A nuclear fission reaction requires oxygen

Nuclear reactor

The “CROCUS” nuclear reactor developed by EPFL in Switzerland for research purposes.
Stylized nuclear reactor on a postage stamp of the Deutsche Bundespost (1964)

A Nuclear reactor (also Nuclear reactor or Nuclear pile, out of date Nuclear burner) is a facility in which a nuclear fission reaction takes place continuously on a macroscopic, technical scale.

Are common worldwide Power reactors, that is to say, nuclear reactor facilities that are caused by the fission fission) first generate heat from uranium or plutonium and mostly electrical energy from it (see nuclear power plant). Serve against it Research reactors for the production of free neutrons, e.g. for the purposes of materials research, or for the production of certain radioactive nuclides, e.g. for medical purposes. In ancient times, natural nuclear reactors were formed repeatedly.

A nuclear power plant often has several reactors. This often leads to imprecise statements. For example, the statement “in Germany 17 nuclear power plants were running before the nuclear phase-out” means that 17 nuclear reactors were running at significantly fewer locations.

Most nuclear reactors are fixed installations. There are also nuclear reactors in submarines and other ships. For example

  • the US has some nuclear-powered aircraft carriers and France has one,
  • six nuclear powers have nuclear-powered submarines, for example nuclear-powered hunting submarines,
  • There were ten nuclear icebreakers and four nuclear-powered freighters in 2011.

In the nuclear euphoria of the late 1950s and early 1960s, the idea of ​​nuclear-powered road vehicles, airplanes or spaceships arose.[1]


The nuclear fission

Very strong attractive forces act between the protons and the neutrons of an atomic nucleus, but these have only a very limited range. Therefore, this nuclear force essentially acts on the nearest neighbors - nucleons further away only contribute to the attractive force to a small extent. As long as the nuclear force is greater than the repulsive Coulomb force between the positively charged protons, the nucleus holds together. Small atomic nuclei are stable if they contain one neutron per proton: 40Ca is the largest stable isotope with the same number of protons and neutrons. With an increasing number of protons, an ever higher surplus of neutrons is required for stability, because the attractive nuclear force of the additional neutrons compensates for the repulsive Coulomb force of the protons. The heaviest stable core is the lead isotope 208Pb with 82 protons and 126 neutrons.

Even heavier nuclei, such as uranium or plutonium, are radioactive, i.e. unstable. Such nuclei do not become stable even with additional neutrons: catches one of these heavy nuclei, such as the uranium isotope235U or the isotope of plutonium 239Pu, a neutron one, so he gains binding energy. This transforms it into a highly excited, unstable state of the nucleus 236U respectively 240Pooh around. Such highly excited, heavy nuclei dissipate through nuclear fission with extremely short half-lives. As a result of the neutron absorption, the nucleus starts to vibrate like an impacted drop of water and tears into two fragments (with a mass ratio of about 2 to 3) and about two to three fast Neutrons. These new neutrons are available for further nuclear fission; that is the basis of the nuclear chain reaction.

Energy release in nuclear fission

The newly formed nuclei of medium mass, the so-called fission products, have a greater binding energy per nucleon than the original heavy nucleus. The difference between the binding energies is converted, among other things, into kinetic energy of the fission products (calculation). These give off the energy as heat through collisions with the surrounding material. The heat is dissipated by a coolant and can be used, for example, for heating, as process heat, for example for seawater desalination or to generate electricity.

About 6% of the total energy released in a nuclear reactor is released in the form of electron antineutrinos, which escape practically unhindered from the reactor's fissure zone and penetrate all of the surrounding material. The neutrinos have no noticeable effects because they hardly react with matter.

Taken together, the approximately 440 nuclear reactors of the 210 nuclear power plants currently in operation in 30 countries worldwide have the capacity to provide approximately 370 gigawatts of electrical power, from which 15% of the total electrical energy is generated worldwide (as of 2009).[2]

Thermal neutrons and the moderator

A fuel rod and uranium oxide pellets, the fuel in most power reactors

The gap cross-section of the isotope, for example 235U increases with decreasing energy and therefore with decreasing speed of the neutron. The slower the neutron, the more likely it is that it will be absorbed by a uranium-235 core and that it will then split. Therefore, in most reactors, the fast neutrons from nuclear fission are slowed down by means of a moderator. This is a material such as graphite, heavy or normal water that contains light atomic nuclei (smaller mass number) and has a very low absorption cross-section for neutrons. The first property means that the neutrons are slowed down as much as possible by collisions with these atomic nuclei. The second property has the consequence that the neutrons are not already absorbed in the moderator and are therefore still available for the chain reaction. The neutrons can be slowed down to the speeds of the moderator's nuclei; their average speed is given by the temperature of the moderator according to the theory of Brownian motion. So there is thermalization. One speaks therefore not of decelerated, but of thermal neutrons, because the neutrons then have a similar thermal energy distribution as that of the molecules of the moderator. A reactor that uses thermal neutrons for nuclear fission is accordingly referred to as a "thermal reactor". In contrast, a “fast” reactor uses the fast neutrons that have not been slowed down for fission (hence the name “fast breeder”).

Initiation and control of the chain reaction

When switched off, i.e. with the control rods retracted, the reactor is subcritical. Some free neutrons are always present in the reactor, released for example by the spontaneous fission of atomic nuclei of the nuclear fuel. If one of these neutrons now triggers a fission chain reaction, it quickly goes out again. To "start up" the reactor, neutron-absorbing material (the control rods) is drawn more or less far out of the reactor core while constantly measuring the neutron flux, until slight supercriticality is reached due to delayed neutrons, i.e. a self-sustaining chain reaction with a gradually increasing reaction rate. Neutron flux and heat output of the reactor are proportional to the reaction rate and therefore increase with it. By means of the control rods, the neutron flux is regulated to the required flux or power level in the critical state and kept constant; the multiplication factork is then equal to 1.0. Any changes to k Rising temperature or other influences are compensated for by adjusting the control rods. In practically all reactors, this is done by an automatic control that reacts to the measured neutron flux.

The multiplication factor 1.0 means that, on average, just one of the neutrons released per nuclear fission triggers another nuclear fission. All other neutrons are either absorbed - partly unavoidable in the structural material (steel etc.) and in non-fissile fuel components, partly in the absorber material of the control rods, mostly boron or cadmium - or escape from the reactor to the outside (“leakage”).

To reduce the power and to switch off the reactor, the control rods are moved in, which makes it subcritical again, the multiplication factor sinks to values ​​below 1.0; the rate of reaction decreases and the chain reaction ends.

A delayed supercritical reactor increases its output comparatively "slowly", over a period of several seconds. If the active control fails in the case of water-moderated reactors, i.e. the criticality is not regulated back to 1, the output increases beyond the nominal value. The moderator heats up and as a result expands or vaporizes. However, since moderating water is necessary to keep the chain reaction going, the reactor will turn back - if that is the case just the water evaporates, but the spatial arrangement of the fuel is still preserved - back into the subcritical area. This behavior is called inherently stable.

This behavior does not apply, for example, to graphite-moderated reactor types, since graphite retains its moderating properties with increasing temperature. If such a reactor falls into the delayed supercritical range due to failure of the control systems, the chain reaction does not come to a standstill and this leads to overheating and destruction of the reactor. This behavior is known as Notinherently stable or unstable.

In contrast to the delayed supercritical reactor is a prompt Supercritical reactor no longer controllable and serious accidents can occur. The neutron flux and thus the thermal output of the reactor increases exponentially with a doubling time in the range of 10−4 Seconds on. The power achieved can exceed the nominal power for a few milliseconds by over 1000 times until it is slowed down again by the Doppler broadening in the fuel heated in this way. As a result of this power excursion, the fuel rods are suddenly heated to temperatures above 1000 ° C. Depending on the design and the exact circumstances of the accident, this can lead to severe damage to the reactor, especially due to suddenly evaporating (cooling) water. Examples of rapidly supercritical light water reactors and the consequences are shown by the BORAX experiments or the accident in the US research reactor SL-1. The biggest accident to date, caused by a reactor that was at least partially supercritical, was the Chernobyl catastrophe, in which immediately after the power excursion, suddenly evaporating liquids, metals and the subsequent graphite fire led to a widespread distribution of the radioactive inventory.

The automatic interruption of the chain reaction in water-moderated reactors is, contrary to what is sometimes claimed, no Guarantee that there will be no core meltdown, as the decay heat is sufficient to cause this to happen if active cooling systems fail. For this reason, the cooling systems are designed to be redundant and diverse. A core meltdown has been taken into account as a design basis accident since the accidents in Three Mile Island in the planning of nuclear power plants and is in principle controllable. Due to the changed geometric arrangement, however, renewed criticality cannot be ruled out in principle.

Sub-critical reactors

A chain reaction with a constant reaction rate can also be achieved in a subcritical reactor by feeding in free neutrons from an independent neutron source. Such a system is sometimes called a driven Designated reactor. If the neutron source is based on a particle accelerator, i.e. can be switched off at any time, the principle offers improved security against reactivity accidents. The decay heat (see below) occurs here just like in the critically operating reactor; Precautions to control loss of cooling accidents are just as necessary here as with the usual reactors. Driven reactors have occasionally been built and operated for experimental purposes and are also designed as large-scale plants for the transmutation of reactor waste (see Accelerator Driven System).

Decay heat

If a reactor is shut down, the radioactive decay of the fission products continues to produce heat. The performance of this so-called Decay heat initially corresponds to about 5–10% of the thermal output of the reactor in normal operation and largely subsides over a period of a few days. The term "Residual heat", Which is misleading, because it is not about the remaining current heat of the reactor core, but about additional energy that is released by ongoing decay reactions.

In order to be able to safely dissipate the decay heat in emergencies (if the main cooling system fails), all nuclear power plants have a complex Emergency and post cooling system. However, if these systems should also fail, the rising temperatures can lead to a core meltdown in which structural parts of the reactor core and, under certain circumstances, parts of the nuclear fuel melt.


When fuel rods melt down, creating an agglomeration of fuel, the multiplication factor increases and rapid, uncontrolled heating can occur. In order to prevent or at least delay this process, the materials processed in the reactor core in some reactors are selected in such a way that their neutron absorption capacity increases with increasing temperature, i.e. the reactivity decreases. The case of the meltdown is called worst case (GAU), i.e. as the most serious accident that must be taken into account when designing the system and which it must withstand without damage to the environment. Such an accident happened, for example, at the Three Mile Island nuclear power plant.

The worst case, for example, that the reactor building cannot withstand and a larger amount of radioactive substances that far exceeds the permissible limit values ​​escapes, is referred to as a worst-case scenario. This happened, for example, in 1986 with the Chernobyl disaster and in 2011 with the Fukushima disaster.

With the current state of the art, only certain high-temperature reactors with a lower power density, in which a core meltdown is fundamentally impossible, are therefore considered to be inherently safe. The power density is given in MW / m³, i.e. in megawatts of thermal power per cubic meter of reactor core. This information allows a statement to be made about the technical precautions that must be taken in order to dissipate the decay heat generated in the event of malfunctions or rapid shutdowns.

Typical power densities are: for gas-cooled high-temperature reactors 6 MW / m³, for boiling water reactors 50 MW / m³ and for pressurized water reactors 100 MW / m³.

The European pressurized water reactor (EPR) has a specially shaped ceramic basin, the Core catcher. In this the melted material of the reactor core is to be caught and cooled by a special cooling system.

Reactor types

The first experimental reactors were simple layers of fissile material. An example of this is the Chicago Pile reactor, where the first controlled nuclear fission took place. Modern reactors are divided according to the type of cooling, moderation, fuel used and construction.

Light water reactor

Reactions moderated with normal light water take place in the light water reactor LWR, which is called Boiling water reactor or Pressurized water reactor can be designed. A further development of the pre-convoy, convoy and the N4 is the European pressurized water reactor (EPR). A Russian pressurized water reactor is that VVER. Light water reactors require enriched uranium, plutonium or mixed oxides (MOX) as fuel. The Oklo natural reactor was also a light water reactor. The fuel elements of the LWR are sensitive to thermodynamic and mechanical loads. In order to avoid this, sophisticated, technical and operational protective measures are required, which shape the design of the nuclear power plant in its entirety. The same applies to the reactor pressure vessel with its risk of bursting. The remaining risks of the core meltdown of the fuel assemblies and the bursting of the reactor pressure vessel were declared to be irrelevant because of their improbability of occurrence, for example by Heinrich Mandel [3]

Heavy water reactor

Moderated with heavy water Heavy water reactors require a large amount of the expensive heavy water, but can run on natural, unenriched uranium. The best-known representative of this type is the CANDU reactor developed in Canada.

Graphite reactor

Gas-cooled, graphite-moderated reactors were developed as early as the 1950s, initially primarily for military purposes (plutonium production). They are the oldest commercially used nuclear reactors; the coolant in this case is carbon dioxide. A number of these systems are still in operation in Great Britain (2011).[4] This type of reactor is called because of the fuel rod cladding made from a magnesium alloy Magnox reactor. Similar systems were also operated in France, but have now all been switched off. On October 17, 1969, shortly after the reactor was put into operation, 50 kg of fuel melted in the gas-cooled graphite reactor of the French nuclear power plant Saint-Laurent A1 (450 MWel).[5] The reactor was then shut down in 1969 (today's reactors of the nuclear power plant are pressurized water reactors).

A successor to the Magnox reactors is the one developed in Great Britain Advanced gas-cooled reactor. In contrast to the Magnox reactors, it uses slightly enriched uranium dioxide instead of uranium metal as fuel. This enables higher power densities and coolant outlet temperatures and thus better thermal efficiency.

High temperature reactors HTR also use graphite as a moderator; helium gas is used as a coolant. One possible design of the high-temperature reactor is Pebble bed reactor according to Rudolf Schulten, in which the fuel is completely enclosed in graphite. This type of reactor is considered to be one of the safest, because even if the emergency and after-cooling systems fail, a core meltdown is impossible due to the high melting point of the graphite. A number of practical problems have prevented the concept from being commercially implemented. Added to this was. that at that time the plant costs of the HTR were higher than those of the light water reactor. In Germany, research was carried out at the test nuclear power plant AVR (Jülich) and the prototype power plant THTR-300 was built in Hamm-Uentrop, the latter with a reactor pressure vessel made of prestressed concrete, in order to prevent the reactor pressure vessel from bursting.

The Soviet type reactors RBMK also use graphite as a moderator, but light water as a coolant. Here the graphite is in blocks, through which hundreds to thousands (depending on the performance of the reactor) are drilled channels in which there are pressure tubes with the fuel elements and the water cooling. This type of reactor is sluggish (it takes a lot of time to regulate) and more unsafe than other types, since a loss of coolant does not mean a loss of moderator (i.e. it does not reduce reactivity) and since the amount of combustible graphite is very large. The wrecked reactor in Chernobyl was of this type.


There are also breeder reactors (Fast breeders), in which in addition to the release of energy 238U so in 239Pu is converted so that more new fissile material is created than is consumed at the same time. This technology is more demanding (also in terms of safety) than that of the other types. Their advantage is that with it the uranium reserves of the earth can be used many times better than if only that 235U is "burned". Breeder reactors work with fast neutrons and use liquid metal such as sodium as a coolant.

Smaller non-breeding reactors with liquid metal cooling (lead-bismuth alloy) were used in Soviet submarines.

Molten salt reactor

In a molten salt reactor (English MSR for molten salt reactor or LFTR for Liquid Fluoride Thorium Reactor) a molten salt containing the nuclear fuel (for example thorium and uranium) is circulated in a circuit. The melt is at the same time fuel, coolant and, in the case of thorium, also breeding material.

Various safety arguments have been put forward in favor of molten salt reactors. However, despite some positive results, development was abandoned around 1975, mainly due to corrosion problems. It was not until the 2000s that the concept was taken up again, among other things. also in the Generation IVConcepts.

Special types

There are also some special types for special applications. For example, small reactors with highly enriched fuel were designed to power spacecraft that do not require liquid coolant. These reactors are not to be confused with the isotope batteries. Air-cooled reactors, which always require highly enriched fuel, were also built, for example for physical experiments in the BREN Tower in Nevada. Reactors were constructed for the propulsion of space vehicles, in which liquid hydrogen is used to cool the fuel. However, this work did not go beyond soil tests (NERVA project, Timberwind project). Reactors in which the fuel is in gaseous form (gas nuclear reactor) also did not get beyond the experimental stage. The molten salt reactor uses a molten uranium salt, usually uranium hexafluoride (UF6) or uranium tetrafluoride (UF4), as fuel and heat transfer medium and graphite as moderator. These reactors were developed in the USA in the 1960s to power aircraft.

We are currently working actively on new reactor concepts around the world, the Generation IV-Concepts, especially with a view to the expected growing energy demand. According to the presentation of the U.S. Department of Energy will be used from 2030.

Another type of reactor that is currently still in the experimental stage is the rotating shaft reactor. If the implementation is successful, this concept promises a much more efficient use of the nuclear fuel and a massive reduction in the problem of radioactive waste, since a traveling wave reactor could be operated with radioactive waste and would systematically use it up in the process.

  • Reactor of the Vienna Atomic Institute (research reactor)

  • Nuclear reactor from inside (active, illuminated)

  • Nuclear reactor from inside (without lighting)

  • View into a nuclear reactor. The bluish Cherenkov radiation is clearly visible

Natural nuclear reactor

A nuclear fission chain reaction can not only be achieved through complex technical systems, but also occurred in nature under certain - albeit rare - circumstances. In 1972, French researchers discovered the remains of the natural Oklo nuclear reactor in the Oklo region of the West African country of Gabon, which was created by natural processes about two billion years ago, in the Proterozoic. In total, evidence of previous fission reactions has been found at 17 sites in Oklo and a neighboring uranium deposit to date.

A prerequisite for the occurrence of the naturally occurring fission chain reactions was the much higher natural proportion of fissile material in ancient times 235U in uranium, at that time it was around 3%. Due to the shorter half-life of 235U opposite 238U is the natural content of 235U in uranium is currently only about 0.7%. With this low content of fissile material, new critical fission chain reactions can no longer occur naturally on earth.

The starting point for the discovery of the Oklo reactor was the observation that the uranium ore from the Oklo mine had a slightly lower content of the isotope uranium-235 than expected. The scientists then used a mass spectrometer to determine the amounts of various noble gas isotopes that were enclosed in a material sample from the Oklo mine. From the distribution of the different xenon isotopes formed during uranium cleavage in the sample, it was found that the reaction took place in pulses. The original uranium content of the rock and the moderating effect of the water present in the cracks in the uranium rock led to criticality. The heat released in this way in the uranium rock heated the water in the crevices until it finally evaporated and escaped like a geyser. As a result, the water could no longer act as a moderator, so that the nuclear reaction came to a standstill (resting phase). The temperature then dropped again so that fresh water could seep in and fill the crevices again. This created the conditions for renewed criticality and the cycle could start over. Calculations show that the active phase (power generation), which lasted around 30 minutes, was followed by a rest phase that lasted more than two hours. In this way, natural fission was kept going for about 500,000 years, consuming over 5 tons of uranium-235. The output of the reactor was a low 100 kilowatts compared to today's megawatt reactors.

The Oklo natural reactor is also important for assessing the safety of final disposal for radionuclides (“nuclear waste”). The very low migration of fission products and the breeding plutonium observed there over billions of years allows the conclusion that nuclear repositories can exist that are sufficiently safe over long periods of time.


Most nuclear reactors are used to generate electrical (rarely: only thermal) energy in nuclear power plants. In addition, nuclear reactors are also used to generate radionuclides, for example for use in radioisotope generators or in nuclear medicine. The desired nuclides are either extracted from the spent fuel or specifically generated by exposing stable isotopes of the same elements to the neutron radiation prevailing in the nuclear reactor (see nuclear reaction, neutron deposition). Theoretically, gold could also be produced in a reactor (gold synthesis), which, however, would be very uneconomical.

In addition to the generation of fission products, the most important reaction carried out in the reactor for the conversion of substances is the generation (called incubation) of plutonium-239 from uranium-238, the most common uranium isotope. Furthermore, nuclear reactors serve as an intensely controllable neutron source for physical investigations of all kinds. Another application of nuclear reactors is the propulsion of vehicles (nuclear energy propulsion) and the energy supply of some spacecraft. Such reactors should not be confused with isotope batteries.

Security and Politics

After years of euphoria since the 1970s, the potential danger posed by nuclear reactors and the hitherto unresolved question of the storage of the radioactive waste generated have led to protests by opponents of nuclear power and to a reassessment of nuclear energy in many countries. While the phase-out of nuclear energy was propagated in Germany in the 1990s in particular, an attempt took place between 2000 and 2010 against the background of fading memories of the risks (the Chernobyl catastrophe was now more than 20 years ago) To make nuclear power socially acceptable again. The reason for this is the reduction in CO required by international treaties2-Emissions from burning fossil fuels. This is offset by a growing energy demand in emerging economies such as China.

For these reasons, some European countries decided to invest in new nuclear power plants. For example, the German Siemens group and the French Areva group are building a pressurized water reactor of the type EPR in Olkiluoto, Finland, which is due to go online in 2013. Russia wants to renew its old and partially ailing nuclear power plants and start building a new reactor every year for at least ten years. Negotiations are also under way in France to build a new reactor. Sweden stopped its nuclear phase-out plans. There are also smaller and larger new construction projects in Iran, the People's Republic of China, India, North Korea, Turkey and other countries. (Main article: Nuclear power by country)

The nuclear accidents in the Japanese power plants Fukushima-Daiichi and Tokai in the wake of the magnitude 9 earthquake on March 11, 2011 triggered new considerations almost everywhere.

The lifespan of nuclear reactors is not unlimited. The reactor pressure vessel in particular is exposed to constant neutron radiation, which leads to the embrittlement of the material. How quickly this happens depends, among other things, on how the fuel assemblies are arranged in the reactor and what distance they are from the reactor pressure vessel. The Stade and Obrigheim nuclear power plants were also the first to be taken off the grid because this distance was smaller than with other, newer nuclear reactors. At the moment, the operators of nuclear power plants are trying to reduce the neutron load on the reactor pressure vessel through a clever loading of fuel elements and additional moderator rods. The Helmholtz Center Dresden-Rossendorf, among others, is researching this problem.[6]

See also


  • Dieter Smidt: Reactor technology. 2 volumes, Karlsruhe 1976, ISBN 3-7650-2018-4
  • Dieter Emendörfer, Karl-Heinz Höcker: Nuclear reactor theory. Mannheim / Vienna / Zurich 1982, ISBN 3-411-01599-3
  • A. P. Meshik et al .: Record of Cycling Operation of the Natural Nuclear Reactor in the Oklo / Okelobondo Area in Gabon. Phys. Rev. Lett. 93, 182302 (2004)

Web links

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Individual evidence

  1. ↑ Atomic euphoria in the 1950s; for more see nuclear energy by country # history
  2. Gerstner, E .: Nuclear energy: The hybrid returns. In: Nature. 460, 2009, p. 25. doi: 10.1038 / 460025a
  3. ↑ Mandel, Heinrich: Location issues with nuclear power plants, atw atomwirtschaft 1/1971, pages 22 - 26
  4. ^ Announcement from February 10, 2011
  5. Accidents: 1960's. In: Nuclear Age Peace Foundation. March 14, 2011, accessed March 14, 2011. As Nuclear Power in Switzerland. In: World Nuclear Association. March 14, 2011, accessed March 14, 2011.
  6. ^ Press release from the FZD from August 9, 2010