# Are Both Cernavoda Reactors Now Offline Due to Low Danube Levels?

Yes. As of 13 August 2026, both units of Romania's Cernavoda nuclear power plant are in controlled shutdown — the first time the country's only nuclear facility has been fully offline due to a hydrological event. The Danube reached 182 centimetres on the morning of 12 August, dropping below the 185-centimetre threshold required for normal plant operation. Nuclearelectrica began the controlled shutdown of unit 2 at 7:00am on 13 August, completing disconnection from the National Energy System at 10:53am. Unit 1 had already been taken offline on 28 July. Together, the two 650 MWe CANDU reactors normally supply roughly one-fifth of Romania's electricity — approximately 1,300 MWe of [baseload power](https://smrintel.com/glossary/baseload) that must now be replaced through imports, hydro, wind, and reserve capacity. Romania's Ministry of Energy says the National Energy System remains stable, but the outage represents a significant test of grid resilience at peak summer demand.

Across the border in Hungary, three of four units at the Paks plant also went offline due to low Danube levels, though unit 2 returned to full nominal power on the evening of 10 August after operating at 50% capacity for eleven days.

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## How Cernavoda's Shutdown Unfolded

The sequence of events illustrates how quickly a drought-driven water level decline can overwhelm emergency mitigation efforts.

Nuclearelectrica shut unit 1 and disconnected it from the grid on 28 July, citing the "unprecedentedly low level of the Danube caused by severe drought." With unit 2 still operating, Romania's energy ministry launched emergency river engineering: explosives were used to remove a rock obstruction, the riverbed was dredged, and four rock-filled barges were sunk to redirect flow. The ministry reported on 9 August that these interventions raised the water level around unit 2 by 4 centimetres — enough, based on available forecasts, to allow the reactor to function for at least nine more days.

It wasn't enough. Three days later, the river was at 182 centimetres and falling. Nuclearelectrica CEO Cosmin Ghita confirmed in a letter to the Bucharest Stock Exchange that unit 2's controlled shutdown would proceed "in accordance with procedures for such situations, based on strict monitoring of safety margins, equipment, and the INHGA forecast." He emphasised that keeping both units in safe shutdown has "no impact on nuclear safety parameters, and therefore on personnel, the environment, or the population."

The CANDU reactor design — used at Cernavoda since unit 1 entered commercial operation in 1996 and unit 2 in 2007 — uses heavy water as both moderator and coolant in a pressurised circuit, but still depends on large volumes of river water for secondary-side cooling and heat rejection. When river intake falls below minimum levels, the plant cannot safely reject decay heat at full power.

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## Hungary's Infrastructure Response at Paks

Hungary's approach to the same problem has been more aggressive and structurally ambitious. Operator Magyar Villamos Művek (MVM) confirmed on 10 August that unit 2 of Paks — which had been running at 50% capacity for the previous eleven days — began ramping back toward full output at 6:00pm that Monday, reaching nominal power just before 10:30pm.

Units 1, 3, and 4 at Paks remain offline.

The Hungarian government announced on 12 August a two-track infrastructure programme. The near-term measure: two 80-metre-long barges placed on standby at Paks, with controlled sinking capable of raising the Danube level by up to 20 centimetres in the short term. The first barge was due to arrive from Mohács on Wednesday 13 August; the second on Thursday.

The longer-term measure is the construction of a permanent bottom sill — effectively a submerged weir — between the Paks plant and the village of Dunaszentbenedek. The government said the design was undertaken with involvement from Budapest University of Technology. Using 35,000 cubic metres of stone in the first round, followed by 110,000 cubic metres, the government stated it can "be ensured within a few weeks that the water level in the Danube at the cooling channel of the nuclear power plant will not drop below -90 cm." The bottom sill could raise the Danube by up to one metre at the intake point without creating flood risk during high-water periods, the government said.

Coordination was assigned to General Pál Kádár, head of the Defence Administration Office, with approximately one hundred national guardsmen from Budapest and Szentes supporting construction.

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## Broader Industry Implications

This dual-country shutdown deserves attention beyond the immediate grid management story. Several structural points stand out:

**Cooling water dependency is a first-order operational risk.** Both Cernavoda and Paks sit on the same river, creating correlated exposure. A single regional drought event has simultaneously threatened over 2,500 MWe of generation capacity across two EU member states. As climate-driven drought frequency increases across Central and Eastern Europe, any nuclear fleet assessment in the region must now model Danube flow probability distributions alongside standard capacity factor calculations.

**The 185cm threshold at Cernavoda is a hard operational limit.** The fact that a 4-centimetre gain from emergency river engineering bought only three days of additional margin — before the river fell 3 centimetres below the minimum — illustrates how thin the operating envelope had become. Emergency barge-sinking is not a substitute for upstream infrastructure planning.

**Hungary's permanent weir response is the right architecture but carries its own risks.** Constructing a submerged sill in a navigable international river within weeks — even with military coordination — is an engineering challenge at pace. The government's claim that it poses "no danger during a possible later flood" will require independent hydrological validation; downstream flow dynamics from a permanent sill are non-trivial.

**For SMR siting analysis, this episode reinforces the appeal of designs with passive or air-cooled decay heat removal.** Reactor concepts that reduce or eliminate dependency on river water for normal and emergency cooling face a structurally different risk profile in drought-prone regions. That siting advantage is increasingly quantifiable in a climate context.

**Romania's grid management is the near-term watch item.** Replacing roughly one-fifth of national electricity generation through imports and variable hydro and wind during peak summer demand is operationally manageable but expensive. The duration of the Cernavoda outage will be determined entirely by Danube levels — a variable outside Nuclearelectrica's control.

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## Key Takeaways

- Both Cernavoda CANDU units (650 MWe each) are in controlled shutdown as of 13 August 2026, with the Danube at 182cm — below the 185cm operational minimum.
- Unit 1 went offline 28 July; unit 2 began controlled shutdown at 7:00am on 13 August and was fully disconnected at 10:53am.
- Emergency river engineering — explosives, dredging, and four rock-filled barges — achieved only a 4cm water level gain, buying fewer than nine days before the river fell further.
- Romania's Ministry of Energy says the National Energy System is stable, with imports, hydro, wind, and reserves covering the shortfall.
- In Hungary, Paks unit 2 returned to full nominal power on 10 August after eleven days at 50% capacity; units 1, 3, and 4 remain offline.
- The Hungarian government is deploying two 80-metre barges as a short-term measure and has initiated construction of a permanent bottom sill to ensure cooling water supply — a project involving the Budapest University of Technology and military coordination.
- The weir is projected to prevent Danube levels at the Paks cooling intake from dropping below -90cm and could raise levels by up to one metre.

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## Frequently Asked Questions

**Why did Cernavoda have to shut down when the Danube water level dropped?**
Cernavoda's two CANDU reactors require river water for secondary-side cooling and heat rejection. When the Danube fell to 182 centimetres on 12 August 2026 — below the 185-centimetre threshold for normal plant operation — Nuclearelectrica determined it could no longer safely operate unit 2 and initiated a controlled shutdown beginning at 7:00am on 13 August.

**Is there a nuclear safety risk from the Cernavoda shutdown?**
Nuclearelectrica CEO Cosmin Ghita explicitly stated that keeping both units in safe shutdown has "no impact on nuclear safety parameters, and therefore on personnel, the environment, or the population." Controlled shutdown is a standard operational procedure, and decay heat management in shutdown CANDU reactors does not require the same water volumes as full-power operation.

**What is Hungary doing to keep the Paks plant running during the drought?**
The Hungarian government has approved two measures: placing two 80-metre barges on standby for controlled sinking to raise Danube levels by up to 20 centimetres as a short-term fix, and constructing a permanent submerged weir between the Paks plant and Dunaszentbenedek using up to 110,000 cubic metres of stone to ensure the cooling intake level does not fall below -90cm.

**How much of Romania's electricity does Cernavoda normally provide?**
The two Cernavoda units together generate approximately one-fifth of Romania's electricity. With both units offline, Romania's Ministry of Energy says the grid is currently covered through a combination of domestic production, imports, hydro, wind, and reserve capacities.

**What does this event mean for nuclear plant siting near rivers in Europe?**
The simultaneous shutdown of capacity across Romania and Hungary from a single regional drought event highlights cooling water dependency as a correlated, climate-sensitive operational risk. It strengthens the siting case for reactor designs with passive or air-cooled cooling systems, and suggests that river-dependent nuclear plants in Central and Eastern Europe need more robust hydrological infrastructure planning than has historically been required.