Slovenské elektrárne has completed the loading of nuclear fuel into the reactor of Mochovce Unit 4. The process began on Monday, 29 June 2026, at 16:04 with the insertion of the first fuel assembly, officially making Unit 4 a nuclear installation. The final fuel assembly was loaded by company personnel on Friday, 3 July 2026. Completion of fuel loading represents another major milestone on the path towards commissioning the last unit at Mochovce.
Each fuel assembly is loaded into the reactor core in accordance with a precisely defined and approved loading programme specifying the position of every individual fuel assembly within the reactor. Fuel loading also marks the transition from the construction phase to the physical and power start-up phases of the new nuclear generating unit.
The entire process is carried out under the strict supervision of the Nuclear Regulatory Authority of the Slovak Republic (ÚJD SR). The authority's inspectors have been verifying the unit's readiness for the commencement of active testing throughout the construction phase, equipment installation, functional testing, and personnel qualification process. Nuclear safety remains the absolute priority throughout the subsequent stages of the commissioning programme.
The fuel used in VVER-440 reactors is uranium dioxide (UO₂) enriched to 5% U-235. The fuel is manufactured in the form of ceramic pellets weighing approximately 5 grams each and contained within fuel rods. A bundle of 126 fuel rods forms a single hexagonal fuel assembly containing approximately 120 kg of nuclear fuel.
The reactor contains 349 assemblies, comprising 312 fuel assemblies and 37 control assemblies. In total, the reactor contains approximately 42 tonnes of nuclear fuel. Once a year, during a planned outage, the reactor vessel is opened and approximately one-fifth of the fuel inventory, around 8 tonnes, is replaced. A single fuel assembly remains in the reactor core for approximately five years.
Following the completion of fuel loading, pre-critical tests, reactor assembly activities and additional tests required to achieve the first controlled fission reaction will take place. These will be followed by tests to verify the reactor core characteristics, known as neutron-physical characteristics.
During the subsequent power start-up phase, reactor power will be gradually increased until the nominal power level is reached. Only then will the new unit begin supplying its first megawatt-hours of electricity to the Slovak power system.
With an installed capacity of 471 MW, Mochovce Unit 4 will cover approximately 13% of Slovakia's electricity consumption. Once commissioned, nuclear energy will account for approximately 77.5% of the country's electricity consumption, placing Slovakia among the countries with the highest share of nuclear power worldwide and ahead of France, where nuclear energy accounts for approximately 67% of electricity consumption.
The completion of the Mochovce 3 and 4 Project will also bring to a close one of the largest investments in Slovakia's modern history, with a total value of approximately EUR 6.7 billion. Unit 3 was connected to the grid in 2023 and, following the start-up of Unit 4, Slovakia will become a long-term net exporter of electricity.
"The completion of fuel loading into Unit 4 represents another significant step towards the completion of the Mochovce Project. With this unit, we are bringing one of the most important chapters in Slovak energy history to a close. The outcome will be a stable and reliable source of low-emission electricity for households, industry and future generations. It is a success achieved by thousands of people who have contributed to the project over many years," said Martin Mráz, Mochovce Project Director, Slovenské elektrárne.
Learn More About the Mochovce Project and Slovenské elektrárne's Generation Fleet
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. It operates five nuclear units, 31 hydroelectric power plants and two photovoltaic power plants.
The company's commitment to generating electricity from low-carbon sources makes it a leader in sustainable power generation 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.
