The day the Danube switched off a reactor

A reactor runs out of river

Cernavodă is cooled by the Danube. Through the summer of 2026 the river fell to a historic low — around 1,370 m³/s at the point it enters Romania, roughly a third of the August average. Unit 1 had already come offline at the end of July. When the water level reached the −230 cm threshold, Unit 2 followed, in a controlled shutdown begun on the morning of 13 August. Nuclearelectrica invoked force majeure on its electricity delivery contracts.

Two units of roughly 700 MW each, normally about a fifth of national generation, gone because of the weather. Return dates were given as indicative only: everything depends on the river.

What that looks like in the data is below. At 08:05 the reactor was at 649 MW; an hour later it was at 496. The decline is almost perfectly linear at about 2.5 MW per minute — the signature of a controlled ramp-down rather than a trip, which would be a vertical drop in a single reading.

The interesting part of the chart is not the coloured line. It is the three grey ones. Gas, coal and hydro barely move while 150 MW of baseload walks off the system.

The sun takes the shift

Zoom out to the whole day and the ramp-down stops looking like an isolated event. Between 06:00 and 11:00 the system loses nuclear entirely — and wind at the same time, from 1,387 MW down to 690. Close to 2,000 MW of supply disappears in five hours.

Over exactly the same hours, solar goes from 7 MW to 2,148. The handover is so clean that gas never has to move: it sits near 1,300 MW all morning and then falls. Coal drifts down too. The most expensive units on the system responded to losing a reactor by producing less.

That is the merit order doing its job. Solar bids at essentially zero marginal cost, so it clears first and pushes everything above it down the stack — including the gap the reactor left behind.

Hydro gets out of the way

By 11:00 something more abrupt happens. In a single hour, hydro output collapses from 1,233 MW to 445. By 15:00 it is at 262 — from a morning peak of 1,695. Nearly 1,400 MW of hydro leaves the system at exactly the moment solar peaks.

In a drought year the obvious explanation is that the water simply is not there. The data rules that out for this particular collapse. Romanian hydro reports separately as run-of-river and reservoir, and the two behave very differently:

Reservoir hydro goes from 1,063 MW at 09:00 to 50 MW at 15:00 — a 95% shutdown — and then back to 825 MW by 20:00, sixteen times its afternoon output, as the evening peak arrives. A drought lowers the whole day; it does not carve a hole in the middle of it and refill it after sunset. What changed between nine in the morning and three in the afternoon was not the water available but the value of using it.

A reservoir is a battery. Water spilled through a turbine at midday, when solar has driven the price to the floor, earns far less than the same water released into the evening peak. So the operators close the gates and wait, then open them again after sunset. You are watching an economic decision, taken thousands of times a day across a national system, show up as a line on a chart — and in a low-water August, water is exactly the thing worth being careful with.

Run-of-river falls too, from 670 MW to 211. That is less obvious, since a river cannot be switched off — but most Romanian run-of-river plants, on the Olt and Bistrița cascades, have small compensation basins that let them shift output within a day even though they cannot store from one day to the next.

The bill arrives after sunset

Up to here the story reads as a happy one: a reactor left, the sun covered it, and nothing costly had to start. That reading is true of the morning and wrong about the day.

Solar falls from 2,264 MW at 16:00 to 17 MW at 20:00. Demand does the opposite, climbing from 4,826 MW to 6,611 as the country comes home. Nuclear, which would normally have been sitting underneath all of it, is at zero.

Everything flexible responds. Gas climbs from 989 MW to 1,410. Coal goes from 601 to 840. Reservoir hydro returns from 50 to 825. The batteries discharge 668 MW at 21:00, after reading almost nothing all afternoon. And it is still not enough: domestic generation reaches only 4,826 MW against 6,611 MW of demand.

The remaining 1,784 MW — 27% of everything Romania consumed at that hour — came across the border. The country had spent the middle of the day exporting its solar surplus at 471 MW and spent the evening importing nearly four times as much.

Nothing here failed. This is a market working exactly as designed: cheap energy is used where and when it exists, and scarcity is covered by whoever has spare capacity, including neighbours. But it locates the cost of a dry river precisely. It is not paid at noon. It is paid at nine in the evening, at whatever the neighbours are charging.

Where the evening came from

The official plan for the shutdown period was to increase imports, mainly from Bulgaria and Ukraine. The border-by-border measurements say something more specific.

At 21:00 the four Bulgarian interconnectors — Dobrudja, Varna and the two Kozloduj lines — were carrying 2,496 MW into Romania. That is more than the entire net import, because at the same moment Romania was still sending 596 MW to Moldova, which depends on it.

Ukraine, named alongside Bulgaria in the official plan, did contribute — but at the wrong end of the day for the peak. The Mukacevo line ran at +414 MW into Romania at 14:00, in the middle of the solar surplus, and was exporting 77 MW at nine in the evening. On this day, the peak was covered by Bulgaria almost alone.

There is an irony worth noticing in those line names. Kozloduj is Bulgaria's nuclear plant, and it sits on the same river, upstream. The Danube took one country's reactor out of the market and left the other's running.

What the day shows

Climate risk arrives as market risk. Nothing broke at Cernavodă. The plant is fine; the river is not. A weather variable turned into 1,400 MW of missing capacity, a force majeure clause, and a quarter of an evening peak bought abroad. That chain is the whole story, and every link in it is measurable.

Timing matters more than capacity. The same 686 MW that cost nothing to lose at ten in the morning was unreplaceable at nine in the evening. Averaged over the day, the reactor's absence looks modest. Hour by hour, it is the whole story.

Flexibility is worth more than energy. Hydro's contribution that day was not the megawatt-hours it produced. It was the megawatt-hours it declined to produce at midday and saved for the evening. In a system with a lot of solar, the ability to move output in time is the scarce commodity — which is why reservoirs and batteries did the same thing on different clocks.

Interconnection is not optional. A 2,255 MW swing from export to import inside eight hours is only survivable because the borders are open. The geography question from section 1.4 stops being abstract when a quarter of the evening peak arrives on it.