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Pressurised water reactors

Like boiling water reactors, pressurised water reactors (PWR) belong to light-water reactors.

Schematic diagram of a pressurized water reactor (PWR) - German only Pressurized water reactorSchematic diagram of a pressurized water reactor (PWR) - German only Source: Deutsches Atomforum e. V.

The reactor pressure vessel in pressurised water reactors is under a pressure of about 160 bar. This high pressure prevents the water in the reactor coolant circuit (also termed primary circuit) from boiling despite a temperature of about 320 degrees Celsius (°C).

Steam generators produce the steam required for power generation in another circuit – the secondary circuit – which is then led to the steam turbine.

Primary circuit - Main coolant circuit of the PWR

Reactor coolant pumps pump the water of the primary circuit into the reactor pressure vessel, where it flows through the reactor core from bottom to top. The heated water leaves the reactor pressure vessel and flows in a circuit through the steam generator tubes back to the reactor coolant pumps.

Secondary circuit of the PWR

Info: Light-water reactors

The difference between the various reactor types is in the coolant used (water, gas or liquid metal) and the moderator used (a substance that slows down fast neutrons, thus enabling and maintaining the chain reaction – thermal fission). Water or carbon in the form of graphite can be used as moderators.

Light-water reactors

Today, light-water reactors are used in Germany, which are the most common types of reactors used world-wide. Among light-water reactors are pressurised water reactors and boiling water reactors. In light-water reactors, normal water (light water) is used as coolant. At the same time the water serves as moderator.

One molecule of water (H2O) consists of two hydrogen atoms and one oxygen atom. If both hydrogen atoms (H) do have only one proton (positively charged module) in the core but no neutron (uncharged module of the nucleus), the combination with oxygen is termed "light water".

In the case of "heavy water", on the other hand, both hydrogen atoms in the core have one proton and one neutron each. These hydrogen atoms are also termed deuterium – an isotope of hydrogen.

The number of protons and neutrons in the core determine the mass number of a nucleus. The hydrogen atoms of heavy water show a larger mass (u≈2) than the hydrogen atoms in light water (u≈1).

The water in the secondary circuit takes up the heat of the primary circuit via the steam generators and is thus heated to about 280°C. As there is low pressure (about 60 bar) in the secondary circuit, the water boils. The generating steam of the secondary circuit drives the steam turbine which is connected with a generator.

Third circuit of the PWR

The steam of the secondary circuit gives off its energy to the turbine and, in a condenser, condenses again to water which is fed back to the steam generators. The released heat in the condenser is discharged via a third circuit, the cooling water system, to the river or the cooling tower.

Radioactive materials only in the primary circuit

The reactor pressure vessel and all other components of the primary circuit are located in the containment. The separation of reactor coolant and secondary circuit in the PWR by means of steam generators prevents radioactive materials from being released from the primary circuit.

The power house containing the secondary circuit, the turbine and the generator does not contain radioactive materials. In the case of an accident, safety devices effect an immediate closure of the containment building.

Control of nuclear fission in the PWR

The number of nuclear fissions can be limited by neutron-absorbing material. The control rods containing neutron-absorbing material are loaded electrically (normal drive) into the reactor core from above and regulate the reactor via the penetration depth. In case of a reactor trip, the control rods drop to the reactor core due to gravity and terminate the chain reaction.

Apart from the control rods, boric acid is added to the primary circuit to regulate the reactivity  in the reactor core of a pressurised water reactor. Boron absorbs neutrons, i.e. the reactor can be regulated by changing the boric acid concentration.

Futher Information

State of 2018.02.09

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