Danube drought cuts Bulgaria’s nuclear buffer

Bulgaria's Kozloduy nuclear plant reduced electricity generation for the first time in its 52-year history because the Danube dropped too low to cool its turbines properly. The cut is modest. But it removed the region's largest source of spare electricity at the exact moment Romania and Hungary were already fighting to keep their own Danube-cooled reactors running.

Why low water means less electricity, not reactor danger

Bulgaria's Energy Ministry announced on 20 August that Unit 5 would drop by about 120 MW as a preventive measure against what it called "continuing unprecedented and critical" Danube levels (Bulgarian Energy Ministry, 20 Aug). By 26 August, plant data showed Unit 5 had fallen from around 1,010 MW to 850 MW, roughly a 15% decline (SEGA, Fakti). Energy Minister Iva Petrova said there was no crisis (Bulgarian Energy Ministry, 21 Aug).

That is technically accurate but narrow. Danube water does not cool the reactor core. It cools steam in condensers on the turbine side, the part that actually generates electricity. When river levels drop, intake pumps struggle, the cooling loop loses efficiency, and the plant produces less power even though nuclear safety systems remain separate and intact (Actualno, Fakti EN). This is an electricity problem, not a nuclear-accident problem.

What makes it significant is that the same drought hit three countries' nuclear plants within weeks. As we reported last week, Romania ordered emergency dredging and reservoir releases to keep water reaching Cernavodă, which normally supplies about a fifth of Romanian electricity (Agerpres, World Nuclear Association). Hungary spent HUF 6.1 billion on stone barriers and sunken barges to physically raise the water level at Paks (kormany.hu). Each plant failed differently. The same river created three distinct engineering emergencies.

The buffer that shrank

Bulgaria was exporting roughly 30,000 MWh a day against domestic consumption of about 90,000 MWh when the cut came (Bulgarian Energy Ministry, 21 Aug). That surplus was already plugging holes next door. On 6 August, Romania imported a net 2,240 MW during the evening peak, with Bulgaria supplying the bulk of the physical flow (Newscenter). Kozloduy's reduction did not create Romania's import dependence. It shrank the supply Romania was already leaning on.

The price signal is visible across the region. On 26 August, day-ahead wholesale prices (what generators and large buyers pay for next-day electricity) clustered tightly: Bulgaria at 172 EUR/MWh, Romania 175, Hungary 172, Greece 173 (euenergy.live). Germany-Luxembourg, further from the stress, sat at 126 EUR/MWh (euenergy.live). That gap measures how much more south-east European buyers pay for replacement power when steady nuclear output, the kind of generation that runs around the clock, drops out.

Who gains, who pays

Slovakia is the clearest winner. Its nuclear plants at Bohunice and Mochovce use cooling towers instead of drawing directly from the river, so low Danube levels barely touch them (Denník E). During stress periods, Slovak power flows to Hungary exceeded 2.5 GW in some hours (SITA).

The losers sit on both sides of the meter. Nuclear operators locked into regulated or pre-agreed prices lose margin when they must buy expensive replacement power to cover shortfalls. Industrial users face interruption risk first: Hungary's emergency rules allow grid operator MAVIR to cut power to factories drawing more than 0.5 MW before touching household supply (HVG). Romania's Nuclearelectrica sent customers a legal warning that it may not be able to honour contracts because of events outside its control (ZF).

The European Commission said on 11 August that Europe's electricity system remained stable despite extreme weather (European Commission). At the continental level, that is true. At the regional level, stability required dredging, barges, stone barriers, emergency reservoir releases and expensive evening imports. The system held. The next useful disclosure is not another reassurance. It is the water level at which each operator must cut output, published before the next drought forces improvisation again.