In principle, a nuclear power plant works with a nuclear section (nuclear fission) and a conventional section (energy generation). A total of three cycles take place here: the primary and secondary cycles and the cooling cycle.

The nuclear section contains the water-filled reactor pressure vessel with the fuel rods, in which thermal energy is generated by nuclear fission. Control elements in the form of control rods made of boron and cadmium regulate the flow of fission neutrons in such a way that sufficient reactive neutrons are available for a controlled chain reaction to split the uranium-235 nuclei.

The nuclear fission chain reaction generates heat in the reactor, which must be dissipated. This is done by the primary circuit, which cools the fission process in the reactor pressure vessel and can therefore be used as a heat source for steam generation. A steam generator is located next to the pressure vessel. This uses the heat from the primary circuit to generate steam, which runs through a standard cycle in the secondary circuit with a steam turbine.

In the secondary, non-nuclear cycle, the steam produced flows through a turbine that drives a generator to produce electricity. The generator converts the kinetic energy of the turbine into electrical energy. The electricity produced is then fed into the power network via a transformer.

The tertiary circuit is the cooling circuit of the steam system. It absorbs the remaining thermal energy of the steam leaving the turbine in the condenser and discharges it. The steam condenses completely into water and can be fed back into the steam generator for re-evaporation. The excess energy is discharged into a cooling tower.

Strict separation of the primary circuit from the other circuits is essential to avoid that radioactively contaminated water from the primary circuit escapes into the environment.

Typical field of application of self-acting control valves in nuclear power plants:

• Pressure reducer DM 652 - compressed air supply
• Bleeding and venting valve EB 1.12 - cooling circuit
• Pressure reducer DM 510 - cooling circuit
• Pressure reducer DM 505 - nitrogen supply

Different types of reactors are used in nuclear power plants. These are primarily differentiated by the nuclear fuels, cooling circuits and moderators used.

The most important are:

The light water reactor (LWR) uses normal "light" water (H2O) as reactor coolant and moderator. Pure uranium oxide or a uranium-plutonium mixed oxide is used as the nuclear fuel. Light water reactors exist in the pressurized water reactor (PWR) and boiling water reactor (BWR) variants. In PWRs, the heat from the fission zone is dissipated by water under high pressure (approx. 160 bar), resulting in a high water temperature in the primary circuit. In the BWR, part of the cooling water boils inside the reactor and drives the turbines directly. 
 
The heavy water reactor (HWR) uses "heavy" water (D2O) as reactor coolant and moderator. This water has good braking properties and low neutron absorption. As a result, natural uranium with a mass fraction of 235U of approx. 0.7 % can be used as fuel. The RBMK is a boiling water reactor with pressurized tubes that uses two moderators: water and graphite. Therefore, uranium with the natural isotope distribution can be used for operation. The plant allows the fuel elements to be replaced during operation. It is a Soviet reactor type that has not been rebuilt since the Chernobyl disaster.

The breeder reactor (fast breeder reactor) produces fissile plutonium from natural uranium during operation and thus enables higher fuel utilization. Liquid sodium is used as a coolant instead of water, as fast neutrons are required for this type of reactor. This causes major safety problems, as sodium reacts strongly exothermically with water and can ignite.

The high-temperature reactor (HTR) works at a comparatively high temperature (up to 950 °C). Here, the fuel is contained in tennis ball-sized graphite, which serves as a moderator. Helium is used for cooling. A core meltdown is considered impossible with this model, but this type of reactor did not catch on.

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