Mochovce 3&4 construction

Construction of the Mochovce Nuclear Power Plant Units 3&4 is unique not only in Slovakia, but also in the EU, where 4 new units are being built currently – and two of them at Mochovce. The electric power of each unit will be 471 MWe and will cover 13% of electricity demand in Slovakia. Compared to coal and gas power plants, Units 3 and 4 of Mochovce NPP prevent the release of at least 5.2 million tons of CO2 emissions annually. So about as many as if more than two million cars and trucks had disappeared from Slovak roads, thus significantly contributing to the EU plan to reach carbon neutrality by 2050.

The new unit will compensate for the capacity to be shut down at Novaky lignite-burning plant that will terminate electricity generation in 2022, as well as negative balance of Slovakia in electricity imports/exports. Slovakia will again become self-contained in terms of domestic electricity supplies.

Mochovce 3&4 Project Completion (03/2021)

Construction of the new units is the largest private investment in the history of Slovakia, with Slovak and renowned foreign companies working on the construction site. Mochovce 3 and 4 is one of the largest and most important projects that have created a number of jobs in Slovakia. There were nearly 7,000 workers on the site at peak. Mochovce has altogether creates 15,000 jobs – direct, indirect and induced. Occupational safety at construction site is the top priority; over 100 million manhours were worked there and the project’s occupation injury statistics are more than a half better than average in the EU’s construction industry. In May 2020, we reached 10 million manhours from the last registered occupational injury.

Mochovce 3&4 project progress: Cummulative man-hours

The construction project is in finals and the date of initial fuel loading into the reactor is approaching. In February 2020, the Slovak Nuclear Regulatory Authority published baselines for their decision based on an application by Slovenské elektrárne, a.s. in the following proceedings:

  • permits for commissioning of a nuclear facility in the scope of facilities and equipment for the Mochovce Unit 3 operation and facilities and equipment common to Units 3 and 4 used for the Unit 3 operation;
  • permits for radioactive waste and spent fuel management;
  • permits for early use of the building.

Progress of Unit 3&4 construction

Key milestones reached in 2020


  • Completion of primary circuit re-heating.
  • 3-week international review of readiness for commissioning by a team of IAEA experts Pre-OSART.
  • Rectification of pending issues from last year’s hydrotesting and inspections.
  • Replacement of pressurizer electric heaters.
  • Completion of cooling tower distribution channel reinforcement.
  • Completed cabling for Unit 3 operation.
  • Repair of welds at service water piping.
  • Completed upgrade and testing of control room simulator for personnel training.
  • Confirmation of HVAC system tests at conventional part.
  • EMC (electromagnetic compatibility) tests.
  • Verification of some small diameter piping components with spectrum analysers, as well as all material certificates.
  • Draft permit of the Nuclear Regulatory Authority for fuel loading at the Unit 3.

Completion progress as of December 2019

  • Unit 3: 99.3 %
  • Unit 4: 87.1 %

Key milestones reached in 2019


  • Hot hydro – all planned tests performed.
  • Extended revision – inspection and evaluation of primary circuit equipment successfully completed.
  • Application for preliminary use of civil structures – all local inspections performed by relevant authorities.
  • Handover of systems to future Operation team in progress.
  • Fresh nuclear fuel storage authorized by the Nuclear Regulatory Authority (ÚJD SR).
  • Turbines in turning gear ready to receive steam.
  • Boric acid preparation for fuel loading.
  • Public consulting with on-site walkdown by stakeholders.
  • Review of readiness for fuel loading by the International Atomic Energy Agency (Pre-OSART Mission).


  • More than 1000 km of cables pulled in 2019.
  • All electrical panels of Distributed Control System installed and tested.
  • Installation of fire detection and fire-fighting systems needed for energization.
  • Essential service water mechanical assembly completed.
  • 250 tons of steel structures installed for seismic reinforcement in conventional island.

Completion progress as of December 2018

  • Unit 3: 98,23 %
  • Unit 4: 86,6 %

Key milestones reached in 2018


  • NPP physical security system in operation
  • Cold Hydro tests successfully completed
  • Fresh Fuel Storage ready for fuel delivery
  • All loop checks for Cold and Hot hydro tests completed
  • Small revision (consisting mainly of disassembly, pre-service inspection and assembly of reactor, pressurizer and steamgenerators)
  • Start of Hot Hydrotest


  • Completion of 110kV/400kV substation and transformer area
  • Small bore piping completed in Nuclear Island
  • More than 1000 km of cables pulled

Completion progress as of December 2017

  • Unit 3: 96.1%
  • Unit 4: 84.5 %

Key milestones reached in 2017


  • Primary (reactor coolant) circuit pressure test
  • Hydrogen recombiner system flushing
  • Integrated thermal cycle flushing completed
  • Chemical and volume control system flushing
  • Fire water station takeover
  • Takeover of key emergency cooling and spray systems
  • Connection to 400kV power transmission system
  • Initial phase of the cold hydro testing of the primary circuit


  • Assembly of high-pressure air system
  • Reactor control assembly
  • Large bore piping erection
  • Civil readiness for distributed control system rooms
  • Completion of main cable trays in conventional island

1st quarter

  • Erection of fire detection system in the fresh fuel storage.
  • Revisions and cleanings of the main circulation pumps at Unit 3.
  • Turnover Low Pressure Air system.
  • Turnover DemiWater distribution system.

2nd quarter

  • Reactor control assembly completed.
  • Flushing of thermal cycle started.
  • Functional Tests of Medium Voltage switchboards completed.
  • Installation of the fuel loading rails at Unit 4.
  • Installation of impulse pipelines in the hydrogen recombination system.
  • Insulation of low-pressure reheaters at the Unit 3 turbine hall.
  • Flushing of pure condensate distribution system completed.
  • At Unit 4, the main circulation pumps were transported.

3rd quarter

  • Successfully passed functional tests in Unit 3: 110 kV substation for Units 2 and 4, fire protection system and HVAC unit in room No. 1305
  • Radiation monitoring system finished in several rooms of the power block.
  • Lengthwise side electrical building of Unit 3: painting of steel sections, plasters, disassembly of scaffolding, lying of cables, recovery of partitions
  • Crosswise side electrical building of Unit 3: installation of pipes and supports, installation of channels
  • Unit 3 commissioning: tests and installation works at the reactor and facilities of the concrete reactor cavity finished, testing of 19 home consumption power switchboards, put of other electronic fire protection system detectors into operation to activate the power home consumption  
  • Unit 4: installation of steel structures of HVAC
  • Completion status: Unit 3: 93.7 %, Unit 4: 79.2 %

 4th quarter

  • Finishing of the boron acid treatment plant.
  • Diesel generator station of Unit 3: installation of switchboards, measuring and excitation transformer and cable trenches
  • Unit 3 commissioning: functional tests of 6 kV switchboards, functional tests of secondary and section switchboards
  • Turbine hall of Units 3&4: insulation of pipes, flushing, conformity check, installation of small diameter pipes, lying of cables, individual tests of switchboards, connection of switchboards
  • Refurbishment of the cooling tower pool.
  • Completion status: Unit 3: 94.1 %; Unit 4: 80.7 %
  • Insulation of tanks for the Standby water system.
  • Erection and welding of stainless steel lining of the Unit 3 spent fuel pool.
  • Installation of 36.000 new anchoring plates for Unit 3 completed.
  • Demineralized water system was installed for turbogenerator TG41.
  • Construction test and hydrotest of the emergency feedwater system tank Erection of anti-aircraft barriers
  • Delivery of a new TG32 turbogenerator for unit 3.
  • The control room of unit 3 was energized, and the nuclear project entered the testing and commissioning phase. Individual tests of the I&C system commenced.
  • Installation of common diesel generator.
  • Under the supervision of the Nuclear Regulatory Authority of the Slovak Republic, Slovenské elektrárne verified the special professional skills of its shift engineers, unit heads and unit operators. The verification has to be done one year before the commencement of physical commissioning of the reactor unit.In unit 3, conditions for clean assembly in the reactor hall were reached.
  • Completion of Full Scope Simulator site acceptance testing.
  • Commencement of functional tests of medium voltage switchboards at Mochovce Power Plant Unit 3.
  • Unit 3 Control Rooms (main control room, emergency control room, common control room) assembled for start of Commissioning activities.
  • Start of Mochovce 3&4 operators training at the Full Scope Simulator; following the completion of licensing process for operators training by the Nuclear Regulatory Authority of the Slovak Republic.
  • Submission of "Pre-Operational Safety Analysis Report" to the Nuclear Regulatory Authority of the Slovak Republic. The document is the key for authorization of Fuel Loading to Unit 3.
  • Unit 3 Main Control System: 90 % of internal cabling of Unit 3 Main Control System (total: 9.000 cables) completed.
  • Unit 3 Nuclear Island: Civil Readiness of over 90 % of NI rooms needed for Unit 3 Commissioning. Instrumentation room (1 out of 3) with impulse lines, penetrations impulse lines and racks completed. The room is ready for the installation of the instruments. Radioactive waste treatment piping installed.
  • Reactor hall civil works completed which is the first step towards Strict Clean Conditions required for Reactor internals works.
  • Unit 3 Conventional Island: First phase of 6kV busbars commissioning completed. Mechanical completion of the main systems (non-essential and essential service water, main steam, feed-water, condensate, high and low pressure air systems). Up to 100 km of main cable trays erected (100 %).
  • As of the end of 2014, the Unit 3 was 81% and the Unit 4 was 63% completed.
  • Civil readiness of Unit 3 Nuclear island critical rooms.
  • Completed the Detailed Design of Unit 3 Anchoring Plates.
  • Start of mechanical erection activities in the Nuclear island critical rooms.
  • TG31 and TG32 Final assembly, bearing closing, turning gear installation, insulation assembly.
  • Completion of Mechanical Erection of Unit 3 Main Steam and Feedwater systems.
  • Individial tests of Unit 3 LV/MV Cabinets.
  • Unit 3 – Distributed control system power up.
  • Full Scope Simulator
    • Application Software Installed and Tested;
    • Site Acceptance Tests.
  • Refurbishment of hermetic cable penetrations at the Units 3&4.
  • Fire protection lining and painting in auxiliary nuclear building
  • Civil readiness for foundations under pipe bridge
  • Concrete floor drainage in room A217
  • Transport and assembly of 7th section of spent fuel pool in Unit 4
  • Installation of instrumentation and control system cabinets
  • Welding of stainless-steel lining floors on Unit 4 in A201/2, A301/2
  • Grouting activities - works under the tripods of primary circuit of Unit 4
  • Assembly of penetrations on the shaft no. 1 of Unit 4
  • Concreting of floors and vertical elements in the vent stack
  • Modernization of main circulation pump 3, 6
  • Refurbishment of main circulation piping
  • Erection of diagnostic system on steam generator 6
  • Welding of support ring under core basket
  • Erection of removable part of main circulation pump 2-6
  • Three stand supports straining of main circulation pumps 1-6
  • Transport of frames and pumps of sprinkler and low pressure emergency injection system
  • Erection of bellows
  • Blowing through the steam generator 1
  • Commissioning of 110 kV back-up electric source
  • Concreting of Unit 3 reactor postament
  • Installation of 400kV electric switch yard
  • Fresh nuclear fuel storage prepared for technology installation
  • Assembly of all four steam turbines of Units 3 & 4 completed
  • Safe Enterprise certificate for SE-MO34 site
  • Refurbishment of internal walls of vent stack;
  • Refurbishment of roof of nuclear auxiliary building;
  • Refurbishment works of condensate tanks on the roof of auxiliary nuclear building;
  • Seismic reinforcement of reactor building;
  • Seismic reinforcement of internal walls of auxiliary nuclear building;
  • Refurbishment of internal walls of connecting bridge;
  • Installation of earthing networks of vent stack;
  • Civil readiness of the emergency room in a simulator building;
  • Hydrotest of spent fuel pool of Unit3 (shaft No. 1);
  • Grouting of mechanical equipments (primary coolant pumps);
  • Civil readiness for refurbishment of reactor upper block;
  • Refurbishment of hermetic and non-hermetic zone in reactor building (Unit 3,4);
  • Foundations for pipe bridge; 
  • Refurbishment of external walls of auxiliary nuclear building;
  • Internal seismic reinforcement of connection bridge;
  • Erection of fire protection lining in auxiliary nuclear building;
  • Concreting works on postament of unit 3;
  • Heavy concreting on postament of Unit 4;
  • Reinforcement on postament of Unit 4 around shaft no.1;
  • Concreting works around reactor pit of Unit 3;
  • Erection and welding of main circulation piping in the steam generator box;
  • Reactor pressure vessel of unit 4 transport and placement (2-3/12/2011);
  • First turbine of Unit 3 installation (December 2011).
  • Seismic reinforcement of reactor building roof;
  • Seismic reinforcement of external structures of reactor building;
  • Seismic reinforcement of steel structures of the turbine hall;
  • Erection of new bridge crane in turbine hall;
  • Refurbishment of turbines supports;
  • Seismic reinforcement of internal steel structures of auxiliary nuclear building;
  • Refurbishment of steam generators (unit 3);
  • Refurbishment of 250t crane in the reactor hall;
  • Reinforcement of roads, corridors and slabs for the reactor pressure vessel transport;
  • Refurbishment of auxiliary structures for transport of reactor pressure vessel;
  • Refurbishment of external walls of vent stack;
  • Refurbishment of cooling towers;
  • Reactor pressure vessel of unit 3 transport and placement (3-4/9/2010);
  • Refurbishment of anchoring structures of steam generators (unit 3);
  • Refurbishment of bubble condenser (unit 3);
  • Refurbishment of external walls of reactor building;
  • Removal of old moisture separators and reheaters (unit 3);
  • Refurbishment of hermetic penetrations;
  • Refurbished rooms in reactor building, number of rooms ready for mechanical erection:
    • Unit 3: 328 out of 353;
    • Unit 4: 197 out of 239.
  • Main nuclear island contracts, EPCM contract for the conventional island and main I&C Engineering contract were signed and activities have started;
  • Contractors' mobilization works completed (site services, site offices, ...);
  • Evaluation of existing components;
  • Refurbishment of reactor hall roof;
  • Seismic reinforcement of the external walls of the reactor building.
20000BC: Inhabited by humans, stone axe found 20 000 BC
Inhabited by humans, stone axe found
1 259 AD
First written mention of a settlement called Mohi
1 259 AD: First written mention of a settlement called Mohi
1 981: Start of construction works at Mochovce – ground works 1 981
Start of construction works
at Mochovce NPP – ground works
Start of construction works on Mochovce NPP unit 1 and 2
Start of construction works on Mochovce NPP unit 3 and 4
Replacement of the instrumentation and control system
Halt of the construction works
Re-start of conrtruction works on unit 1 and 2
Commissioning of Mochovce NPP unit 1 and 2 1998
Commissioning of unit 1

Commissioning of unit 2
Enel acquired 66 % of Slovenské elektrárne
Unit 1 and 2 installed output increased to 107%
Start of the Mochovce NPP 3&4 completion project


Main suppliers

Civil works: Inžinierske stavby Košice

Mechanical systems: Škoda JS, ENSECO, VUJE

Electrical systems and equipment: PPA

Control system: Areva / Siemens

Other suppliers: ASE, Rolls-Royce, GSE

Civil works: ZIPP

Mechanical systems: Škoda Power, Modrany Power

Electrical systems and components: ČKD, PPA, Energo & Schneider, Brush SEM, Techimp