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Mochovce Unit 4 began its staged power start-up at 15:33 on 21 September 2026. The 471-megawatt VVER-440/V213 pressurized-water reactor is not automatically at full power: engineers are raising output in test levels, checking the reactor, turbine, generator and safety systems before a required 144-hour proof run.
Residents were warned on 2 October that testing steam-generator safety valves could produce loud bursts of noise. A claim that the unit was still drawing 17.7 megawatts from the grid about three weeks after start-up has not been verified by an operator, regulator or grid-operator record.
What happened at Mochovce 4
Mochovce 4 (MO34) is the fourth unit at Slovakia’s Mochovce nuclear power station and is operated by Slovenské elektrárne. It is a VVER-440/V213 design, a pressurized-water reactor whose installed electrical capacity is 471 MW.
The Nuclear News Network and Enerdata reported that the power-start-up phase began at 15:33 on 21 September 2026. That milestone followed the earlier physical start-up work: the first fuel assembly was loaded on 29 June, and the American Nuclear Society reported that the reactor reached criticality at 10:10 p.m. local time on 6 August. Minimum controlled power followed 21 minutes later.
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This is Unit 4, not Mochovce Unit 3. The start-up is a regulated commissioning sequence rather than a single switch from construction to commercial operation.
Commissioning milestones and what each one means
| Date or stage | Milestone | Meaning |
|---|---|---|
| 29 June 2026 | First fuel assembly loaded | Fuel loading began the physical commissioning sequence. |
| 6 August 2026 | Criticality at 22:10 local time | A controlled nuclear chain reaction was established; minimum controlled power followed 21 minutes later. |
| 21 September 2026, 15:33 | Power start-up began | Output started rising through defined test levels while plant systems were checked. |
| About 20% power | Planned turbine-generator synchronization point | Steam production should be sufficient to synchronize the generators with Slovakia’s electricity system and send the first megawatt-hours to the grid. |
| Final energy-start-up stage | 144-hour full-power proof run | The unit must demonstrate sustained operation before commissioning can be completed and later approvals considered. |
The Nuclear Regulatory Authority of the Slovak Republic says approval of later stages depends on evaluating test results and the accompanying documentation. A synchronization event therefore does not, by itself, mean that the reactor has completed commissioning.
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Why the safety-valve tests can be so loud
In its 2 October public notice, Slovenské elektrárne said engineers were testing safety valves used to discharge steam from the steam generators. Opening or actuating these valves can release high-pressure steam and create a pronounced noise that carries beyond the plant boundary.
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The operator described the work as standard during power start-up and said similar inspections occur when Slovak nuclear units return from refuelling and maintenance outages. The notice explains the expected sound; it does not announce an emergency or an unplanned release.
How the staged climb toward full power works
Physical start-up
Physical start-up covers fuel loading, achieving criticality and bringing the reactor to controlled low-power conditions. Engineers verify that instrumentation, control systems and protection functions respond as designed before increasing output.
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Energy start-up
During energy start-up, reactor power is increased through defined test levels. At each level, teams examine the reactor, steam generators, turbine, generator and safety systems, recording operating data and resolving findings before moving higher.
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Grid synchronization near 20%
Slovenské elektrárne’s commissioning explanation identifies roughly 20% reactor power as the point at which steam production should support turbine-generator synchronization with the Slovak grid. The first electricity exported at that point is an intermediate commissioning milestone, not proof that the unit is producing its rated output.
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Is Mochovce 4 already operating at full power?
The available milestones do not establish that it has reached full power. The regulator’s process still includes review of test results and documentation, followed by the final energy-start-up stage and its 144-hour full-power proof run.
At full output, the Nuclear Regulatory Authority estimates that the unit could supply approximately 13% of Slovakia’s electricity consumption. That is an estimate of its potential contribution when operating at its installed capacity, not a statement that the reactor was delivering that share during the early power-ascension tests.
What the reported 17.7 MW grid figure does—and does not—show
No primary source identified for this account verifies that Mochovce 4 was still drawing 17.7 MW from the grid three weeks after power start-up. The figure should therefore be treated as unresolved, not as a confirmed operating measurement.
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What residents should expect next
- Additional controlled increases in reactor power and associated equipment tests.
- Further noise when steam-generator safety valves are actuated during testing.
- A possible grid-synchronization milestone once the unit reaches the relevant test level.
- Regulatory review of operating data and documentation before later commissioning approvals.
- A 144-hour full-power proof run before the final energy-start-up stage is complete.
Mochovce 4’s 21 September start was the beginning of a measured commissioning climb. The loud valve testing is part of that process, while the 17.7 MW grid-import claim remains unconfirmed until a primary operating record defines and verifies it.
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