Bulgaria has reduced output from Kozloduy nuclear unit 5 to around 850 MW as low Danube levels continue to restrict cooling-water availability, extending drought-related pressure on firm generation across central and southeast Europe.
The reactor had been operating near 1,010 MW before hydrological restrictions began, meaning output has now fallen by roughly 15%.
Unit 6 remains unaffected.
Bulgarian authorities have announced plans to construct a 250-300 metre levee near the plant to improve access to cooling water, with works expected to take around 10-15 days.
The deeper reduction at unit 5 suggests hydrological conditions remain difficult before the emergency works can be completed.
The curtailment is significant because Kozloduy is one of Bulgaria’s main sources of low-marginal-cost baseload supply.
The loss of around 160 MW comes as other major Danube-based nuclear plants have also faced water-related constraints.
Romania’s Cernavodă station has experienced prolonged disruption, while Hungary’s Paks plant cut output sharply earlier in August before recovering.
At the same time, weak hydrology has reduced hydro availability in several southeast European markets.
The combination has helped keep regional power prices elevated.
Central SEE day-ahead markets have repeatedly traded in the €180-190/MWh range during August, while evening hours in some countries have exceeded €250/MWh.
Paks output has largely recovered, but the regional premium has persisted, suggesting scarcity has broadened beyond the Hungarian nuclear outage.
Reduced Kozloduy availability adds to that pressure.
The current conditions also underline a growing climate-risk issue for nuclear generation.
Nuclear plants are normally considered firm supply, but conventional reactors remain dependent on reliable cooling-water access.
Extreme heat can therefore create a double effect: electricity demand rises because of cooling loads while river conditions simultaneously reduce generation availability.
The likely response will require both site-specific adaptation and wider system flexibility.
For nuclear operators, this means more investment in water-management and cooling infrastructure.
For power systems, it increases the value of storage, interconnection, hydro flexibility, demand response and fast-start thermal generation.
The Kozloduy levee represents a relatively modest infrastructure intervention, but it is significant because it shows climate adaptation moving directly into nuclear operating expenditure.
If low-water events become more frequent, utilities may need to incorporate such investment permanently into asset-management plans.
The Danube is therefore becoming an increasingly important regional power-market variable.
It affects hydro output, nuclear cooling and energy logistics simultaneously.
For traders, river conditions can no longer be treated as a secondary hydrological indicator.
They are increasingly part of the short-term availability outlook for some of southeast Europe’s largest generation assets.
