Pressurised water reactor (PWR)
The pressurised water reactor belongs to the class of light water reactors. In Germany, the pressurised water reactor is the most widely used type of reactor: a total of seven of the nine nuclear power plants in operation are equipped with a pressurised water reactor. The newest type of pressurised water reactor currently being built is the European Pressurised Water Reactor (EPR).
How does the pressurised water reactor work?
The pressurised water reactor has two separate circuits: the primary circuit and the secondary circuit. In the primary circuit, the water flows through the so-called reactor core. There, it is heated from 291 °C to 326 °C (in the so-called reactor pressure vessel) by the energy generated by the fuel elements from nuclear fission. Since the water is under a pressure of 157 bar, it does not boil even at these high temperatures (in contrast to the boiling water reactor (BWR)).
The heated water from the reactor pressure vessel is fed into the tubes of the so-called steam generator. The steam generator is the interface between the primary and the secondary circuit. The heat is thus transferred to the water in the secondary circuit, which flows on the outside of the steam generator tubes. This prevents the contaminated water of the primary circuit from entering the secondary circuit. After heat transfer in the steam generator, the cooled water in the primary circuit is pumped back into the reactor pressure vessel. Since the pressure on the secondary side of the steam generator is only 64.5 bar, the water evaporates there at 280.5 °C. The resulting steam is passed through turbines. A downstream generator converts the energy produced there into electricity. The steam is then cooled down, condensed in a condenser, and returned to the steam generator as water.
Special features of the pressurised water reactor
In contrast to the boiling water reactor, boric acid - which absorbs neutrons - is added to the water in the primary circuit of the pressurised water reactor. By changing the amount of boric acid (boric acid concentration), the intensity of nuclear fission and thus the power of the reactor can slowly be regulated.
In addition, control rods, which also contain neutron absorbers, are used for fast power control. In the pressurised water reactor, the control rods are inserted into the reactor core from above, whereas in the boiling water reactor they are inserted from below. In the pressurised water reactor, the control rods are held electromagnetically in a position above the reactor core. In the event of a reactor shutdown, the control rods drop into the core due to the effect of gravity. The chain reaction is thus interrupted and the reactor shut down.
In contrast to the boiling water reactor, the turbine and the turbine building in the pressurised water reactor are not contaminated due to the separate water circuits. The separate circuits thus ensure easier maintenance compared to the boiling water reactor.