Slovak utility Slovenské elektrárne sealed the reactor vessel of Mochovce Nuclear Power Plant’s fourth unit on July 20, 2026, and announced that physical start-up testing has begun — the final preparatory phase before the unit achieves its first controlled fission reaction, according to World Nuclear News. When the 471-megawatt reactor eventually reaches full commercial operation, Slovakia will supply the equivalent of 77.5% of its electricity consumption from nuclear sources — the highest proportion for any nation on Earth, as confirmed by World Nuclear News.

That record will not be instantaneous. Physical start-up tests are a multi-stage process that will take weeks to months, followed by an approach to criticality, a stepwise power ascension program, and a mandatory 144-hour continuous full-power endurance run — the last gate before commercial operation is declared, as the Idaho National Laboratory explains. The company is targeting first electricity to the grid by the end of summer 2026, but full commercial operation — what determines Slovakia’s final nuclear share — will almost certainly follow in 2027, based on the Mochovce 3 precedent.

“By completing the assembly of the reactor, we have concluded one of the key stages of physical start-up,” said Martin Mráz, director of the MO34 project. “Ahead of us are strength and leak tests, pre-critical tests and then the first criticality — the first controlled fission chain reaction,” Mráz told World Nuclear News.

What Physical Start-Up Tests Actually Measure

The sequence now underway at Mochovce 4 is a systematic verification that the as-built reactor core will behave as designed — before any nuclear heat is generated. The VVER-440/V-213 reactor (the V-213 designation distinguishes the upgraded, post-Chernobyl model from its older V-230 predecessor) uses six primary coolant loops, each equipped with a horizontal steam generator — a configuration that differs from most Western pressurized water reactors, which typically use four vertical steam generators, as detailed by Wikipedia’s VVER entry.

After hermetic sealing of the reactor pressure vessel, engineers run hydraulic pressure and leak tests on the primary circuit. Only after those pass do pre-critical measurements begin: neutron sources are used to characterize the reactor’s reactivity behavior and predict, with precision, exactly where in the control-rod withdrawal sequence criticality will occur. “We will go critical where we say we will” is the engineering standard — and verifying that prediction against actual measurements is precisely the point of the pre-critical phase, according to INL’s startup physics testing research.

Criticality itself — the moment when the fission chain reaction becomes self-sustaining, with each splitting atom producing on average exactly one neutron that triggers another split — is not the moment electricity flows, as the Idaho National Laboratory’s criticality explainer notes. At initial criticality, the reactor operates at near-zero power: enough neutrons to sustain the reaction, but far too little heat to drive a turbine. Power ascension begins afterward, with output raised in steps — at 5%, 25%, 50%, 75%, and full capacity, each level requiring regulatory sign-off from the Nuclear Regulatory Authority of the Slovak Republic (ÚJD SR) before the next increment, per the utility’s announcement.

The 144-hour continuous run at 100% output is the proof-of-performance that closes the commissioning sequence. Any automatic shutdown during that six-day run requires returning to a lower hold point and ascending again. Only after a clean 144-hour run can the 471 MW unit be declared commercially operational.

What the VVER-440 V-213 Is — and What It Is Not

The reactor at the heart of this milestone is a Soviet-designed pressurized water reactor, but calling it simply “Soviet” obscures the engineering history. The VVER-440’s V-213 model was the first Soviet design to incorporate emergency core cooling systems and a dedicated accident localization structure — a bubble condenser tower that suppresses steam pressure during a loss-of-coolant scenario without releasing it externally, as described in VVER design documentation. It is mechanically different from the older V-230 units that the European Union required Slovakia to shut down at Bohunice as a condition of EU accession: those lacked adequate accident mitigation systems. The V-213 units at Mochovce passed EU safety assessments. The Mochovce 3 and 4 units are further designated V-213+ to reflect safety enhancements made during the multi-decade completion project.

The thermal efficiency of the VVER-440 is approximately 29.7% — meaning that for every 1,375 megawatts of thermal energy produced by fission in the reactor core, around 440 megawatts emerges as electricity, per VVER technical specifications. The remainder is released as waste heat through cooling towers. Slovenské elektrárne has outlined a plan to capture part of that waste heat from Mochovce for district heating in the nearby town of Tlmače, with implementation scheduled for 2027–2028, according to the World Nuclear Association.

The fuel inside Mochovce 4 — all 42 tonnes of it, loaded across 349 assemblies between June 29 and July 3, 2026 — is manufactured by Russian supplier TVEL, as reported by Balkan Green Energy News. Each assembly holds 126 fuel rods containing ceramic uranium dioxide pellets enriched to approximately 3.5% uranium-235. The assemblies are designed to remain in the core for approximately five years. This means Mochovce 4 will operate on Russian fuel for at least its first one to two fuel cycles. European alternatives are coming: in April 2026, Framatome signed a long-term cooperation agreement with Slovenské elektrárne, ČEZ (Czech Republic), Fortum (Finland), and MVM Paks NPP (Hungary) to develop the VERA-440 — a fully European VVER-440 fuel design manufactured in Framatome’s French and German fabrication facilities. The company aims to produce its first own-design lead fuel assemblies starting in 2028, with regular supply expected in the early 2030s, per the Framatome press release.

Forty Years in One Milestone

The physical start-up announcement marks the culmination of a construction history that spans the fall of communism, the dissolution of Czechoslovakia, two decades of dormancy, a decade of contested construction revival, and a cost that grew from an original estimate of approximately €2.8 billion to a final tally of €6.7 billion, as World Nuclear News confirms.

Construction of units 3 and 4 at Mochovce started in 1986, three years after site work began for units 1 and 2. Work stopped in 1992 as the post-communist political and economic transition reshaped Czechoslovakia and then newly independent Slovakia. The concrete shells of units 3 and 4 sat dormant for nearly 17 years. A formal completion project launched around 2009. Unit 3 finally achieved first criticality in October 2022 and entered commercial operation in October 2023 — a roughly 12-month gap that illustrates what lies between “first reaction” and “commercial power” in practice. Unit 4 is following approximately three years later.

Slovenské elektrárne CEO Branislav Strýček described the moment in terms of its distance from completion, not its distance from origin: “Mochovce has once again come close to its goal. In recent days, we have successfully completed two key operations — the loading of nuclear fuel and the assembly of the reactor. Each of these milestones brings us closer to the moment when Slovakia will fully utilise the potential of the completed Mochovce and the new unit will begin generating electricity for households and businesses for decades to come,” Strýček said in the World Nuclear News announcement.

How to Interpret the 77.5% Figure

When Mochovce 4 reaches full commercial operation, the combined output of Slovakia’s six nuclear reactors — four at Mochovce and two at Jaslovské Bohunice — will be equivalent to 77.5% of Slovakia’s electricity consumption, per World Nuclear News. Each individual Mochovce unit provides approximately 13% of national consumption at full capacity.

That figure is expressed as a production-to-consumption ratio, not a generation mix share in the conventional sense. Slovakia already produces more electricity than it consumes with five operational reactors; with all six running, it will be a significant net exporter. The fourth reactor alone is projected to increase annual production to approximately 37 terawatt-hours against a national consumption of roughly 28 terawatt-hours, reinforcing Slovakia’s position as an electricity exporter to neighboring EU markets, according to Balkan Green Energy News.

France, widely understood as the world’s most nuclear-dependent major economy, derived around 67% of its electricity from nuclear in 2024, per IAEA data. Slovakia’s projected 77.5% would represent the highest share for any nation, ahead of France and Ukraine — the only other countries exceeding 50%, based on 2025 tracking data. Units 3 and 4 together, with a combined design life of roughly 60 years, are projected to supply approximately 26% of Slovakia’s total electricity consumption over their operating lives, as Nuclear Engineering International reports.

Slovakia Is Not Done Building

The hermetic sealing of Mochovce 4 is the final chapter of one story and the opening paragraph of another. Slovakia’s government has already moved to expand its nuclear fleet beyond the Mochovce completion.

In January 2026, Slovakia and the United States signed an intergovernmental agreement for a new large Westinghouse unit — over 1,000 MW — at Bohunice, per the World Nuclear Association. A January 2026 feasibility study confirmed four Slovak sites — Bohunice, Mochovce, Vojany, and the U.S. Steel Košice industrial site — as suitable for small modular reactor deployment, with Slovenské elektrárne indicating SMRs could be operational in Slovakia as early as 2035, according to the World Nuclear Association’s Slovakia country profile.

The broader European picture is undergoing its own acceleration. The European Commission adopted a Small Modular Reactor Strategy in March 2026, backed by an investment guarantee to accelerate deployment. Nuclear plant construction is currently under way in only two EU member states: Slovakia with Mochovce 4, and Hungary, where first concrete for the Paks II expansion was poured in February 2026. The rarity of active nuclear construction in the EU — against a backdrop of rising electricity demand from data centers and electrification — gives both projects outsized strategic significance.

The majority shareholder in Slovenské elektrárne is Slovak Power Holding BV, part of the Czech energy group Energetický a průmyslový holding (EPH). The Slovak state retains a 34% stake, as confirmed in the World Nuclear News announcement.

What Comes Next, and When

Physical start-up tests have no fixed end date. They proceed step by step, with ÚJD SR review required at each stage. Based on the Mochovce 3 precedent — first criticality in October 2022, commercial operation in October 2023 — the path from “tests now underway” to “commercially operational” will most likely extend through the first half of 2027.

The company’s stated target of delivering first electricity to the grid by the end of summer 2026 is plausible for the turbogenerator synchronization milestone — when the reactor is generating electricity at low power — but that milestone alone does not establish the 77.5% nuclear share. Full commercial operation, with the 144-hour continuous-run endurance test as the final gate, is the milestone that closes the book on the Mochovce 4 project.

The sequence ahead: strength and leak tests on the primary circuit; pre-critical neutron measurements; first controlled fission chain reaction; zero-power physics verification; stepwise power ascension with regulatory sign-off at each level; and finally a six-day continuous full-power run. When that run completes without interruption, a reactor that was begun under a communist Czechoslovak government in 1986, frozen by revolution in 1992, and restarted in 2009 will at last be generating electricity for the small Central European country that spent four decades completing it.

Frequently Asked QuestionsWhat is the difference between “first electricity to grid” and full commercial operation at Mochovce 4?

First electricity to the grid means the turbogenerator has been synchronized and the reactor is producing some electricity — but at low power, early in the power ascension sequence. Commercial operation means the reactor has completed the full power ascension program, including all hold-point tests, and has run continuously at 100% output for 144 hours. The two milestones can be separated by many months. At Mochovce 3, first criticality occurred in October 2022 and commercial operation followed in October 2023. Slovenské elektrárne is targeting “end of summer 2026” for first electricity; commercial operation will most likely follow in 2027.

How is the VVER-440 reactor used at Mochovce different from Western nuclear plant designs?

The VVER-440/V-213 is a Soviet-designed pressurized water reactor with several distinctive engineering features. It uses six primary coolant loops instead of the four used in most Western PWRs, with horizontal rather than vertical steam generators. Its containment system uses a “bubble condenser tower” that suppresses steam pressure in a loss-of-coolant accident — different from the full concrete containment buildings used in Western designs such as the AP1000. The V-213 model added emergency core cooling and accident localization systems that the older V-230 model lacked; EU accession required Slovakia to close its V-230 units at Bohunice. The V-213+ designation at Mochovce 3 and 4 reflects further safety enhancements made during the extended completion project.

Will Slovakia’s Mochovce 4 reactor run on Russian fuel?

Yes, for its initial fuel cycles. The 349 fuel assemblies loaded in late June and early July 2026 are manufactured by Russia’s TVEL corporation. Each assembly is designed to remain in the core for approximately five years. Framatome signed an agreement in April 2026 with Slovenské elektrárne and three other Central European utilities to develop the VERA-440 — a fully European VVER-440 fuel design — with first own-design lead assemblies targeted for 2028 and regular supply expected in the early 2030s. Until then, Mochovce 4 will rely on Russian-origin fuel.

Why does Slovakia have the world’s highest nuclear electricity share?

Slovakia’s nuclear dominance is the result of energy policy decisions made during the Soviet era, when the country built four VVER reactors at Mochovce and additional units at Bohunice, plus its small population and modest electricity consumption (~28 terawatt-hours per year). With five reactors already operating, Slovakia generates more electricity from nuclear than any other country as a share of consumption. Adding Mochovce 4’s 471 MW output pushes the total equivalent to 77.5% of national consumption — ahead of France (roughly 67% in 2024) and every other nation. Slovakia will export the surplus to neighboring EU markets.