Mochovce 4 starts generating, taking Slovak nuclear towards 77.5%
Serguey Asael Shinder2 min readSlovakia's Mochovce 4 began generating power at 15:33 on Monday, a step operator Slovenské elektrárne describes as the last major stage before the unit enters operation. World Nuclear News reported it on 22 September 2026.
The unit reached criticality and minimum controlled power at the start of August. The Slovak Republic's Nuclear Regulatory Authority carried out inspections and assessed documentation before clearing this stage. The switch from physical to energy start-up means reactor heat now raises steam to turn the turbines.
Power will be increased in stages — 5%, 20%, 35%, 55%, 75% and 90% — with checks and tests at each level. The unit joins Slovakia's electricity system at 20%, and the final test is a 144-hour run at 100%.
The history is long. Construction of the first two VVER-440 units at Mochovce began in 1982; work on units 3 and 4 started in 1986 and stalled in 1992. Units 1 and 2 were completed in 1998 and 1999, and the project to finish 3 and 4 resumed ten years later at an estimated EUR 6.7 billion (USD 7.6 billion). Mochovce 3 entered commercial operation in October 2023.
Each unit can supply 13% of Slovakia's electricity at full capacity. With the 471 MW unit 4 running, nuclear will provide the equivalent of 77.5% of Slovak electricity consumption, which World Nuclear News describes as the highest proportion of any country. Slovakia currently operates five reactors producing about half its electricity, and the government has plans for a new large unit at Bohunice alongside exploratory work on small modular reactors.
What it means
The staged power ascension is the part worth understanding, because it is where a first-of-its- kind completion either proves itself or does not. Each plateau is a test campaign, not a formality, and the 144-hour full-power run at the end is the demonstration that the plant can hold output without intervention. Announcements at this stage describe a schedule; commercial operation is what closes it.
The 77.5% figure needs its qualifier kept attached. It is the equivalent of national consumption at full capacity — a ratio of potential output to demand, not a statement that the grid will run 77.5% nuclear in practice, which depends on dispatch, exports and outages.
Read the dates as the real story: 1986 to 2026 for unit 4, with a fourteen-year stall in the middle. That, rather than the capacity, is the number that recurs in every discussion of what new nuclear costs.