Mochovce Unit 4 reaches 35% reactor power

Slovenské elektrárne has increased reactor power at Mochovce Unit 4 to 35%, advancing the Slovak nuclear unit through the next defined stage of its startup programme. The reactor reached that level at 02:35 on 12 November, with operators continuing a series of measurements and functional tests before any further power increase. The next planned plateau is 55%, followed by 75%, 90%, and ultimately nominal reactor power.

The milestone follows the unit’s first synchronisation with Slovakia’s electricity system on 15 October 2026. At that point the reactor was operating at approximately 20% power and had produced sufficient steam to bring one of its two turbine generators into service. Synchronisation requires the generator’s electrical frequency, voltage, and phase relationship to match the grid before the connection is closed and electricity begins flowing into the system.

Since that initial connection, Unit 4 has supplied around 16.2GWh of electricity while undergoing commissioning. The operator reported 16,184.77MWh of cumulative output by 8 November, a quantity it compared with the annual electricity consumption of approximately 6,500 households. This is an energy figure, rather than an instantaneous power rating, and includes production during the staged startup process. The 35% milestone refers to reactor operating power.

The reactor’s next power increases will alter thermal conditions throughout the primary and secondary circuits. Steam production, turbine loading, control response, and electrical output have to be assessed as the unit moves between operating levels. Slovenské elektrárne is therefore conducting measurements and functional tests at defined plateaux rather than moving directly from initial synchronisation to full reactor power. Results from the current 35% stage must be assessed before operators advance to 55%.

Martin Mráz, project director for the completion of the Mochovce units at Slovenské elektrárne, said each level has a defined set of tests to verify the plant’s behaviour under its actual operating conditions. Progression to higher power takes place only after those tests have been completed and evaluated. This makes the latest achievement an intermediate commissioning result, not evidence that all systems have passed their final acceptance criteria.

The unit is a VVER-440 pressurised water reactor and forms part of the Mochovce 3 and 4 expansion. Each of the two newer units has an installed electrical capacity of 471MW, giving a combined 942MW. The company expects Unit 4, once fully operational, to produce approximately 3.5 million MWh annually, a figure it says is equivalent to about 13% of Slovakia’s present electricity consumption. That annual output remains a forecast for operation after the commissioning programme is complete.

As Unit 4 began producing electricity, Mochovce reached a separate operating milestone: all four units at the site were connected to the national power system for the first time, bringing eight turbine generators into the wider generating fleet. Connection does not mean that all units are at maximum output simultaneously. Each generator operates within the conditions permitted by the relevant unit and the needs of the electricity network.

During these tests, engineers check the response of the reactor and its supporting systems to changing temperatures, pressures, steam flow, and load conditions. Turbine regulation and generator protection must remain compatible with the permitted operating envelope, while the reactor’s safety and control systems are evaluated under the commissioning programme. Increasing electricity sent to the grid is one outcome of a higher power stage, but it does not replace the separate tests needed to demonstrate stable and compliant operation.

Slovenské elektrárne has described the subsequent stages as 55%, 75%, 90%, and 100% power. After the unit reaches nominal output, a 144-hour demonstration run is planned. That final six-day test requires operation at the specified design parameters and forms part of the evidence supporting completion of the startup process. The company has not announced that the test has begun or that commercial operation has been authorised.

Mochovce Unit 3 underwent its own staged power startup in 2023, providing a recent operational precedent at the same site. Unit 4 nevertheless has to demonstrate the performance of its own equipment and systems. Commissioning results are specific to the installed unit, and the successful operating history of a sister reactor does not replace the measurements and regulatory approvals required for the new one.

Once fully commissioned, the 471MW unit would add a substantial source of dispatchable electricity to Slovakia’s generating fleet. Slovenské elektrárne expects annual production of around 3.5TWh after completion, although realised output will depend on availability, planned outages, and operating conditions. That forecast should be kept separate from the 16.2GWh already generated during startup, when the reactor is still working through successive test stages and has not reached full power.

The next permitted increase is to 55% reactor power once measurements at 35% have been completed and evaluated. Further checks at 75%, 90%, and 100% are planned before the full-power 144-hour trial. Unit 4’s position on that commissioning schedule, rather than its cumulative generation to date, will establish when the reactor is ready for regular commercial service.