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How do the different reactor types work?

Pressurized water reactor

In a pressurized water reactor, a pressurizer keeps the water in the reactor pressure vessel always sufficiently pressurized so that it does not boil and vaporize at the prevailing operating temperatures. The water heated in the reactor core circulates in a closed circuit (primary circuit). It flows to a steam generator where it transfers its thermal energy to a separate water-steam circuit, the secondary circuit. The pressure is lower here; the feedwater in the steam generator in the secondary circuit therefore begins to boil and evaporate.

In the secondary circuit, the energy is converted in the same way as in a fossil-fueled thermal power plant: The generated steam is fed through the live steam line into a high-pressure turbine and then into a low-pressure turbine. There, the thermal energy contained in it is converted into rotational energy. The steam emerging from the low-pressure turbine condenses on the tubes with cooling water flowing through it in the condenser. With the help of condenser and feed-water pumps, the water is fed into the steam generator once more, where it evaporates again.

The turbine drives the generator. It converts the rotational energy of the turbine shaft into electrical power. The power produced in the generator is transformed by large transformers from 27 kilovolt to 400 kilovolt and fed into the supply grid via the high-voltage switchgear.

Pressurized water reactors are the most frequent commercially used reactor type: They account for approximately two thirds of the capacity presently installed in nuclear power plants.

Boiling water reactor

In boiling water reactors, the water in the reactor pressure vessel partially evaporates due to the heat created by the fission process. Circulating pumps lead non-evaporated water back into the reactor core, thus supporting natural circulation. The generated steam is transportet directly into the turbine and goes from there into a condenser, where it cools down and forms water. This then goes back into the reactor.

Heavy water reactor

Heavy water (D2O) contains the heavy water isotope deuterium (D) instead of common hydrogen (H). Normal water (H2O) is often referred to as “light water” for this reason.

The advantage of heavy water? It is a less powerful neutron absorber than normal water. This allows natural uranium to be used as fuel: The uranium used in the reactor does not have to be enriched as a result. At the same time, the heavy water can be used as coolant, because its other physical properties are very similar to those of normal water.

Reactors employing heavy water as coolant and moderator were mainly developed and built in Canada with the so-called CANDU reactors.

Fast breeder

Contrary to light water and heavy water reactors, the neutrons released during fission are not slowed down here. Either liquid metal (often sodium) or inert gas (helium) is used as coolant. The nuclear fuel contains plutonium, an element, which is fissile just as the isotope uranium-235.

By now, fast breeders have left the prototype phase behind. They are one reactor type selected for the next generation of nuclear power plants. This reactor generation IV is set to be available by 2040.