New Slovak nuclear unit Mochovce 4 reaches criticality for the first time

The fourth unit of Slovakia's Mochovce nuclear power plant has moved significantly closer to entering operation. The VVER-440 reactor has reached criticality and minimum controlled power for the first time. The unit will now undergo tests at various power levels. The new installed capacity of 471 MW (gross) will place Slovakia among the countries with the highest share of nuclear power in their energy mix.

Slovak energy company Slovenské elektrárne announced that its fourth unit at the Mochovce nuclear power plant had reached criticality for the first time. Technicians began commissioning the unit on 6 August; at 22:10, the reactor reached criticality for the first time, and 21 minutes later it reached minimum controlled power. This is one of the key commissioning milestones for every new unit.

"Achieving criticality and minimum controlled power is the result of the work of thousands of experts involved in the preparation, construction, inspection and commissioning of the fourth unit. It confirms that the plant is ready to proceed to the next phase of testing before electricity generation begins," said Mochovce plant project director Martin Mráz.

The VVER-440 nuclear unit will now undergo a series of tests at various power levels. The aim of the tests is to verify the behaviour of the unit and its systems and prepare it for grid connection. The company said commissioning was proceeding exactly according to schedule. After completing a 144-hour test at full power, it plans to bring the unit into commercial operation later this year.

High share of nuclear generation

Mochovce unit 4 is the second of two new nuclear units being built at the plant site. Construction of the first two units at the four-unit plant began in 1982. Slovakia started work on units 3 and 4 in 1986, but suspended it for around 16 years in 1992. Technicians brought Mochovce unit 3 into commercial operation at the beginning of 2023.

"The new unit will also strengthen domestic electricity generation and contribute to the country's long-term energy security," Slovenské elektrárne said in a press release.

Once the new unit enters operation, the installed capacity of Slovakia's nuclear units will rise to 2.75 GW. Slovakia will also become the country with the highest share of nuclear electricity generation in domestic electricity consumption, which is expected to reach just under 80%. The country then plans to commission another large conventional nuclear unit in the 2040s.