Slovenské elektrárne begins cold tests on new Mochovce 4 nuclear unit

The new unit at Slovakia’s Mochovce nuclear power plant has reached an important commissioning milestone. Technicians have begun cold tests, which will include the first comprehensive testing of the unit’s systems. Slovakia resumed work on the mothballed construction of Mochovce 3 and 4 in 2009. The sister unit, Mochovce 3, with a capacity of just under 500 MW, has been generating electricity commercially since autumn 2023.

At the end of last year, Slovenské elektrárne announced the start of cold tests at the Mochovce 4 nuclear unit, which is nearing completion. The VVER-440 unit has a capacity of 471 MW (gross) and 440 MW (net) and, once completed, will cover around 13% of the country’s electricity consumption.

"Following the successful completion of inactive functional tests, we are entering the second phase of inactive tests in the plant commissioning process. These are known as staged inactive test programmes. The first is the cold hydrostatic test of the primary circuit," said Peter Andraško, deputy project director for the completion of Mochovce 4.

Cold tests involve comprehensive testing of the unit’s components and systems under operating conditions. During hydrostatic tests, the plant’s cooling circuit is filled with water and pressurised to operating levels. Technicians then check the integrity of welds, joints and other components in the cooling and high-pressure systems.

"Next will come a minor inspection, a hot hydrostatic test and a major inspection, which will bring the inactive tests to an end. Once the Slovak Nuclear Regulatory Authority approves our application to load fuel, we will load nuclear fuel into the reactor and begin active tests," explained Martin Mráz, director of completion and system start-up for Mochovce 4.

Completion of units started last century

Czechoslovakia began construction of Mochovce 3 and 4 in Slovakia in 1987. After around six years, however, work was halted for more than 15 years. Slovenské elektrárne decided to complete both units in 2007.

Mochovce 3 has been in commercial operation  since October 2023 and is helping Slovakia meet its electricity demand. In 2023, the country became a net exporter of electricity, with generation exceeding consumption by around 3.5 TWh. Completion of Mochovce 4 will replace capacity taken offline at coal-fired power plants, meet rising demand and help Slovakia maintain its self-sufficiency in electricity generation.

In 2024, nuclear power accounted for more than 60% of the electricity generation mix, and this share is expected to rise further. In addition to the new Mochovce unit, the country is planning to build a large unit at Jaslovské Bohunice with a capacity of around 1200 MW, for which it wants to launch a tender later this year.