Slovakia’s newest reactor achieves criticality

The fourth unit of Slovakia's Mochovce nuclear power plant has achieved criticality, operator Slovenské Elektrárne has announced. Unit 4 reached the milestone at 10:10 p.m. local time on August 6. Twenty-one minutes later, the reactor reached its minimum controlled power level.

Officials welcomed the news, seeing it as proof the reactor was ready for the next stages of testing. Next up for Mochovce-4 are physical tests and tests at different performance levels to determine how the power plant systems behave when operational, moving it closer to connecting to the grid. The final test of the 471-MWe pressurized water reactor will last 144 hours at full power.

"The achievement of the critical state and the minimum controlled performance is the result of the work of thousands of experts who participated in the preparation, construction, inspection and commissioning of the fourth unit. It confirms that the power plant is ready to proceed to the next phase of testing before the start of electricity production," said Martin Mráz, project director of Mochovce, in the announcement (translated from the original Slovak by Microsoft translator).

About Slovakia's reactors: When Mochovce-4 enters commercial operation, Slovakia will have six operating PWRs. The country's Nuclear Regulatory Authority issued a commissioning permit for Unit 4 on May 22 which went into effect on June 24. The first fuel assembly was loaded into the reactor on June 29.

Currently, there are already three operating Russian-designed VVERs at Mochovce, which is Slovakia's second nuclear power station. Mochovce-1 came on line in 1998 and generates 467 MWe; Mochovce-2 came on line in 2000 and generates 469 MWe. The 471-MWe Mochovce-3, meanwhile, has been producing power commercially since the fall of 2023 under a commissioning license. Unit 4 is still listed as "under construction" by Slovenské Elektrárne.

The third and fourth reactors at Mochovce have been in the works since the 1980s, according to Nuclear News and other publications. Work has been halted multiple times over the years. According to Slovenské Elektrárne, one reactor will cover 13 percent of Slovakia's electricity needs.

Slovakia's other nuclear power plant, Bohunice, has two operating reactors. Bohunice-3 and -4, both 466-MWe Russian-designed VVERs, came on line in 1985. Bohunice-A1, a 104-MWe gas-cooled heavy water reactor, was Slovakia's first commercial nuclear reactor when it came on line in 1972. Bohunice-1 and -2, both of which were 408-MWe PWRs, followed in 1980 and 1981. Bohunice-A1, -1, and -2 are now shut down.