are noted as potentially significant in cost relative to economic impacts from prolonged shutdowns at plants providing a large share of national electricity supply.
A resilience model focused on correlated resource reductions rather than single-reactor failure
The main risk identified is not failure of one reactor because systems are designed to withstand individual outages. Instead vulnerability comes from simultaneous reduction of several resources: nuclear availability, hydropower production, thermal capacity, import capability and evening renewable output.
A severe drought could shift national strategies into regional competition for limited supply unless coordinated investment supports storage deployment, demand response programmes, transmission infrastructure improvements, flexible generation options and climate adaptation for existing assets.
The next stage of Southeast Europe’s energy transition is therefore framed around building flexibility needed to maintain reliability under increasingly extreme operating conditions rather than adding renewable capacity alone; this includes ensuring that dispatchable resources can be sustained when water constraints affect multiple technologies at once.
