First criticality achieved for Mochovce 4

Operator Slovenské elektrárne said it achieved the landmarks at 22:10 and 22:31, respectively, on Thursday 6 August. Criticality is the point at which a nuclear reactor sustains a controlled, self-supporting chain reaction.

Mochovce project director Martin Mráz said: "Achieving criticality and minimum controlled output is the result of the work of thousands of experts who participated in the preparation, construction, control and start-up of the fourth unit. It confirms that the power plant is ready to continue into the next phase of testing before the start of electricity production."

There will be a series of tests and trials now carried out before the power level is slowly increased in steps as part of the commissioning process.

These tests are designed to ensure everything is working and behaving as it should at each power level before it is increased. The culmination of the process is a 144-hour demonstration run at full power.

Background

Construction of the first two VVER-440 units at the four-unit Mochovce plant started in 1982. Work began on units 3 and 4 in 1986, but stalled in 1992. The first two reactors were completed and came into operation in 1998 and 1999, respectively, with a project to complete units 3 and 4 beginning ten years later at an estimated cost of EUR6.7 billion (USD7.6 billion).

Mochovce 3 entered commercial operation in October 2023. Each of the units can provide 13% of Slovakia's electricity needs when operating at full capacity and when the 471 MW-capacity unit 4 is operating, nuclear will be providing the equivalent of 77.5% of Slovakia’s electricity consumption, the highest proportion for any country.

Slovenské elektrárne's majority shareholder is Slovak Power Holding BV, which is owned by the Czech energy group Energetický a průmyslový. The second shareholder is the Slovak Republic, which has a 34% stake.

Slovakia currently has five nuclear reactors generating about half its electricity. As well as the Mochovce units, there are two at Bohunice, which went into commercial operation in 1984 and 1985, respectively. The Slovak government has plans for a new large unit at Bohunice, and has also been exploring the potential for small modular reactors in the country.