Mochovce Unit 4 completes reactor assembly and heads for start-up

Slovakia has moved closer to completing a nuclear project worth approximately EUR 6.7 billion. Under the current schedule, Mochovce Unit 4 is expected to deliver its first electricity to the grid at the end of summer 2026.

Slovenské elektrárne has completed the assembly and hermetic sealing of the reactor at Unit 4 of the Mochovce nuclear power plant. Following the loading of all 349 fuel assemblies, technicians installed and connected the plant systems, closed the reactor vessel and carried out inspections which, according to the operator, confirmed that the primary circuit is ready for the next phase of start-up.

The completion of assembly marks the end of one of the key stages of physical start-up. This will be followed by strength and leak-tightness tests of the primary circuit, pre-critical tests, and then the achievement of first criticality and subsequent stabilisation of the reactor at minimum controlled power (MCP).

Output will be increased gradually

During physical start-up, specialists will verify the neutron-physical characteristics of the reactor core and other reactor parameters. The transition to the subsequent power start-up phase will require the approval of the Slovak Nuclear Regulatory Authority, which will assess the results of the tests performed and the plant's readiness.

During power start-up, the unit's output will be increased in individual power stages, with further tests carried out at each level. Slovenské elektrárne expects the turbogenerator to be synchronised with the grid and for the unit to supply its first electricity at the end of summer 2026. The start-up process will subsequently culminate in a 144-hour demonstration run at full output. The stated timeline depends on the progress and results of the individual tests.

Start-up authorised by nuclear regulator

The Nuclear Regulatory Authority of the Slovak Republic issued authorisation to bring Unit 4 into operation on 22 May 2026. As no objections were filed against the decision, it became legally effective on 24 June. Fuel loading began on 29 June and was completed on 3 July. The regulator also noted that the transition between individual start-up stages is subject to its separate approval.

Unit 4 uses a VVER-440 pressurised water reactor and has an installed electrical capacity of 471 MW. According to the operator, it is expected to supply an amount of electricity equivalent to approximately 13 % of Slovakia's electricity consumption. Once it enters operation, Slovak nuclear power plants should cover approximately 77.5 % of domestic electricity consumption.

The project began in the last century

Mochovce's sister Unit 3 began commercial operation in October 2023. Construction of Units 3 and 4 began in 1986 but was suspended in 1992. Completion work resumed only after a lengthy hiatus in 2008. Slovenské elektrárne puts the value of the entire project at approximately EUR 6.7 billion, or roughly CZK 162 billion.

Once Unit 4 is completed, Slovakia will have six power reactors in operation. Slovakia is also preparing another major nuclear power source at the Jaslovské Bohunice site. The final capacity and other project parameters have not yet been definitively determined.