Unit 4 of Slovakia's Mochovce Nuclear Power Plant Reaches First Criticality

Slovenské Elektrárne stated that Unit 4 of the Mochovce Nuclear Power Plant reached first reactor criticality at 22:10 local time on August 6, and achieved minimum controllable power at 22:31. This is a key milestone in the pre-commissioning work of the unit, indicating that the reactor is ready for subsequent testing.

First criticality refers to the first time the nuclear fuel in the core achieves a self-sustaining fission chain reaction. Slovenské Elektrárne stated that first criticality and reaching minimum controllable power are important steps in the commissioning process of a nuclear power plant, used to confirm the unit's readiness to enter the next phase of testing.

Martin Mráz, Director of the Mochovce project, stated that achieving criticality and minimum controllable power at Unit 4 is the result of the work of thousands of experts involved in preparation, construction, inspection, and startup, demonstrating that the unit is ready for the next phase of testing before power generation.

According to the plan, Unit 4 of Mochovce will next undergo physical tests and related testing at various power levels to verify the performance of the plant's systems under operating conditions and prepare the unit for grid connection.

Unit 4 of Mochovce uses a Russian-designed VVER-440/V213 pressurized water reactor. The project is located near Nitra in southwestern Slovakia. Construction of Units 3 and 4 began in 1987 but was halted in 1991 due to insufficient funding, and resumed in November 2008 after extensive modernization to meet modern EU nuclear safety standards. Unit 3 of Mochovce entered commercial operation in June 2024.

Units 3 and 4 of Mochovce each have an installed capacity of 471 megawatts. Once both units are operational, they are expected to meet approximately 26% of Slovakia's electricity demand. Currently, Slovakia has five operating nuclear units across the Mochovce and Bohunice sites. According to International Atomic Energy Agency (IAEA) data, nuclear power accounted for approximately 65% of Slovakia's electricity generation in 2025, a share second only to France in Europe. Slovenské Elektrárne stated that the commissioning of Unit 4 at Mochovce will further enhance the country's domestic power supply capacity and strengthen long-term energy security.