One country in Europe is about to pull more than three out of every four watts on its grid from a single power source, and it isn’t gas, coal, or wind. The Mochovce 4 power startup sent electricity from a nuclear reactor into Slovakia’s grid for the first time on September 22, and once the unit reaches full output, nuclear plants will supply an estimated 77.5% of the country’s electricity: a figure Slovenske elektrarne, the plant’s operator, says would be the highest share of any nation.
Inside the Mochovce 4 Power Startup
Workers broke ground on Mochovce 4 back in 1986. Money ran out after Slovakia split from Czechoslovakia, and the project sat half-built for more than a decade. Construction picked back up in 2008, this time with modern safety systems folded into the old design. Engineers finally loaded fuel into the reactor this past June. Now the unit is climbing through power levels in stages: first 5%, then 20%, then higher. Safety checks happen at every step, and before regulators sign off for good, the reactor has to run flawlessly for 144 straight hours at full strength.
The unit itself generates 471 megawatts using a VVER-440 design. That’s the same type as Mochovce 3, which came online in 2023. Slovenske elektrarne CEO Branislav Strycek called the project “proof that even smaller countries can achieve great things.” After a 40-year build, that’s not a small claim.
What 77.5% Nuclear Actually Looks Like
Most grids treat nuclear power as one ingredient among several. France has topped the nuclear-share rankings for decades. It gets about 67% of its electricity from reactors. Slovakia is about to clear that bar by a wide margin.
| Country | Share of electricity from nuclear power |
|---|---|
| Slovakia (after Mochovce 4) | ~77.5% |
| France | ~67% |
| United States | ~19% |
| World average | ~9% |
Leaning that heavily on one source sounds risky until you remember what it replaces. For Slovakia, that mostly means imported natural gas. That’s the same fuel that sent European electricity prices spiking more than tenfold during the 2022 energy crunch, as the World Nuclear Association’s Slovakia profile lays out in more detail.
The Bill Some Families Might Not Get
Picture Tomas, a nurse who works night shifts at a hospital in Kosice. In the winter of 2022, his family’s electricity bill jumped so fast that he picked up extra weekend shifts just to keep pace. Nuclear power won’t make energy free. But it locks in a cost that doesn’t swing wildly every time a pipeline dispute or a cold snap hits the continent.
Slovenske elektrarne expects Mochovce 4 alone to cover roughly 13% of the country’s electricity demand. Run the math on that output, and it comes out to enough steady, low-carbon power for well over a million homes a year, by my rough estimate based on the country’s average household use. For someone like Tomas, that’s one less monthly gamble to plan around.
Worth Watching Next
Slovakia is a country of 5.4 million people, not a global superpower with money to burn on mega-projects. What it had was patience. It took 40 years from groundbreaking to power startup, and regulators are still watching the safety record closely as testing continues. Poland and Turkey are both studying small reactor designs right now, partly because a small country just showed the long game can pay off. The real test comes over the next few months, when Mochovce 4 either clears that 144-hour run at full power or hits a snag nobody saw coming. Either way, it’s worth watching.