MVM's Paks Reactors Came Within 10 Centimetres of a Full Stop. Hungary's HUF 6bn Fix Rebuilds the Danube, Not the Plant

A 2,000 MW nuclear station lost most of its output this month. There was no fault, no fuel problem and no order from the regulator. The river simply dropped about 10 centimetres too far.

The margin was measured in centimetres

On 3 August 2026 the Danube gauge at Paks read minus 140 centimetres, 43 centimetres below the 2018 record of minus 97 centimetres. Attila Aszódi, professor at the Budapest University of Technology and Economics (BME), set out what that meant in Portfolio.hu on 14 August 2026: at that level the cooling-water pump of the last running turbine sat a few centimetres above its operability limit, and the pumps of the safety cooling-water system had just 10 centimetres of margin left.

MVM Paksi Atomerőmű Zrt. runs four VVER-440 units for 2,000 MW, close to half of Hungary's electricity generation. Output fell to 965 MW at the start of August. By 18 August only Unit 2 was still turning, at half load, with restart of the idle turbines flagged for the following Sunday. It is the first time in the plant's operating life that a complete stop has been on the table.

Rebuilding the river was cheaper than rebuilding the plant

The intuitive engineering answer is to lower the pumps. Aszódi is blunt about that route: dropping the cooling-water pumps to the lowest practical level buys at most 50 centimetres of headroom, and the work runs to several years. Inside a 30-day window it is not an option at all.

So Hungary went the other way and re-engineered the river. A bottom sill is being built at Danube river-kilometre 1526.33, between Paks and Dunaszentbenedek, across roughly 300 metres of reach between the plant's cold-water and warm-water channels. Cost is put at around HUF 6bn. Against that, Prime Minister Péter Magyar put a full plant shutdown at a minimum HUF 50bn a month to the budget, a prime-ministerial estimate rather than a booked loss, but the ratio is what drove the decision. The first phase places 35,000 cubic metres of rock, with a further 110,000 to follow, and the target is to hold the level at the intake above minus 90 centimetres.

Two construction methods, 30 days, no tender

The programme splits at the centreline. On the Paks side, where the bed is deeper and the flow stronger, quarry stone of 40 to 200 kg is trucked in, stockpiled on the bank and released into the bed from vessels. That section is a submerged sill about 200 metres long with a 150-metre-wide base, its crest kept at least half a metre below the current low-water surface so the international navigation channel survives. From Dunaszentbenedek on the left bank, army engineers advance dry: trucks reverse and tip, dozers level, and the plant drives forward on the rock it has just laid. That side becomes a 300-metre stone wall standing 50 to 60 centimetres above current low water, founded on a one-metre-thick, 40-metre-wide stone blanket over geotextile, with a 6-metre crown.

BME modelled the structure at 600 cubic metres a second, below both the 720 m³/s all-time minimum flow and the roughly 800 m³/s running during construction. The same modelling shows no meaningful obstruction at a 7,500 m³/s flood. Work runs around the clock for 30 days, with stability widening continuing into October. Two 80-metre barges sit on standby to be sunk at 45 degrees if the gauge falls below minus 132 centimetres, worth roughly 20 centimetres of immediate lift.

The supply chain turned up before the paperwork

Delivery sits with the national water directorate OVF on the technical side, under József Gacsályi, and with the Defence Administration Office on coordination, under Brigadier General Pál Kádár, with 100 soldiers deployed and BME on design. The materials came from the contracting market, and they came free. Magyar Építők reported on 13 August 2026 that STRABAG offered technology, haulage and material at no charge, that KÉSZ Zrt. made a similar approach, that Market Építő committed 10,000 tonnes of stone delivered on its own fleet, that Duna Aszfalt released a barge from the Mohács Danube bridge site, and that Bayer Construct offered 25,000 to 30,000 tonnes of graded gravel from its Kiskunlacháza pit.

Note what is missing from that list. There is no tender, no bill of quantities, no priced risk allocation, no FIDIC clause governing who carries a washout, and no defects liability period on a structure that will sit in a live navigation channel for years. Emergency powers and goodwill got stone into the water within a day. Neither is a procurement model, and neither can be relied on a second time.

What riverside assets are worth is being repriced

Paks is not an isolated case. Romania prepared Cernavodă Unit 2 for controlled shutdown over low Danube levels in the same fortnight. The prime minister pointed to a 125-year record at the Paks gauge showing summer levels trending down, with the decline steepening after 2010, and the bed at Paks is incising as well. Aszódi's conclusion is that 2018 was not an outlier.

For lenders and owners the diligence question shifts from how much water a river carries to what level it carries it at, because pumps respond to elevation, not volume. For contractors, the work that follows is river engineering rather than plant engineering: sills, dredging, bank protection and intake reconstruction, procured by water authorities on flood-defence timelines rather than by utilities on outage schedules. And for Hungary specifically, Paks II is being built on the same reach of the same river, with an intake that will have to work at levels the 2010s did not contemplate.