What is a nuclear reactor

How does a nuclear power plant work?

Nuclear power plants make use of the fact that when atomic nuclei are fissioned, a tiny part of the mass is converted into energy. Unlike other steam-powered power plants in which fossil fuels - such as coal, gas or oil - are burned, no chemical reactions take place in nuclear power plants. That is why there is no CO2. What these types of power plants have in common is that they use the energy generated to heat water. The hot steam can then be used to drive a turbine and generate electricity.

The heart of a nuclear power plant is the reactor: This is where the fuel elements are located, embedded in the water to be heated. Fuel elements are fuel rods grouped into square or hexagonal units, which in turn consist of an extremely solid shell and the enclosed fuel. The primary fuel used is the chemical element uranium, which occurs naturally in different variants - so-called isotopes. The most common uranium isotope is uranium-238 with 92 protons and 146 neutrons in the core. In nuclear power plants, on the other hand, the isotope uranium-235 is used, which has three fewer neutrons and is particularly easy to split. Uranium-235 is only found in natural minerals in amounts of less than one percent. In the fuel elements, the proportion is enriched to three to five percent.

In addition to uranium-235, some other heavy elements are also suitable as nuclear fuel, such as plutonium-239. In order to be considered for use in nuclear power plants, chemical elements must have several properties at the same time: First, they should be available on the world market at affordable prices. Second, they must be easily fissionable by neutrons. And third, they have to release several neutrons themselves in the event of a nuclear fission. This is the only way to maintain a chain reaction in which, on average, at least one of the released neutrons splits another atomic nucleus.

Nuclear power plant with pressurized water reactor

If a neutron hits a uranium-235 nucleus, it can decay - usually into two fragments and two to three individual neutrons. The fragments then fly apart at high speeds. In fact, the neutrons released during nuclear fission are initially so fast that they react poorly with other atomic nuclei. For this reason, the fuel rods are usually surrounded by water: when they collide with the hydrogen atoms, the neutrons lose some of their energy and are thus slowed down. The slow neutrons have a much higher probability of triggering a nuclear fission. But braking has another effect: The energy lost in the collisions is converted into heat - this heats up the surrounding cooling water.

So-called control and emergency rods can be inserted between the fuel rods of a nuclear reactor in order to be able to influence the chain reaction in a targeted manner and, if necessary, to break it off. These rods contain highly neutron-absorbing materials such as cadmium or boron. The deeper the control rods are inserted into the reactor core, the more neutrons are intercepted and cannot trigger any further fission. This reduces the reaction rate.

Reactor types

The nuclear power plants active in Germany are so-called pressurized water and boiling water reactors. The main difference between the two types of reactor is where the hot water vapor is generated, which ultimately drives the turbine.

Boiling water reactor

In boiling water reactors, the fuel assemblies are located in a pressure vessel - usually a cylindrical container made of thick special steel - about two-thirds of which is filled with water. The chain reaction heats the water to more than 280 degrees Celsius, with a pressure of around seventy bar. That corresponds to seventy times atmospheric pressure. Due to the high temperature, part of the water in the reactor pressure vessel evaporates, hence the name boiling water reactor.

The hot steam is directed to a turbine that drives a generator. Behind the turbine, the water vapor has to be cooled and liquefied in a large condenser before high-pressure pumps pump this water back into the reactor pressure vessel. In this way, the water cycle remains closed and any radioactive particles that may escape cannot get into the environment or contaminate parts of the power plant site.

In contrast to boiling water reactors, pressurized water reactors have a second water circuit that is decoupled from the reactor and drives the turbine. In addition, the pressure in the reactor pressure vessel rises, at around 150 bar, significantly higher than in the boiling water reactor. This means that the water inside does not boil and remains liquid despite the higher temperature of 325 degrees Celsius. The steam generation only takes place in the secondary circuit, which is coupled to the primary circuit via a heat exchanger. This means that all radioactive substances remain in the central primary circuit, so that the turbine and all parts connected to it do not belong to the radioactive control area.

In addition to pressurized water and boiling water reactors, there are other types of power plants. So-called research reactors are relevant for science and medicine: Instead of hot water vapor, neutrons are generated here, with which, for example, various materials can be screened or radionuclides produced for medical or scientific applications. There are currently three such research reactors in Germany, including the FRM II neutron source in Garching near Munich.