Mochovce 4 connects to Slovakia's grid for the first time

Slovakia's Mochovce 4 nuclear unit delivered electricity to the national grid for the first time on 15 October, when operator Slovenské elektrárne synchronised the first of its two turbine generators at 20:41 with the reactor at 20% power. The 471-MW unit is expected to cover about 13% of Slovak consumption, but full-power testing still lies ahead.

Key facts

• The first of Mochovce 4's two turbine generators was synchronised with the grid at 20:41 on 15 October 2026, at 20% reactor power, according to Slovenské elektrárne.
• The VVER-440 unit has an installed capacity of 471 MW and is expected to generate about 3.5 million MWh a year, roughly 13% of Slovakia's current consumption.
• Synchronisation came two days after a 13 October postponement triggered by an unusual response in voltage-measurement circuits on the non-nuclear side of the plant.
• Power ascension still has to finish with a 144-hour run at 100% power.
• The completion project for Mochovce 3 and 4 was estimated at EUR6.7 billion, according to World Nuclear News.

What happened

Slovenské elektrárne said on 15 October that its Mochovce Unit 4 had begun supplying electricity to the Slovak power system for the first time. The first of the unit's two turbine generators was brought into phase with the grid at 20:41 local time, with the reactor operating at 20% of its power, the operator said.

"For the first time in history, Mochovce unit 4 is supplying electricity to the Slovak electricity system," Branislav Strýček, chairman and chief executive of Slovenské elektrárne, said, as reported by World Nuclear News. In the company's Slovak-language release, he called it a moment generations had waited for and a new low-emission source for the country.

The milestone arrived two days later than planned. On 13 October the company postponed the synchronisation and cancelled a press event after primary-voltage tests on the unit's generator produced an unusual response in voltage-measurement circuits. Those circuits sit in the non-nuclear part of the plant. The operator repeated part of the technical tests, said the event had no effect on nuclear safety, and described the step as standard practice during commissioning.

Why it matters

Grid synchronisation is the point at which a commissioning reactor stops being a test rig and becomes a generator on the national system. For Mochovce 4, it closes the gap between first criticality on 6 August and the start of power generation in late September, which NNN covered in Mochovce 4 starts power generation.

The scale matters for Slovakia. Slovenské elektrárne expects the unit to produce about 3.5 million MWh a year at full operation, about 13% of current national consumption. Nuclear already generated about 65% of Slovakia's electricity in 2025, citing IAEA data, from five units at Mochovce and Bohunice. A sixth unit would strengthen Slovakia's position among the most nuclear-reliant power systems in the world and reduce its need for imports.

It also matters for Europe's construction record. Mochovce 3 and 4 have been the region's longest-running completion project. A grid-connected Mochovce 4 shows that a long-stalled Soviet-era VVER-440 can be finished and brought to power under a modern EU regulatory regime, even if the schedule and cost have been far longer and higher than originally planned.

The 13 October delay is a reminder that commissioning is still under way. The fault was on the conventional side of the plant and was resolved within two days, but every step from here involves tests the regulator must accept before the unit moves on.

Background

Mochovce sits in western Slovakia and hosts the country's VVER-440/V-213 pressurised water reactors. Units 1 and 2 entered operation in 1998 and 1999. Construction of units 3 and 4 began in the 1980s and stalled in the early 1990s for lack of funding; completion work resumed in 2008. Mochovce 3 entered commercial operation in October 2023.

Unit 4 followed a long commissioning path in 2026. Fuel loading began in June, the reactor was sealed for physical start-up tests in July (NNN coverage), it reached criticality on 6 August and began generating power in September. NNN's Mochovce 4 tracker records each step with its source. Readers following new nuclear builds and SMRs can set the unit against other plants moving through commissioning.

Slovenské elektrárne produces about 66% of Slovakia's electricity and operates five nuclear units, 31 hydroelectric plants and two solar plants. The Slovak Republic holds a 34% stake in the company; the majority shareholder is Slovak Power Holding, owned by the Czech group Energetický a průmyslový holding.

What's next

The operator's next step is to synchronise the second turbine generator. After that, the unit will run prescribed tests at gradually increasing power until it reaches nominal output, under the supervision of the Nuclear Regulatory Authority of the Slovak Republic (ÚJD). The operator set out staged test points at 5%, 20%, 35%, 55%, 75% and 90% when power generation began (NNN coverage), and the final test will be a 144-hour run at 100% power.

Slovenské elektrárne has not announced a date for commercial operation. The milestones to watch are the second turbine's synchronisation, the first sustained operation at full power and the completion of the 144-hour run. Any further conventional-island issues like the one on 13 October would push those dates, but would not by themselves indicate a nuclear safety problem.