Just a few days after the loading of the last of the 349 fuel assemblies, Mochovce Unit 4 reached another major milestone: reactor assembly. During this operation, specialists progressively installed and connected dozens of tonnes of technological equipment, closed the reactor vessel, and carried out a series of inspections and tests confirming the readiness of the primary circuit for the subsequent stages of start-up. The new nuclear unit is now moving into the phase of physical tests leading to the first controlled fission reaction.
“Mochovce has once again moved closer to its goal. Over the past few days, we have successfully completed two key operations: nuclear fuel loading and reactor assembly. We are now entering the start-up phase, which will determine the commissioning of the new unit. Each of these milestones brings us closer to the moment when Slovakia will fully utilise the potential of the completed Mochovce project and the new unit will begin generating electricity for households and businesses for decades to come.”
Branislav Strýček, Chairman of the Board and Chief Executive Officer, Slovenské elektrárne
The reactor is securely enclosed within the hermetic zone inside the containment structure, which consists of the reinforced concrete protective envelope of the nuclear island. Following the closure and sealing of the reactor vessel, we completed a series of inspections and tests that confirmed the readiness of the primary circuit for the next phases of start-up.
“With the completion of reactor assembly, we have concluded one of the key stages of physical start-up. Ahead of us are pressure and leak-tightness tests, pre-critical tests and, subsequently, first criticality, that is, the first controlled self-sustaining fission chain reaction. All activities are being carried out in accordance with the approved programme and with a strong emphasis on nuclear safety, which remains our absolute priority,” said Martin Mráz, Project Director of MO34.
Provided the start-up programme continues according to schedule, Unit 4 is expected to deliver its first megawatt-hours of low-emission electricity to the grid following the synchronisation of the turbine generator at the end of summer 2026. Reactor power will subsequently be increased gradually to the nominal power level, with the process culminating in a 144-hour full-power trial operation.
For decades, a simple rule has been passed down through generations of nuclear power plant operators. The final principle of the so-called Operator's Ten Commandments, published as early as 1989 in the technical handbook Fundamentals of Operating Modes of VVER-440 Nuclear Power Plants, states succinctly:
This is not merely a professional curiosity. It reflects respect, discipline and humility towards a technology whose safe operation requires the highest level of expertise and strict adherence to procedures. These same principles also guide the commissioning of Mochovce Unit 4.
Once completed, the new unit, as part of the Mochovce and Bohunice nuclear fleet, will strengthen Slovakia's energy self-sufficiency, enhance the stability of the national power system, and reinforce the country's position among European leaders in the share of nuclear energy in electricity generation.
With a 66% share of Slovakia's total electricity generation, Slovenské elektrárne, a.s. is the country's largest electricity producer. Following the shutdown of the last coal-fired power plant at the end of the first quarter of 2024, the company now generates all of its electricity without direct CO₂ emissions. Slovenské elektrárne currently operates five nuclear units, 31 hydroelectric power plants and two photovoltaic power plants. The flexibility of the energy system is further supported by two battery energy storage systems: a 3.2 MW facility at the Dobšiná hydropower plant and a 36 MW facility in Nováky, the largest battery energy storage system in Slovakia.
The company's commitment to generating electricity from low-carbon sources makes it a leader in sustainable energy production in Central and Eastern Europe. The company has been included five times in the prestigious Europe's Climate Leaders ranking, compiled annually by The Financial Times and Statista since 2021.
Slovenské elektrárne, a.s. has two shareholders. The majority shareholder, holding a 66% stake, is Slovak Power Holding B.V. (SPH), which is owned by the Czech energy group Energetický a průmyslový holding (EPH). The second shareholder is the Slovak Republic, which holds a 34% stake in the company.
