Mochovce 4 starts power generation in Slovakia

Slovakia's 471-megawatt Mochovce 4 began generating power on 21 September, moving the country's newest reactor into staged commissioning before grid synchronisation and commercial operation. Its entry into service will add a major low-carbon source to a power system already heavily supplied by nuclear energy.

Key facts

• Mochovce 4 began power generation at 15:33 on 21 September, according to World Nuclear News, citing operator Slovenské elektrárne.
• The VVER-440 unit has an installed capacity of 471 MW, according to Slovenské elektrárne.
• The operator expects the unit to supply about 13% of Slovakia's electricity consumption at full output.
• Grid synchronisation is planned at 20% power, followed by tests at higher output and a 144-hour run at 100% power.

What happened

The power start-up followed fuel loading, pre-critical tests and the reactor's first controlled fission reaction. Mochovce 4 had reached criticality and minimum controlled power in early August, according to World Nuclear News. The Slovak Nuclear Regulatory Authority reviewed documentation and inspected the unit before authorising the next commissioning stage.

Power is rising in a sequence rather than jumping directly to normal output. The operator's reported test points are 5%, 20%, 35%, 55%, 75% and 90%. Engineers check reactor and turbine performance at each level. At 20%, the unit is expected to synchronise with the Slovak electricity system; the later stages test the plant at progressively higher loads. The process ends with a continuous 144-hour run at full power, as reported by World Nuclear News.

That makes Monday's event a commissioning milestone, not a declaration that the unit has completed testing or entered commercial operation. The operator has not announced a date for the 20% grid milestone or the start of commercial service. Those dates depend on test results and regulatory oversight.

The unit uses a VVER-440 pressurised-water reactor design. Its 471-MW capacity is similar to that of Mochovce 3, which entered commercial operation in 2023. Slovenské elektrárne says each unit can cover about 13% of national electricity consumption when operating at full output, although actual generation depends on availability, demand and the unit's operating profile (operator information).

Why it matters

Mochovce 4 changes Slovakia's supply outlook because it turns a long-running construction project into a reactor producing electricity during commissioning. When it completes its tests and reaches full commercial operation, the 471-MW unit should reduce the amount of electricity Slovakia needs from other sources. It also extends the operating life of a project whose two newer units took decades to complete.

The operator says nuclear generation already supplies roughly two-thirds of Slovakia's electricity consumption, counting Mochovce 1 and 2 and the Bohunice plant alongside Mochovce 3 (Slovenské elektrárne). Its claim that the share will rise to 77.5% after Unit 4 starts up is a projection tied to generation and consumption, not a fixed annual outcome. Reactor output, maintenance outages, hydrological conditions and electricity demand can all change the realised share.

The milestone is also a useful reminder of what “commissioning” means in practice. A reactor can be physically complete and reach criticality without being ready for routine commercial output. The staged power ascent checks the interaction of the reactor, steam system, turbine-generator and control systems under real operating conditions. Grid synchronisation marks another step, but not the end of the process.

NNN has tracked the unit's pre-start-up testing and its Mochovce 4 project record. Readers following new nuclear construction and small modular reactors can compare this late-stage commissioning with newer construction milestones: once a plant is built, regulatory verification and operational testing still determine when it can deliver dependable electricity.

Background

Mochovce is a four-unit nuclear station in western Slovakia. Construction of the first two units began in 1982. Work on Units 3 and 4 began in 1986, then stopped in 1992. Slovenské elektrárne restarted completion work in 2008, incorporating updated safety requirements; World Nuclear News reported the combined project cost at about €6.7 billion. Unit 3 later entered commercial operation in October 2023 (World Nuclear News).

The long gap between construction starts and operation has a practical consequence: Unit 4 is not simply another repeat build arriving on a predictable schedule. It has moved through design updates, construction completion, fuel loading and regulatory reviews before reaching power start-up. The regulator's scrutiny remains part of the schedule, not a box to tick after the engineering work.

Slovakia has five operating reactors across Mochovce and Bohunice, and the government has also discussed a future large unit at Bohunice and small modular reactors. Those plans remain separate from the Mochovce 4 commissioning sequence. Unit 4 is a conventional VVER-440 unit, not an SMR, and its current milestone does not itself authorise or finance those prospective projects.

What's next

The immediate watchpoint is whether the unit completes the 20% tests and synchronises with the grid. It then needs to pass further tests at higher output and complete the 144-hour full-power run. Slovenské elektrárne and the regulator have not provided a firm commercial-operation date in the reports reviewed here.

The clean status line is: Mochovce 4 is generating power during commissioning; grid synchronisation, staged testing and the full-power endurance run remain ahead.