How does it work?

Paks Nuclear Power Plant operates four pressurised water reactors consisting of two closed water loops, a primary and a secondary circuit. A specific feature of pressurised water reactors is that the primary water circulates under high pressure (123 bars) preventing boiling of the coolant having the temperature of 300C° and circulating in the water loop.

Primary circuit

The primary circuit is a complex of systems comprising a nuclear reactor, circulating loops, heat-exchanging tubes of steam generators and a pressurizer. Its main task is generation of nuclear-based steam, maintenance of specified pressure and temperature conditions, and prevention of coolant leakage into the environment

A nuclear reactor is a system used for the maintenance of a controlled chain reaction with the use of large quantities of fissile material. For the purposes of chain reaction fast neutrons have to be slowed down with the help of a moderator. In the case of pressurised water reactors, the role of moderator is played by ordinary water. A greater part of the energy released in the process of nuclear fission is then carried by fissile nuclei in the form of kinetic energy; upon colliding with other atoms of the nuclear fuel they lose their energy, which appears in the form of heat and is removed from the reactor with the help of the coolant. The quantity of neutrons (and, correspondingly, neutron power) is controlled by means of absorber, control and scram rods.

Paks Nuclear Power Plant operates four power units with VVER-440/213 type nuclear reactors belonging to the group of pressurised water reactors (PWR). As we can see from the type designation, their originally designed nominal electric power was 440 MW; however, by now – through a number of developments – this value has been increased to 500 MW. As of today, the overall electric capacity of the nuclear power plant is 2000 MW, reactor thermal power accounts for 1485 MW.

The part of the reactor where the chain reaction takes place is called the reactor core. The reactor core is made up of 312 nuclear fuel assemblies, 37 control assemblies (absorber rods) and cooling water functioning as moderator.

Reactor vessel Steam generator Refuelling machine
Cooling pond Radiation shield Supplementary feed watersytem
Reactor Localization tower Bubbler trays
10 Deaerator 11 Aerator 12 Turbine
13 Condenser 14 Turbine hall 15 Degasser feed water tank
16 Feed water pre-heater 17 Turbine Hall Overhead 18 Instrumentation and Control room

If there is an incident, control rods are automatically lowered into the reactor core and serve to stop the chain reaction, i.e. shut down the reactor, within 12-13 seconds.

Secondary circuit

The secondary circuit is a complex of systems comprising feed water equipment of steam generators, a main steam system, a turbine and its low-pressure components, a condenser and feed water systems. Its main task is to convert the energy of circulating steam into rotary motion, which ensures the propulsion of turbines and generator.

1485 MW of thermal power generated by the nuclear reactor is delivered to steam generators with the help of water circulating in the closed system where the primary water generates steam through heat transfer tubes in another closed water loop under the pressure of 46 bars and temperature of 260C°. The quantity of generated steam is 240 tonnes per hour and provides motion for two items of large-size equipment, turbines at 300 rpm. This rotary motion, through mechanical linkages, produces electricity in generators with the voltage of 15,750 volts based on the magnetic induction principle. The electricity is transferred to the national grid with the help of connecting equipment and transformers, with the voltage of 120 and 400kV. The main equipment also comprises a number of auxiliary systems and components that fulfil safety functions, improve the plant’s efficiency and ensure continuous cleaning of the water loops.

Having performed its function, the dry steam again turns into water using the cooling effect of the Danube. This water loop is of an open type and the water taken from the Danube in the volume of 100 cubic meters per hour returns to the river heated by 8C° on average.

There are eight turbines in total operating at the nuclear power plant, meaning that the energy generated in one reactor drives two turbines through steam generators.

Having performed its function the steam goes to the condenser where the water from the Danube circulates in almost 11,000 tubes. The steam in these tubes is condensed at the temperature of about 25C°. Each low-pressure turbine unit is supplemented with two condenser modules with the pressure maintained at the level of 0,035 bar (vacuum).

The liquid working medium is returned to the steam generator by feed pumps through different cleaning and pre-heating equipment. Pre-heating is required in order to ensure better efficiency of the power plant – this task is performed by the steam coming from the turbine; the water with the temperature of 25C° coming from the condenser is heated up to 224-225C° in nine heat exchangers. The feed water enters the steam generator heated up to this temperature and can again receive the heat of the primary water.

Types of nuclear power plants:

Various types of nuclear power plants are used in different parts of the world to generate electricity.

Today the most frequently used reactor types are as follows:

  • Light water reactors: here the role of both moderator and coolant is played by light water (H2O). This type includes pressurised water reactors (PWR: Pressurized Water Reactor) and boiling water reactors (BWR: Boiling Water Reactor).
  • Heavy water reactors (e.g. CANDU): heavy water (D2O) is both moderator and coolant.
  • Graphite-moderated reactors: these include gas-cooled reactors (GCR: Gas Cooled Reactor) and light water-cooled reactors (RBMK).
  • Exotic reactors (fast breeders and other experimental installations).
  • New generation reactors of: reactors of the future.


Safety is the key word in the nuclear industry and involves the quality of operation and maintenance, personnel qualification and commitment, the technical condition of equipment and seismic stability. 

Most of the equipment applied by the nuclear power plant is involved with generating electricity, so a question arises with regard to the possibility of power loss. Each reactor unit is equipped with three diesel generators, which automatically come into operation in the case of serious incidents and ensure electricity supply to the main consumers.

Routine electricity generation is supported by professional safety evaluations providing a comprehensive picture of the activities carried out by the company with the purpose of safety enhancement and maintenance. Safety of operation and maintenance, personnel performance, and operation and availability of safety systems are subject to continuous monitoring and inspections. Areas for improvement are determined based on the comparison of our data with international data and recommendations.

The purpose of international peer reviews is to provide an intrinsic picture of all its activities to the nuclear power plant being reviewed. The International Atomic Energy Agency carries out its activity as the professional organization of the UN. The IAEA issues recommendations, organises supporting programmes, carries out oversight of nuclear fuel sales and use, performs legal activities and provides financial support for certain programmes.

The purpose of the peer review programme implemented by the World Association of Nuclear Operators (WANO) is to provide the possibility for the member power plants to compare their activities with the best international practices – in the course of information exchange and peer reviews undertaken by the team of external experts from different nuclear power plants of the world.

The measurement results and related data do not support fears and a negative attitude vis-à-vis nuclear power plants. The population living in the vicinity of Paks Nuclear Power Plant is annually effected at a maximum by an extra dose corresponding to the natural radiation dose per two hours. This extra dose is so insignificant (compared to natural dose exposure during 8760 hours annually) that it cannot lead to any health problems.

More info:

1. Nuclear reactor

2. Structure of Paks NPP

7.1 Life of uranium 1.

7.2 Life of uranium 2.

8. Safety

9. Waste managemant

10. Nuclear thesaurus

Radiation Health Effects



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