Mochovce Unit 4 in Slovakia achieves first criticality, reaching minimum controlled power level

At 10:10 p.m. on August 6, Slovakia’s Mochovce Unit 4 reactor achieved criticality—the point at which a nuclear chain reaction becomes self-sustaining. Twenty-one minutes later, operator Slovenské Elektrárne confirmed the reactor had reached its minimum controlled power level, clearing a key milestone in the unit’s commissioning process.

Mochovce Unit 4 reaches criticality milestone

On August 6, Slovenské Elektrárne announced something the plant had been working toward for years. At exactly 10:10 p.m. local time, Mochovce Unit 4 hit criticality—the moment when the nuclear chain reaction inside the reactor sustains itself without needing an external neutron source to keep it going.

That doesn’t mean electricity was flowing to anyone’s home yet. What it means is the physics checked out. The reactor core can hold a controlled fission reaction on its own, and that’s the foundational requirement for everything that follows in commissioning.

Slovenské Elektrárne operates all four units here, plus the reactors at Bohunice—a larger nuclear portfolio than the company has managed at any point in its history.

Just 21 minutes after criticality, operators confirmed the reactor had reached its minimum controlled power level—a separate confirmation showing the team can actually manage and stabilize the reaction under controlled conditions, not just trigger it.

Why achieving criticality matters in reactor commissioning

Commissioning a reactor isn’t one big moment. It’s a long, structured sequence of tests, each one building on the last, and criticality is among the earliest and most significant steps because nothing else moves forward without it.

Before a reactor can send electricity to the grid, it first has to prove it can sustain a chain reaction, according to the American Nuclear Society. Reaching minimum controlled power adds another layer—it shows the reactor responds predictably to operator inputs and can hold steady at a low-power state. From there, output increases in stages, performance gets verified at each level, and the regulator has to sign off before anything advances.

That process exists specifically to catch problems before they affect safety or reliability. Slovakia’s nuclear safety authority monitors every stage. The path from minimum controlled power to full commercial operation can take months, depending on test results and the regulator’s schedule.

Unit 4 completes Slovakia’s Mochovce expansion

Unit 4 is the fourth and final reactor at Mochovce, completing an expansion that’s been years in the making. The site now has a full complement of four units, wrapping up what was a long and genuinely complex construction effort.

Mochovce Unit 3—the sister reactor—went through its own commissioning before entering commercial operation, already a significant moment for Slovakia’s nuclear program. Unit 4 is now following the same path. These two newer units represent the expansion phase of the site, added alongside the two original reactors already running there.

Finishing Unit 4 means the full vision for Mochovce has been realized. Slovenské Elektrárne operates all four units here, plus the reactors at Bohunice—a larger nuclear portfolio than the company has managed at any point in its history.

Nuclear power’s role in Slovakia’s energy mix

Nuclear isn’t a marginal technology for Slovakia. Look at how much of the country’s electricity comes from it, and it’s clear this is central to how the grid functions.

Mochovce and Bohunice together form the core of that capacity, with decades of operational history behind them. The infrastructure, workforce, and regulatory frameworks are mature. Adding Unit 4 increases total low-carbon generation—nuclear produces electricity without direct carbon dioxide emissions during operation, which makes it relevant to any serious conversation about reducing a country’s carbon footprint while keeping supply reliable.

Across Europe, the tone around nuclear has shifted noticeably. Several countries that were moving toward phase-outs have reconsidered, citing energy security concerns and the difficulty of hitting decarbonization targets with renewables alone. Slovakia’s continued investment puts it firmly among the European nations actively expanding or maintaining nuclear capacity rather than retreating from it.

A required step in the commissioning sequence

Mochovce Unit 4 achieved criticality on August 6 at 10:10 p.m. local time, with minimum controlled power confirmed 21 minutes later. Slovenské Elektrárne announced both milestones, which together mark a required step in the commissioning sequence before the reactor can generate electricity for the grid.

Unit 4 is the last reactor in Mochovce’s expansion, completing a project that adds to Slovakia’s existing nuclear capacity at both sites. Each phase of commissioning requires regulatory sign-off before the next begins, and full commercial operation remains some months away.

For Slovakia, this adds low-carbon generation to a grid that already leans heavily on nuclear—at a moment when European interest in the technology as part of long-term energy strategy keeps growing.

Kelly Lippke

Kelly is an experienced writer with 15 years of experience exploring the big stories that shape our world, from tech breakthroughs and space exploration to climate, energy, and the fascinating quirks of science. She has a talent for turning complex ideas into sharp, memorable insights that stay with readers long after they’ve finished reading.