Danube Low Slows Paks Reactors

Hungary built much of its electricity system around one plant on one river. Paks, the nuclear station south of Budapest, supplies nearly half the country's electricity (World Nuclear). By 29 July, one of its four reactors had been taken offline for the first time because the Danube had fallen too low (Intellinews).

Prime Minister Péter Magyar warned that the whole plant could be dark by 4 or 5 August, possibly for weeks (Telex). Romania has already shut one reactor at Cernavoda for the same reason (Agerpres). The drought across the Danube basin is taking out cheap, steady power just when summer demand is at its peak.

How a river shuts a power plant

A nuclear plant does not use river water as fuel. It uses it to carry heat away. Steam turns the turbines, and the remaining heat has to go somewhere; at Paks, it is discharged into the Danube.

Hungarian rules say the Danube cannot exceed 30°C within 500 metres downstream of the discharge point (MVM Paks, UNECE). When the river is too shallow or too warm, operators reduce output. Less electricity produced means less waste heat pushed back into the water.

The limit arrived quickly. On 27 July, MVM, Hungary's state energy group, began cutting generation after the Paks gauge hit a record -106 cm (MVM). Two days later, the first reactor was offline.

Romania's Cernavoda faced the same physics. Danube flow at Bazias fell to about 1,630 m³/s, low enough to threaten cooling pumps (HotNews). As we reported when the shutdowns began (To Brief), this is not a safety crisis. Operators stop reactors well before damage thresholds. The problem is the loss of baseload: steady, round-the-clock power when the system is already stretched.

The evening trap

Solar has softened the daytime hit. Hungarian solar parks generated 1,504.8 GWh in June, more than Paks' 1,312.1 GWh (24.hu). That looks reassuring until the evening.

After 18:00, solar falls away while air-conditioning demand remains high. Magyar said Hungary's evening load is about 6,000 MW. Losing the full 2,000 MW Paks plant would remove roughly a third of that (Daily News Hungary).

The replacement power is dearer. Hungary was already importing more than 1,000 MW from neighbours on recent evenings. Day-ahead prices, the price paid for electricity delivered the next day, rose to 122.08 EUR/MWh in Hungary and 132.87 EUR/MWh in Romania after the shutdowns (Serbia Energy).

Romanian Prime Minister Ilie Bolojan identified 20:00-23:00 as the dangerous window and urged households to cut evening use (Agerpres). The grid problem, in other words, is not the whole day. It is the few hours when the sun is gone, people are still cooling homes, and the missing nuclear output has to be found somewhere else.

Who pays when the river drops

Hungary is preparing to order large industrial users, including MOL and Samsung SDI, to cut electricity use while protecting households and hospitals (Portfolio). That protects voters and essential services. It does not make the bill disappear.

The cost moves to factories, state utilities or taxpayers. For industry, the hit is immediate: lost production, higher power prices, or both. For governments, the temptation will be to absorb the shock and argue about the invoice later.

Bulgaria is the short-term winner. Romanian reporting found Bulgarian generators supplying vital evening imports, with Profit.ro observing that "household power in Romania depends paradoxically on Sofia" (Profit.ro). But Bulgaria's advantage rests on the same river. The Danube near Ruse was already at -85 cm, below historical minimums (Club Z). If the drought deepens, Kozloduy, Bulgaria's nuclear plant, faces the same cooling constraint.

Austria adds another squeeze. Vienna's Freudenau hydropower plant on the Danube generated roughly 29% less than its 30-year average (MeinBezirk). When Hungary needs imports to replace nuclear power and Austria has less hydropower to sell, the pool of cheap evening electricity gets smaller.

The European Commission said on 31 July that it was monitoring the situation and was ready to convene the EU Electricity Coordination Group (Euronews). The drought has turned an engineering assumption into a political number: how much cheap evening power in Central Europe still depends on rivers being deep enough, and cool enough, to do the quiet work behind the grid.