Danube Drop Halts Nuclear Reactors

The Danube beside Romania's Cernavoda nuclear plant is now carrying about 1,630 cubic metres of water per second, roughly a third of what would be normal for July (Agerpres, Știrile ProTV). Around 300 kilometres upstream, at Hungary's Paks nuclear plant, the river gauge has fallen to -118 cm, a local record (Telex).

The mechanism is simple enough. Nuclear plants use rivers as part of their cooling system. A reactor converts only part of the heat it produces into electricity; the rest has to be removed safely. If the river is too low, or too warm, the plant cannot discharge that heat within safety limits. Operators then reduce output or shut the reactor.

That is what happened this week at two points in the Danube basin. Romania shut both Cernavoda reactors, taking about 1,300 MW off the grid. President Ilie Bolojan said that was close to a fifth of national electricity consumption (HotNews). In Hungary, state-owned MVM began shutting Paks Unit 3 on 29 July. Paks normally supplies close to half of Hungary's electricity (TVP World, HVG).

Neither case was an accident. Both shutdowns were preventive. Romania's Energy Ministry warned that damage to a single cooling pump could require repairs lasting up to a year (Mediafax). A fortnight without production is costly. A year without a pump is worse.

Cheap power drops out, expensive power fills in

Once a nuclear plant has been built, each extra unit of electricity is cheap to produce. When that power disappears, the gap is filled by gas plants, imports, or other flexible sources that can come online quickly. Those sources cost more.

The price effect is amplified by the way wholesale electricity markets work. The market price is set by the most expensive producer needed to meet demand. So when a cheaper nuclear plant drops out and a dearer gas or import source sets the price, everyone selling into the market receives that higher price.

After Cernavoda went offline, Romania's day-ahead wholesale price rose €22.1 to €132.87 per megawatt-hour. Hungary's equivalent increased €26.50 to €122.08/MWh (Serbia Energy, Serbia Energy).

Romania also moved from net electricity exporter to net importer within hours. The strain is sharpest between 20:00 and 23:00, when households use more power and solar generation has dropped away. During those evening hours, Romania was importing about 1,700 MW, mainly from Bulgaria (Digi24, Profit.ro).

That is a fragile dependence. Bulgarian gauges on the Danube at Ruse already show -85 cm, below previous historic lows (ClubZ). In other words, Romania is leaning on a neighbour exposed to the same river problem.

Who pays, who profits

Romanian households have some short-term protection. Officials said domestic contracts carry fixed prices through the year (Romania Insider). But fixed prices do not make the cost disappear. They push it elsewhere: onto suppliers buying expensive replacement power, into next year's tariff reset, or eventually onto the state budget.

Industrial users face the wholesale spike more directly. Bolojan appealed publicly to both citizens and industry to cut evening consumption. Romania and Hungary also prepared contingency plans to reduce demand from large consumers while exempting hospitals (444.hu, Portfolio).

The winners are gas plant operators and power exporters able to sell during scarcity hours. They receive the higher price set by the shortage. The nuclear operators lose production and revenue.

For Malta, the lesson is familiar even if the geography is different. A small system feels imported price shocks quickly. Malta does not cool nuclear reactors on a river, but it does rely on a narrow set of energy routes: gas-fired generation, the interconnector, and imported fuels. When a larger European market tightens, the effect can reach an island consumer through procurement costs, supply contracts, or the public subsidy needed to keep bills stable.

Not a freak event

This is not an isolated episode. France's Cour des comptes, the national audit office, has documented climate-related nuclear output losses as a recurring problem. French plants lost at least 1 TWh of output in each of 2018, 2019, and 2020 because rivers were too warm or too low (Argus).

The Danube basin is now showing the same pattern on a river that cools reactors in four countries. This matters because nuclear power is supposed to be the dependable low-carbon backbone of many European grids. Its reliability now depends partly on river conditions that are becoming less reliable.

Romania's grid operator estimates Cernavoda Unit 1 could restart around 4 August and Unit 2 around 10 August (Profit.ro). Those dates depend on rainfall that forecasters have not promised.

The wider problem will last beyond this heatwave. Europe is building a cleaner power system, but some of its most reliable low-carbon plants still need cold, abundant river water. Climate change is making that input scarcer, and the price shows up first in the evening market, then in the bill, and finally in the politics of who absorbs the cost.