Danube Drought Squeezes Bulgaria’s Nuclear Cushion

For more than half a century, Bulgaria’s Kozloduy nuclear plant had never had to trim output because the Danube was too low. That changed in August. The reduction was not large, and it did not point to a reactor safety problem. But it took some of south-east Europe’s most useful spare electricity off the system just as Romania and Hungary were already scrambling to keep their own Danube-cooled nuclear stations running.

Why low water means less electricity, not reactor danger

Bulgaria’s Energy Ministry said on 20 August that Unit 5 would be cut by about 120 MW as a preventive step because of what it called "continuing unprecedented and critical" Danube levels (Bulgarian Energy Ministry, 20 Aug). By 26 August, plant data showed Unit 5 down from roughly 1,010 MW to 850 MW, a fall of about 15% (SEGA, Fakti). Energy Minister Iva Petrova said there was no crisis (Bulgarian Energy Ministry, 21 Aug).

That is true, but only if the question is nuclear safety. Danube water is not cooling the reactor core. It cools steam in condensers on the turbine side, where heat is turned into electricity. When the river falls too low, intake pumps have less to work with, the cooling loop becomes less efficient, and the plant has to produce less power while the nuclear safety systems remain separate and intact (Actualno, Fakti EN). It is an electricity problem, not a nuclear accident problem.

The difficulty is that the same river put three nuclear systems under pressure 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 lift the water level at Paks (kormany.hu). The engineering problem was different in each case. The source of pressure was the same.

The buffer that shrank

When the Bulgarian cut came, the country was exporting roughly 30,000 MWh a day, against domestic consumption of about 90,000 MWh (Bulgarian Energy Ministry, 21 Aug). That surplus was already doing work outside Bulgaria. On 6 August, Romania imported a net 2,240 MW during the evening peak, with Bulgaria supplying most of the physical flow (Newscenter). Kozloduy’s reduction did not create Romania’s import dependence. It narrowed the margin Romania was already using.

The market noticed. On 26 August, day-ahead wholesale prices, the next-day electricity prices paid by generators and large buyers, sat in a tight cluster across the region: Bulgaria at 172 EUR/MWh, Romania 175, Hungary 172, and Greece 173 (euenergy.live). Germany-Luxembourg, further from the immediate stress, was at 126 EUR/MWh (euenergy.live). The difference is the cost of replacing steady nuclear output, the kind of round-the-clock generation that usually keeps the system calm.

Who gains, who pays

Slovakia is the obvious winner. Its nuclear plants at Bohunice and Mochovce use cooling towers rather than drawing directly from the river, so low Danube levels do not hit them in the same way (Denník E). During the stressed hours, Slovak power flows into Hungary exceeded 2.5 GW at times (SITA).

The bill falls in several places. Nuclear operators tied into regulated or pre-agreed prices lose margin if they have to buy expensive replacement electricity to cover shortfalls. Big industrial users carry the first interruption risk: Hungary’s emergency rules allow grid operator MAVIR to cut power to factories drawing more than 0.5 MW before households are touched (HVG). In Romania, Nuclearelectrica warned customers 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). Across the continent, it did. In south-east Europe, that stability depended on dredging, barges, stone barriers, emergency reservoir releases and costly evening imports. The useful disclosure now is not another broad reassurance. It is the river level at which each operator must cut output, published before the next drought leaves governments making it up under pressure.