Slovakia’s Mochovce 4 Reaches First Criticality After 39 Years

Slovenské elektrárne says its 471-MWe Mochovce Unit 4 in Slovakia reached first criticality on 6 August, moving a reactor that has been under construction for roughly 39 years into the commissioning sequence. It is a major operating milestone, but not commercial power yet: grid connection and a full-power demonstration still lie ahead.

Key facts

• Mochovce 4 reached first criticality on 6 August 2026, according to Slovenské elektrárne, as reported by NucNet.
• The unit is designed for 471 MWe of electric power, according to the operator.
• Construction of Mochovce Units 3 and 4 began in the 1980s and was later stopped, leaving Unit 4 with a construction timeline of about 39 years.
• Each of the two new units is expected to cover 13% of Slovakia’s electricity demand at full output, according to Slovenské elektrárne.
• The commissioning plan still includes a 144-hour full-power test after grid connection and a gradual increase in reactor power.

What happened

First criticality means the reactor has achieved a controlled, self-sustaining nuclear chain reaction for the first time. It does not mean the turbine is producing commercial electricity. For Mochovce 4, the milestone begins the test sequence that will check the reactor and its supporting systems as power rises.

NucNet reported that Slovenské elektrárne said the unit reached criticality on 6 August. The Russian-designed VVER-440 reactor will now proceed through tests and trials before operation. That sequence is a much more useful marker than another construction update: the project has moved from preparing the hardware to proving that the finished systems work together under nuclear conditions.

The distinction matters because commissioning has several gates. Operators must test the unit at progressively higher power, connect the turbogenerator to the grid and complete the final demonstration run. A reactor can reach first criticality and still face technical, regulatory or scheduling work before it becomes a dependable power station.

Why it matters

Mochovce 4 is one of Europe’s clearest examples of how long a nuclear project can remain unfinished. Slovenské elektrárne says construction of Units 3 and 4 began in the 1980s, stopped after 1992 and later resumed as a completion project. The fourth unit is now approaching operation nearly four decades after the original build began.

That history makes the latest milestone easy to misread. This is not a new reactor order, and it is not a promise that a project will eventually be built. The plant is already assembled, fuelled and in start-up. First criticality is the point where the remaining risks become operating risks rather than only construction risks.

The unit also changes the scale of Slovakia’s electricity system. The operator lists each new Mochovce unit at 471 MWe and says each can cover 13% of national electricity demand. Once Unit 4 completes commissioning, Slovakia will have another large nuclear source alongside the existing Mochovce and Bohunice units. Slovenské elektrárne says Units 3 and 4 together are expected to avoid at least 5 million tonnes of carbon dioxide emissions a year compared with coal and gas generation.

NNN previously covered the approach to this point in Mochovce 4’s physical start-up tests. The new milestone is the next chapter in that same sequence. It also belongs in the wider operating context of advanced reactors reaching criticality in the United States, where first criticality likewise marks a hard technical test rather than a commercial launch.

Background

Mochovce is a four-unit nuclear site in central Slovakia. Units 3 and 4 were designed as VVER-440 reactors, a Soviet-era pressurised-water design used across Central and Eastern Europe. The two units were started during the Czechoslovak period, then left incomplete before Slovakia revived the project and completed Unit 3.

Unit 3 reached commercial operation in 2023. The operator’s construction page says Unit 4 followed a long sequence of revisions and hydrostatic testing, with licensing work required before fuel loading. NucNet reported that fuel loading and reactor sealing preceded the physical start-up phase, which led to the criticality milestone announced this week.

For readers tracking new nuclear construction and small modular reactors, Mochovce offers a useful comparison. The reactor is not small, novel or at the design stage. Its lesson is about finishing a large conventional unit, maintaining a licensing path through a long interruption and then moving from mechanical completion to controlled operation.

What’s next

The immediate watchpoints are the post-criticality tests, the first grid connection and the gradual power increase. Slovenské elektrárne’s stated plan includes a final 144-hour full-power run. Completion of that run would provide a stronger basis for commercial operation than first criticality alone.

The practical question is whether Mochovce 4 can turn this milestone into steady electricity without another long delay. If the remaining tests proceed as planned, Slovakia will gain a 471-MWe addition to its nuclear fleet. For now, the reactor has crossed the most important boundary yet: it has begun operating as a reactor, even though it has not begun operating as a commercial power plant.