# Is Mochovce-4 Ready to Generate Power for Slovakia?

At 10:10 p.m. local time on August 6, 2026, Slovenské Elektrárne's Mochovce Unit 4 — a 471-MWe pressurized water reactor — achieved [nuclear criticality](https://smrintel.com/glossary/criticality), and twenty-one minutes later the reactor reached its minimum controlled power level. The milestone clears the path to a grid connection that will, according to the operator, cover 13 percent of Slovakia's total electricity needs once the unit enters commercial operation.

The commissioning sequence now moves through physical tests and stepped-power-level tests designed to characterize system behavior under operating conditions. The final hurdle is a 144-hour full-power demonstration run. Slovenské Elektrárne still officially lists Unit 4 as "under construction," so commercial operation figures will follow only after that endurance test and subsequent regulatory sign-off.

Slovakia's Nuclear Regulatory Authority issued a commissioning permit for Unit 4 on May 22, which went into effect on June 24. The first fuel assembly was loaded on June 29 — meaning the plant moved from fuel load to first criticality in roughly five weeks, a timeline that will be scrutinized closely by other European utilities managing long-delayed construction projects of their own.

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## A Decades-Long Construction Story Finally Nearing Its End

Mochovce-3 and -4 have been in development since the 1980s, with construction halted multiple times across political and economic cycles. Unit 3, the identical 471-MWe sister reactor, only reached commercial operation in the fall of 2023 under a commissioning license — itself a protracted saga. Unit 4's achievement of criticality in August 2026 caps what has been one of Europe's longest-running nuclear construction projects.

Project director Martin Mráz framed the milestone in explicitly human terms: "The achievement of the critical state and the minimum controlled performance is the result of the work of thousands of experts who participated in the preparation, construction, inspection and commissioning of the fourth unit. It confirms that the power plant is ready to proceed to the next phase of testing before the start of electricity production."

The characterization is accurate but also carefully calibrated. Criticality confirms physics — the reactor can sustain a chain reaction — but it says nothing definitive about cost overruns, final commissioning schedules, or the long-term reliability of aging VVER-design balance-of-plant equipment that was procured across multiple decades of intermittent construction.

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## Slovakia's Nuclear Fleet by the Numbers

When Mochovce-4 enters commercial operation, Slovakia will operate six PWRs across two sites:

**Mochovce (Slovak second nuclear station):**
- Unit 1: 467 MWe, online 1998
- Unit 2: 469 MWe, online 2000
- Unit 3: 471 MWe, commercial operation from fall 2023
- Unit 4: 471 MWe, currently in commissioning

**Bohunice:**
- Unit 3 (Bohunice V2-3): 466 MWe, online 1985
- Unit 4 (Bohunice V2-4): 466 MWe, online 1985

All six are Russian-designed VVERs. Three earlier Bohunice units — the 104-MWe gas-cooled heavy water Bohunice-A1 (Slovakia's first commercial reactor, online 1972), plus the 408-MWe Bohunice-1 and -2 (online 1980 and 1981) — are now permanently shut down.

The six-reactor fleet, concentrated entirely in Soviet-era VVER technology, represents a strategic dependency that Slovak policymakers are acutely aware of. The U.S.-Slovakia intergovernmental nuclear cooperation agreement signed in January 2026 by Energy Secretary Chris Wright and Prime Minister Robert Fico — referenced in related ANS coverage — signals Bratislava's intent to diversify its nuclear supply chain over the longer term, even as it commissions the last of its Russian-design units today.

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## What This Means for the Broader European Nuclear Market

Slovakia's Unit 4 criticality is operationally significant, but its real value is demonstrative: a long-dormant VVER project can be completed and commissioned, even if the timeline stretches across generations of engineers and political administrations. That message lands directly in Prague, where Elektrárna Dukovany II has just received final bids for a fifth reactor, and in Budapest, where the U.S. State Department recently added Hungary to its Central European nuclear cooperation agreements.

For [baseload power](https://smrintel.com/glossary/baseload) planning across Central Europe, the sequencing matters. Mochovce-3 has been generating since late 2023; Unit 4 adds roughly equivalent capacity from the same site infrastructure, with shared grid interconnects and operational staff already trained on the twin design. The incremental [capacity factor](https://smrintel.com/glossary/capacity-factor) economics for Unit 4 should be meaningfully better than Unit 3's, given shared fixed costs — assuming the commissioning tests proceed without incident.

The skeptical read: Europe's energy planners should resist treating Mochovce-4's criticality as confirmation that old-design VVER completions are replicable at scale. This project's cost and schedule history is not a model; it is a cautionary exhibit. The reactors that will actually move the needle on European clean baseload capacity in the 2030s are the new-build programs now entering procurement — Czech Dukovany, Polish first-of-kind decisions, and potential UK projects — not the tail end of 1980s-era construction.

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## Key Takeaways

- Mochovce-4, a 471-MWe VVER operated by Slovenské Elektrárne, achieved criticality at 10:10 p.m. on August 6, 2026, and reached minimum controlled power 21 minutes later.
- Next commissioning steps include physical tests, stepped-power tests, and a final 144-hour full-power run before grid connection.
- Slovakia's Nuclear Regulatory Authority issued the commissioning permit on May 22 (effective June 24); first fuel load was June 29.
- When commercial operation begins, Slovakia will operate six PWRs — all Russian VVER designs — across Mochovce and Bohunice.
- The operator states one reactor will cover 13 percent of Slovakia's electricity needs.
- Construction on Units 3 and 4 began in the 1980s and was halted multiple times; Unit 3 only entered commercial operation in fall 2023.

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## Frequently Asked Questions

**What does it mean that Mochovce-4 achieved criticality?**
Criticality means the reactor is sustaining a self-sustaining nuclear chain reaction. For Mochovce-4, this confirms the core physics work correctly and the plant can proceed to the next commissioning phases — stepped power testing and ultimately a 144-hour full-power run — before connecting to Slovakia's electricity grid.

**When will Mochovce-4 start producing electricity commercially?**
The source does not specify a commercial operation date. The reactor must complete physical tests, performance tests at multiple power levels, and a 144-hour full-power test before commercial operation begins. Slovenské Elektrárne still lists Unit 4 as "under construction."

**How many nuclear reactors will Slovakia have when Mochovce-4 is operational?**
Six operating pressurized water reactors — four at Mochovce (Units 1, 2, 3, and 4) and two at Bohunice (Units 3 and 4). All are Russian-designed VVERs.

**What is the power output of Mochovce-4?**
The reactor is rated at 471 MWe. According to Slovenské Elektrárne, a single unit at this rating will cover approximately 13 percent of Slovakia's electricity needs.

**How long has Mochovce-4 been under construction?**
Planning and initial construction work dates to the 1980s, with multiple halts over the intervening decades. Its sister unit, Mochovce-3, only achieved commercial operation in the fall of 2023, giving a sense of the extended timeline both projects have followed.