Record Low Danube Forces Hungary to Shut Its Only Nuclear Plant
The Paks power station draws cooling water from a river that has dropped below its intake nozzles. Hungary's prime minister says the country's electricity supply could turn critical from Monday, in the middle of a 37C heatwave.
Hungary's only nuclear power station is being switched off completely for the first time in its history. Nothing has broken. No reactor has faulted, no operator has erred, no regulator has intervened. The plant is being shut down because the river beside it has fallen too low for its pumps to reach the water.
Prime Minister Péter Magyar told Hungarians that the country's energy supply could become "critical" from Monday, with the Paks shutdown coinciding with a heatwave that will hold temperatures near 37C for a week. He asked people to avoid energy-hungry activities between 17:00 and 22:00 — charging electric cars, running air conditioning — and said a "rotational shutdown regime", in which parts of the grid are cut in turn, remained possible if voluntary restraint was not enough. The government would draw up a list of electricity and water users whose consumption could be capped.
That is a striking amount of disruption to trace back to a river level. It is worth working out how one leads to the other, because the answer says something uncomfortable about a technology usually sold on its dependability.
The promise of a power source that ignores the weather
Nuclear plants are typically described as baseload generation: steady output, running flat out around the clock, indifferent to whether the wind blows or the sun shines. That reputation is the industry's central argument against relying on renewables, and on its own terms it is fair. A reactor's fuel does not care about the forecast.
But a reactor is only half of a nuclear plant. The other half is a very large machine for moving heat into the environment, and that half is not weather-independent at all. It is a river-dependent machine, and rivers are weather.
Nuclear fission releases enormous quantities of heat, continuously, and that heat must be continuously carried away. This is not a matter of efficiency or output. It is the condition under which the plant is safe to operate at all. Remove the heat and you have a power station; fail to remove it and you have a reactor that overheats and fails catastrophically. Cooling cannot be reduced, substituted or deferred for a few days until the rain comes. At Paks, that heat goes into the Danube — and it gets there through suction nozzles that only work when they are underwater.
A third of the water, and a riverbed in the middle of Budapest
The Danube runs 2,857km through 19 countries, and its level at any point reflects rainfall across the whole basin upstream. This summer that basin has had very little. Much of Europe has seen high temperatures and low rainfall for months, and in Serbia hydrologists point out that the problem did not begin with the current heat: the spring was dry too, leaving nothing in reserve when the heat arrived.
The numbers are stark. Romania's National Institute of Hydrology puts the average July flow where the Danube enters the country at 4,700 cubic metres per second. On Wednesday it measured less than a third of that — 1,600 — and forecast a further fall to 1,500 by Tuesday. Levels across several countries are the lowest in 30 years; one Serbian hydrologist puts them at lows not seen in 50 to 60 years, with no improvement expected before mid-September. In central Budapest, weeks of drought have left large areas of riverbed exposed. Shipping has stopped in Bulgaria, where a river cruise ship ran aground near Vidin and 186 passengers had to be taken off by border police.
At Paks, the water simply dropped below the level the intakes were built to reach. And here the trap closes, because those intakes cannot be moved. Their depth was fixed decades ago by engineers working from the water levels their records showed the Danube to have. The Soviet-era plant's cooling arrangement is a sunk physical decision, poured in concrete, sized for a river that behaved the way the river used to behave. You cannot lower it by a metre and a half during a heatwave. If the water is not there, the only remaining option is to stop the reactor.
The heat that removes the supply also creates the demand
If this were merely a summer inconvenience, it would matter less. What makes it a crisis is that the two halves of the problem arrive together, and cannot arrive apart.
The same prolonged heat that dries the basin is the heat that drives people to switch on air conditioning. Electricity demand does not adjust downward on request, and it peaks hardest precisely when the weather is most extreme — which is also when cooling water is scarcest. A grid loses a large block of generation on exactly the evening it needs the most. That is why Magyar's appeal targets the 17:00 to 22:00 window rather than the whole day, and why voluntary restraint may not be sufficient.
Nor can Hungary lean on its neighbour, because its neighbour is on the same river. In Romania, one of the two 706-megawatt reactors at Cernavoda — a plant supplying about 20% of national electricity — was shut down earlier in the week for the same reason, with the state operator Nuclearelectrica trying to keep the second one running. If it goes, Romania too will be temporarily without any nuclear generation. One drought, one basin, two national nuclear fleets, both offline at once, with no reserve on the other side of the border to borrow from.
Infrastructure built for a climate that has moved
Europe is the fastest-warming continent, heating at roughly twice the global average, according to the Copernicus climate service. That is what turns this from bad luck into a structural problem. Every large piece of energy infrastructure encodes an assumption about the weather — how low a river gets, how often, for how long. When the climate drifts away from the record the assumption was drawn from, the assumption becomes a single point of failure, buried in concrete and impossible to renegotiate in an emergency.
And it fails in the least forgiving way available: by removing supply and adding demand in the same stroke, so that no amount of switching off air conditioners closes the gap.
Which leaves the harder question. If Danube lows like this stop being exceptional and become a normal feature of European summers, what would actually fix it — deeper intakes, different cooling designs, plants sited away from shrinking rivers — and how much would that cost, and how many years would it take to build?
Questions
Why can't the Paks plant just run at reduced power instead of shutting down?
Because the constraint is the cooling water intake, not the electricity output. The suction nozzles that draw river water into the plant only function when they are submerged. Once the Danube falls below them, the plant cannot reliably remove the heat the reactor produces, and heat removal is a safety condition rather than a performance target.
Is this a problem with old Soviet-era reactors specifically?
The design date matters in one respect: the depth of Paks's water intakes was fixed decades ago using the river levels recorded at the time. Any plant that relies on a river for cooling carries the same category of assumption, and the assumption holds only as long as the river behaves the way the historical record says it does.
How is Romania affected?
One of the two 706-megawatt reactors at Cernavoda, a plant supplying roughly 20% of Romania's electricity, was shut down earlier in the week for the same reason. The operator was trying to keep the second running. If it also stops, Romania will temporarily have no nuclear generation at all.
How low is the Danube compared with a normal summer?
Where the river enters Romania, average July flow is about 4,700 cubic metres per second. On Wednesday it was under 1,600, with a forecast fall to 1,500. Several countries are seeing the lowest levels in 30 years, and some hydrologists put them at lows not recorded for 50 to 60 years.