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Plant systems and operation
How does a nuclear power plant work?

The technical part of a nuclear power plant is divided into three basic thermodynamic sets of systems:

  • primary systems,
  • secondary systems, and
  • tertiary systems.

Since water circulates in all three sets of systems, which are separate from each other, we can also refer to them as loops. The first two systems are sealed while the third, which uses water from the River Sava to condense the steam, is connected to the outside environment.

Primary system

The primary system consists of the reactor, two steam generators, two reactor pumps, the pressuriser and piping.

The heat released in the reactor core heats the water circulating in the primary system. The heat of the water is transferred to the water of the secondary system via the walls of the U-tubes in the steam generators.

The circulation of the water in the primary system is facilitated by the reactor coolant pumps. The pressuriser maintains the pressure in the primary system and prevents the water in the core from boiling.

All the components of the primary system are sited in the containment building, whose function it is to isolate the primary system from the environment even in accident conditions.

Secondary system

The secondary system consists of steam generators, turbines, the generator, the condenser, feedwater pumps and piping.

The steam generators are basically boilers – heat exchangers – in which steam is formed from the water of the secondary system and led off to the turbine. In the turbine the steam energy is converted into mechanical energy.

The generator converts this energy into electrical energy and sends it to the transmission grid via transformers.

The used steam from the turbine goes to the condenser, where contact with the cooling pipes in the condenser turns it back into water.

The feedwater pumps force the water from the condenser back into the steam generator, where steam forms once again.

Tertiary system

The tertiary system consists of the condenser, the circulating water (CW) pumps, the cooling towers and piping.

The tertiary system is designed to cool the condenser and dissipate the steam heat that cannot be usefully employed to produce electricity.

The circulating water pumps force water from the Sava into the condenser and back into the river. As it passes through the condenser the river water heats up, because it absorbs heat from the used steam.

Since heating the water of the Sava results in the thermal pollution of the river, administrative regulations specify the permitted increase in temperature and the percentage of the river’s flow that can be diverted for power plant cooling. In the case of unfavourable meteorological conditions, the cooling towers are used. In exceptionally unfavourable conditions the plant power has to be reduced.