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 a single 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 electricity-security challenge.
The stress scenario covers the Paks nuclear power plant in Hungary, Kozloduy in Bulgaria, Cernavodă in Romania and Krško in Slovenia. Together, these 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. Although the scenario is not a forecast, it points to risks from multiple assets relying on interconnected regional climate and infrastructure conditions.
Water-linked cooling constraints across Danube and Sava river systems
The vulnerability is primarily linked to water availability. 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 several nuclear facilities simultaneously. The severity and timing of restrictions could differ between locations even under the same broad weather pattern.
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 present a different challenge for system operators.
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. This includes the ability to maintain supply when multiple generation types are constrained at the same time.
Nuclear shutdowns translate into multi-week energy deficits
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.
The deficit would rise to around 1.8 TWh after two weeks and exceed 3.8 TWh after one month. The replacement challenge extends beyond total capacity because it depends on where and when power can be delivered.
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 exactly when and where required.
If coal and lignite were used instead of nuclear generation, emissions would increase by approximately 4 million tonnes of CO₂ over one month. Gas-based replacement would still add more than 1 million tonnes of CO₂, with additional increases possible if drought reduces hydropower output.
Drought effects extend beyond nuclear to hydro and thermal output
The nuclear outage would represent only an initial layer of the crisis. The same drought conditions affecting cooling could reduce reservoir inflows and river-based generation.
High temperatures could also limit thermal plant performance while increasing electricity demand during peak periods. 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 evening-hours gap is described as the key system-security challenge rather than the nuclear outage alone . The interaction between constrained generation types would affect balancing needs across multiple national systems.
Country-by-country impacts: Hungary, Romania, Bulgaria and Slovenia
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.
An expanding Hungarian solar fleet would reduce daytime pressure but would not address the evening supply challenge when photovoltaic output declines while demand remains high. Batteries could support short-term balancing; however current storage capacity cannot replace continuous nuclear generation during extended low-renewables periods .
Romania would face a double challenge if Cernavodă units were unavailable during severe drought. The country would lose approximately 1.3 GW of nuclear generation alongside weaker hydropower production across its reservoir fleet.
Bulgaria, where Kozloduy provides a major share of output under normal conditions, would lose approximately 2 GW of generation capacity in a shutdown scenario. This would significantly reduce Bulgaria’s ability to export electricity to neighbouring markets .
Bulgaria could raise output from the Maritsa East lignite complex, but ageing infrastructure, coal supply challenges, cooling restrictions and carbon costs would limit compensation potential. Its export role would weaken precisely when neighbours require additional imports.
Krško in Slovenia, jointly owned with Croatia through its ownership structure, would remove around 696 MW. Each country would lose approximately half of its allocated output while also facing additional pressure from weaker hydrology and higher summer electricity demand .
Southeast Europe exposure: Serbia and hydropower-dependent systems
Serbia, despite having no nuclear generation in this scenario set-up, is described as highly exposed through reliance on regional electricity exchanges. Reduced hydro output from the Đerdap complex could coincide with pressure on lignite plants and limited import options as neighbouring markets tighten.
Drought conditions are also expected to reduce hydropower flexibility value for Albania, Montenegro and Bosnia and Herzegovina. For these systems, reservoir management becomes critical to preserve water for highest-value periods rather than maximize short-term generation .
Imports become less effective as scarcity spreads across markets
The report distinguishes between isolated outages and regional weather events for import reliability. Electricity imports are effective during single-country disruptions but become far less reliable when multiple countries experience the same drought conditions.
During a regional drought, Hungary, Romania, Bulgaria and Serbia could all become import-dependent simultaneously . This outcome is expected to create several scarcity zones as congestion between markets limits cross-border power flows.
The price impact described includes prolonged periods above EUR 250-500/MWh, with extreme scarcity hours exceeding EUR 1,000/MWh. A month-long nuclear deficit combined with additional hydropower losses could generate billions of euros in additional procurement costs before accounting for industrial losses and emergency measures .
Operational coordination plus flexibility investments for multi-day stress
The first response described focuses on operational coordination rather than emergency construction. Transmission system operators would need coordinated dispatch procedures alongside demand-response activation and strategic reserve utilisation.
The scenario also highlights flexible industrial consumption as part of balancing measures. Large industrial consumers can provide 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. Storage measures are also positioned as part of daily stress management rather than direct replacement for weeks-long nuclear loss .
Batteries, pumped storage and grid reinforcement requirements
Battery storage 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 is outlined at 8-12 GW of battery power and 30-50 GWh of storage capacity.
Pumped-storage hydropower is identified as providing additional long-duration flexibility through development of several gigawatts of new storage capacity . Generation flexibility alone is described as ineffective without adequate transmission capacity for moving power across constrained areas.
Southeast Europe requires reinforcement of internal networks and cross-border connections along corridors including Hungary-Serbia-Romania, Romania-Bulgaria, Bulgaria-Serbia and Balkan-Italy . These upgrades are framed as necessary to support balancing when multiple regions face correlated constraints.
Nuclear cooling adaptation measures under changing climate conditions
The stress scenario does not reduce the importance attributed to nuclear energy; instead it indicates that nuclear assets need resilience to changing climate conditions . Potential measures listed include improved cooling systems and upgraded water intake infrastructure.
The measures also include additional heat sinks, hybrid cooling technologies and improved river-temperature forecasting . Costs for such upgrades are described as potentially significant but should be compared with economic impacts from prolonged shutdowns at plants that provide a large share of national electricity supply .
A regional resilience model based on correlated resource reductions
The main risk for Southeast Europe is described as not being failure at one reactor because systems are designed to withstand individual outages. Instead vulnerability comes from simultaneous reduction across resources including nuclear availability, hydropower production, thermal capacity, import capability and evening renewable output .
A severe drought could shift national electricity strategies toward competition for limited supply across countries. Avoiding that outcome requires coordinated investment in storage, demand response, transmission infrastructure, flexible generation and climate adaptation of existing assets .
