Mochovce, Slovakia – On August 6, 2026, the Mochovce 4 nuclear reactor achieved a self-sustaining chain reaction for the first time. The 471 MW reactor reached criticality at 10:10 p.m. and attained minimum controlled power 21 minutes later. This marks the start of the next phase of commissioning, although the unit is not yet feeding electricity into the grid. Before grid connection, further physical tests and inspections at various power levels must be successfully completed.
Mochovce 4 Nears Grid Connection: Tests to Determine Next Step
In the context of a nuclear reactor, criticality initially refers simply to a controlled, self-sustaining nuclear fission process. Reactor power remains very low while experts analyze the behavior of the reactor core. Subsequently, power levels are increased in stages, with further tests conducted at each step. The commissioning process concludes with a 144-hour demonstration run at full power.
Image: Shutterstock
Active commissioning for Mochovce Unit 4 began as early as late June with the initial fuel loading. The Slovak nuclear regulatory authority, ÚJD, granted the permit on May 22, and it became legally binding on June 24. The first fuel assembly was inserted into the reactor on June 29, and the full loading process was completed on July 3. The core contains 349 fuel and control assemblies, holding a total of approximately 42 tonnes of nuclear fuel.
471 megawatts of additional capacity for Slovakia
The history of the new reactor spans nearly 40 years. Construction of Units 3 and 4 began in 1986, but the project was halted in 1992. Slovakia resumed work on their completion around a decade into the new millennium. Unit 3 finally entered commercial operation in October 2023.
The total cost for both new units has most recently been cited at around 6.7 billion euros. At full capacity, each reactor can theoretically meet approximately 13 percent of Slovakia’s electricity consumption. With the addition of Unit 4, the site’s installed capacity increases by another 471 megawatts. Furthermore, once fully commissioned, the country will have six operational power reactors across the Mochovce and Bohunice sites.
Electricity generation begins only after testing
Achieving criticality is therefore a significant technical milestone, though it does not yet constitute regular power plant operation. First, the reactor systems must demonstrate their intended performance characteristics as power output increases. In the case of Mochovce Unit 4, the nuclear regulatory authority oversees the transitions between the various commissioning phases. The operator can apply for final operating licenses only after the mandatory tests have been completed and evaluated.
Nevertheless, the new unit is of considerable importance to Slovakia’s electricity supply. Five nuclear reactors already provide a large share of the country’s domestic electricity, and this sixth unit will further boost available generation capacity. Once the new unit is operating at full load, Slovenské elektrárne projects a level of nuclear power generation mathematically equivalent to 77.5 percent of national electricity consumption. However, actual electricity delivery from the new reactor will not commence until after successful grid connection and the subsequent power output tests.
Author: Blackout News
Sources: Nuclear Newswire (12.08.26) – Energynews (11.08.26) – World Nuclear News (10.08.26) – Slovenske Elektrarne (07.08.26)