Danube drought shuts Paks power

The Danube is usually treated as part of the furniture of central Europe: broad, working, dependable. In the last week of July, it stopped being dependable enough to cool two nuclear plants.

Hungary's only nuclear power station, Paks, went fully offline on 2 August after the river fell so low that its cooling pumps could no longer reach the water surface. The plant supplies close to 2 GW of baseload power, about half of Hungary's electricity (MVM, World Nuclear Association). Downstream in Romania, the same drought had already forced one of Cernavodă's two reactors offline, while the second was operating with an estimated four to five days of margin. In the space of a week, two countries found that electricity they treated as permanently available still depends on rain.

One river, two shutdowns

The Paks shutdown unfolded over six days. When the Danube gauge fell to -106 cm on 27 July, MVM, the operator, began cutting output (MVM). By 30 July it was warning that all four units would stop within 24 to 72 hours if the water kept falling (VG).

There was no fault in the reactors. The weakness was more basic. The pump intakes were left sitting above the dropping river surface, and without a steady flow of water a nuclear plant cannot remove the heat it produces. MVM had set -134 cm as the final trigger. The Danube reached it on 2 August (Index, AP).

Romania was exposed to the same river, just further downstream. Apele Române, the national water authority, measured Danube flow at 1,650 m³/s at Romania's border on 28 July, about one-third of the seasonal average and close to the historic low recorded in 1985 (Apele Române). Nuclearelectrica shut Cernavodă's Unit 1 as a precaution, keeping Unit 2 online only after confirming its intake parameters still held (BVB/Nuclearelectrica, Energynomics).

The Romanian government then allocated 7 million lei for temporary barges to redirect river flow at a key bifurcation point upstream of the plant's intake (Agerpres). Keeping even one reactor running had become a civil engineering job.

Who absorbs the shock

When electricity supply disappears this quickly, the grid protects households and hospitals first. The burden then moves to large industrial users, because they are the demand that can be ordered down when the system has to hold.

Hungary made that explicit. A government decree gave MAVIR, the national grid operator, power to order any consumer drawing more than 0.5 MW to cut load, with penalties set at 200% of the spot price (444.hu, HVG). The market moved in the same direction. Hungarian week-ahead power rose to about €250–263/MWh, from €217.90 a day earlier (Argus).

Romania's price move was sharper again. Peak-hour clearing prices on the day-ahead market went from 63.3 lei/MWh at midday to 1,604 lei/MWh by evening, as demand ran into a smaller supply stack (Agerpres). Nuclearelectrica warned customers that force majeure could affect contractual deliveries (ZF).

The immediate winners were neighbouring exporters able to sell into the squeeze. Bulgaria was already supplying roughly 1,700 MW to Romania during peak hours, against Romanian evening consumption above 7,000 MW (Profit.ro). The scarcity rents travelled across the border with the electricity.

A cooling-design problem, not a nuclear one

This was not a reactor failure. Other nuclear plants on the same continent, facing the same heatwave, kept operating because they were built with a different cooling answer.

Slovenia's Krško switched to cooling towers, which release heat into the air rather than relying directly on river flow (Delo). Czech plants at Dukovany and Temelín use similar designs and were less exposed (Seznam Zprávy). France's national audit court has already found that heat and water limits cut at least 1 TWh of French nuclear output in each of 2018, 2019 and 2020 (Cour des comptes). The European Commission said it was monitoring the situation and could convene an emergency electricity coordination meeting (Euronews).

The lesson is awkward for European energy planning. Capacity cannot be counted as firm if summer river levels can remove it from the system. Grid planners now have to price the choice plainly: cooling towers, replacement capacity, or higher scarcity costs in drought years. Paks has put a number on the delay.