Basic definitions

Primary circuit

It includes a system of facilities that enables the acquisition of heat energy from nuclear fuel through controlled chain fission reaction, its continuous removal by means of coolant, and its conversion to a form of heat energy usable in the steam turbine.

Basic facilities of this circuit include:
  • Reactor
  • Steam generators
  • Main circulating pumps
  • Circulation piping of the primary circuit
  • Volume compensator 

As already mentioned above, it is a resource of heat for heating the pressurised water released during the controlled chain fission reaction in the nuclear fuel. Heat released from the reactor core is removed by the forced circulation of coolant that is carried out by the main circulation pumps.

Nuclear reactor

The nuclear reactor is a technical facility (containing nuclear fuel, coolant, moderator, structural materials and control systems) that is intended for maintaining the controlled chain fission reaction and enabling a smooth removal of heat energy released during the fission process. It consists of a steel pressure vessel equipped with a removable cover. Located inside the reactor is a core that contains the nuclear fuel and regulation equipment for control and monitoring the fission reaction.

Circulating (main circulating) pump

The main circulating pump provides circulation of coolant in the primary circuit in a quantity corresponding to the heat capacity of the reactor. In the viewpoint of the design, it is a vertical-shaft centrifugal stuffing-box pump driven by an asynchronous electric motor.

Volume compensator

Despite the fact that the volumetric-thermal expansivity coefficient of water is relatively small, the growth of volume due to temperature influence as the volume of the primary circuit coolant reaches a few hundreds of cubic metres has to be taken into consideration. In case that the growth of coolant volume is not compensated in some way, the volume growth of water causes such a high mechanical stress of the primary circuit facilities that it would result in the disruption and release of radioactive coolant into the primary circuit area. The volume compensator is a vertical steel pressure vessel with a size comparable to the reactor pressure vessel, connected by piping to a hot branch of one of the primary circuit loops. Apart from compensation of volumetric-thermal changes of the coolant, the volume compensator also serves for regulation of the primary coolant pressure by means of the built-in electric heaters or showers. The volume compensator is equipped with safety valves against exceeding the permissible pressure value in the primary circuit.

Steam generator

The horizontal pressure evaporator heat exchanger is where the primary circuit water (flowing in pressure pipes in the steam generator) transfers its heat to the secondary circuit water. As the temperature of water in the primary circuit is higher than temperature of the boiling point of water in the secondary circuit (pressure of water in the primary circuit is in fact more than doubled in comparison with the pressure of water or steam in the secondary circuit), it effects a more intensive generation of steam in the generator that is taken through steam piping to the turbine.

Primary circuit piping

Stainless steel piping of 500 mm diameter and a wall thickness of 32 mm interconnects the reactor, steam generator and circulating pumps. To reduce heat loss but simultaneously to enable control of its condition, this piping is equipped with a removable heat insulation. That part of the piping, located between the reactor and the steam generator in which the heated water flows from the reactor to the steam generator, is called the hot branch, and the outstanding part of the piping that removes water from the steam generator through the circulating pump into the reactor is called the cold branch of the primary circuit.

Secondary circuit

The secondary circuit in the power plant includes a system of facilities that enable the conversion of heat energy from steam to the mechanical energy from the steam turbine rotor.

Basic facilities of this circuit include:
  • Turbine and generator
  • Condenser
  • Condensate and feeding pumps
  • Regenerative heaters
Turbine and generator

Rotary heat motor in which the internal energy of steam is converted into a rotary mechanical energy in the turbine rotor. Upon impulse, the turbines pressure drop of steam changes in the stationary blades of the stator to a kinetic energy of steam that is transferred through the moving blades to the rotor. The turbine rotor is linked with the generator rotor where kinetic energy of the rotor is transformed to electric energy.


Heat exchanger where steam condenses after expansion in the turbine and cooling by cooling water. It is placed in the immediate vicinity of the bottom part of the low-pressure section of the turbine. Steam leaving the turbine passes between pipes in which cooling water is flowing, and condenses on the surface of pipes. The condensed steam (the condensate) is transported by the condensate pumps through the condensate treatment section, regenerative exchangers and degasification section to the steam generator.

Low-pressure and high-pressure regenerative heaters

Heat exchangers where steam from non-regulated regenerative take-offs of the turbine transfers its condensation heat to the condensate or feeding water of the steam generator. In low-pressure regenerative exchangers the condensate is successively heated to boiling point so that the gases dissolved in it can be removed in the degasification tank. In high-pressure regenerative heaters the feeding water, after removal of gases in the degasification tanks, is heated to temperatures close to boiling point in the steam generator.

Condensate and feeding pumps 

Condensate pumps are intended for pumping the condensate from the turbine condensers through low-pressure regenerative heaters into the degasification tank. Feeding pumps transport the degasified feeding water from the degasification tank through high-pressure regenerative heaters to the steam generator, and simultaneously increase the pressure of the degasified feeding water to pressure of the generated steam.

Tertiary circuit

A task of the tertiary circuit is to create the highest vacuum, usable by the turbine, in the condenser to achieve the highest possible efficiency of the turbine. The lower the temperature of the cooling water in the tertiary circuit, the higher the vacuum created in the condenser.

Basic equipment of this circuit includes:
  • Cooling towers
  • Circulating pumps
  • Cooling water piping and channels

Power plants built in the vicinity of the sea or large rivers are not equipped with cooling towers as the condenser can be cooled with the through-flow water, without fears of a negative impact of the heated water to the water ecosystem.

Cooling towers

They represent the dominating feature of power plants but at the same time the cooling towers are subtle constructions made from reinforced concrete in the shape of a hyperboloid of revolution that serve for providing the sufficient draught of cooling air needed for cooling the cooling water, and for mounting built-in structures that ensure the sprinkling of cooling water aimed at the better efficiency of its cooling. A part of the cooling water is evaporated. Latent heat needed for evaporation is the main reason of temperature drop of cooling water. In the bottom section of the cooling tower is a round pool in which the cooled water is collected and transported back by means of the cooling water pumps to the turbine condenser.

Circulating pumps

Centrifugal pumps serving for circulation of cooling water between condensers of turbines and cooling towers. 

Cooling water piping and channels

Cooling water flow can be compared with a flow of water in a river. It is the piping of the largest diameter in the entire power plant.

Containment VVER440/213

One of the principle requirements for radiation safety in nuclear power plants is to prevent an uncontrolled leakage of radioactive substances into the environment. Therefore, radioactive substances are separated from the life environment by a few barriers.

The first barrier is the actual fixation of the radioactive substances in the fuel cells. The second barrier is formed by the hermetically sealed fuel rods in which the cells are sealed. The third barrier represents a tightly sealed primary circuit, and the fourth barrier is the containment.