When people across Slovakia switch on the lights, cook dinner or charge their phones tonight, some of that electricity may already come from Unit 4 of the Mochovce Nuclear Power Plant. At 20:41 on Thursday, 15 October 2026, Slovenské elektrárne successfully connected the new reactor unit to the national transmission system for the first time.
During the process known as grid synchronization, the first of the unit’s two turbine generators was synchronized with the grid and began delivering its first megawatt-hours of electricity.
“For the first time in its history, Mochovce Unit 4 is supplying electricity to Slovakia’s power system. This is a moment generations of people who prepared, built and commissioned this project have been waiting for. Today, we are seeing the result of their work in the form of a new low-carbon source of electricity for Slovakia.”
Branislav Strýček, Chairman and Chief Executive Officer of Slovenské elektrárne.
The synchronization took place while the reactor was operating at 20% power. The first of the unit’s two turbine generators was connected to the grid. Unit 4 has a total installed electrical capacity of 471 MW and, once fully operational, is expected to generate around 3.5 million MWh of electricity annually, enough to cover approximately 13% of Slovakia’s current electricity consumption.
Grid synchronization itself takes only a few moments, but it is preceded by an extensive series of inspections and tests. Specialists must verify generator protection systems, electrical circuits and the readiness of all equipment involved in exporting power to the grid. The electricity produced by the generator must precisely match the voltage, frequency and phase of the transmission system.
The process can be compared to a musician joining an orchestra. Before playing, the musician must tune the instrument, match the tempo and enter at exactly the right moment. Only then can they perform in harmony with the rest of the ensemble. For Mochovce Unit 4, that decisive moment was the connection of the generator to the national grid.
The journey of electricity begins inside the reactor. Heat generated by the controlled nuclear fission process is used in steam generators to produce steam. The steam drives a turbine, the turbine powers a generator, and the generator converts mechanical energy into electricity. Before synchronization could take place, every element of this chain had to be ready to operate together safely and reliably.
“From the main control room, operators manage reactor power and oversee the operation of the steam generators, turbine and generators. Electrical specialists then establish a safe connection to the grid. Today’s milestone lasted only a short time, but it was made possible by the perfectly coordinated work of many different professions,” said Martin Mráz, Project Director for Mochovce Units 3 and 4.
The first synchronization does not mark the end of the commissioning process. The second turbine generator will also be synchronized to the grid, followed by a series of mandatory tests at progressively higher power levels. During these tests, specialists will verify the performance of technological, control and safety systems all the way to nominal power output. The process is being carried out under the supervision of the Nuclear Regulatory Authority of the Slovak Republic.
Mochovce Unit 4 achieved first sustained criticality on 6 August 2026. This was followed by physical tests and power ascension testing, which prepared the unit for electricity generation and grid connection. Grid synchronization represents a major turning point: for the first time, electricity generated by Unit 4 has left the plant and entered the national power system.
After years of development and construction, the project has begun delivering the result for which it was built: reliable electricity for Slovakia.
With a 66% share of Slovakia’s total electricity generation, Slovenské elektrárne is the country’s largest electricity producer. Following the shutdown of Slovakia’s last coal-fired power plant in the first quarter of 2024, the company now generates all of its electricity without direct CO₂ emissions.
Slovenské elektrárne operates five nuclear units, 31 hydroelectric power plants and two photovoltaic power plants. The flexibility of the Slovak power system is also supported by two battery energy storage facilities: a 3.2 MW battery system at Dobšiná and a 36 MW battery storage facility in Nováky, the largest battery installation in Slovakia.
The company’s commitment to low-carbon electricity generation has made it a leader in sustainable energy production in Central and Eastern Europe. Slovenské elektrárne has been included five times in the prestigious Europe’s Climate Leaders ranking, compiled annually by the Financial Times and Statista since 2021.
