# Nuclear reactor

CROCUS teaching reactor at EPFL in Switzerland

A nuclear reactor , also called an atomic reactor or atomic pile, is a system in which a nuclear fission reaction takes place continuously as a chain reaction on a macroscopic, technical scale.

Are distributed worldwide power reactors , nuclear reactor plants, which (by splitting English fission ) of uranium or plutonium first heat and it usually electrical power (see nuclear power plant ) win. Research reactors, on the other hand, are used to generate free neutrons , for example for the purposes of materials research or to produce certain radioactive nuclides for medical or similar purposes.

In ancient times, natural nuclear reactors were formed repeatedly .

A nuclear power plant often has several reactors. The two terms are often used imprecisely. For example, the statement “in Germany there were 17 nuclear power plants running until the nuclear phase-out ” means that 17 nuclear reactors were running at significantly fewer locations. For example, the Gundremmingen nuclear power plant originally consisted of three reactor blocks; each block consists of a reactor with a steam generator and a turbo set .

Most nuclear reactors are fixed installations. In the nuclear euphoria of the late 1950s and early 1960s, the idea of ​​nuclear-powered road vehicles, airplanes or spaceships came up. There are now some nuclear reactors in submarines, surface ships and spacecraft.

## functionality

### The nuclear fission

Very strong attractive forces act between the protons and the neutrons of an atomic nucleus , but they have only a very limited range. Therefore, this nuclear force essentially acts on the closest 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: 40 Ca is the heaviest stable nuclide with the same number of protons and neutrons. With an increasing number of protons, an ever greater excess of neutrons is required for stability; the repulsive Coulomb force between the protons is compensated by the attractive nuclear force of the additional neutrons.

If a very heavy nucleus, such as the uranium isotope 235 U or the plutonium isotope 239 Pu, captures a neutron, it becomes a highly excited, unstable 236 U or 240 Pu nucleus due to the binding energy gained . Such highly excited, heavy nuclei dissipate through nuclear fission with extremely short half-lives . To put it clearly, the neutron absorption causes the nucleus to vibrate like an impacted drop of water and tears into (mostly) two fragments (with a mass ratio of about 2 to 3), which fly apart with high kinetic energy; in addition, about two to three fast neutrons are released. These neutrons are available for further nuclear fission; that is the basis of the nuclear chain reaction.

### Breeding reactions

When neutrons hit nuclear fuel, other nuclear reactions inevitably take place in addition to fission . Of particular interest are reactions in which components of the nuclear fuel that are not themselves fissile are converted into fissile ones. Such reactions are called breeding reactions, the process of breeding or conversion . One speaks of a breeder reactor , however, only when more new fissile material is produced than the reactor itself consumes in the same time, i.e. the conversion rate is over 1.0.

The fuel in almost all nuclear reactors mainly contains uranium. Therefore, the breeding reaction at the non-fissile uranium isotope 238 U is particularly important. The 238 U is transformed by neutron capture in 239 to U. This passes through two successive beta decays into the fissile plutonium isotope 239 Pu:

${\ displaystyle \ mathrm {^ {238} _ {\ 92} U \ + \ _ {0} ^ {1} n \ \ longrightarrow \ _ {\ 92} ^ {239} U \ {\ xrightarrow {\ beta ^ {-}}} \ _ {\ 93} ^ {239} Np \ {\ xrightarrow {\ beta ^ {-}}} \ _ {\ 94} ^ {239} Pu}}$

The 239 Pu is partially digested again still in the reactor, partly it can but separated by reprocessing the spent fuel and be used for other purposes.

If the separated plutonium is to be used for nuclear weapons purposes ( weapons plutonium ), it must be isotopically as pure as possible, ie it must not contain too much 240 Pu. This next heavier plutonium isotope is created when the 239 Pu atomic nucleus captures another neutron. Therefore, weapons-grade plutonium can only be obtained from fuel elements that are removed from the reactor after a relatively short operating time.

In the same way as Pu-239 from U-238, the fissile U-233 from thorium Th-232 can also be “ hatched ”.

### 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 in binding energies occurs mostly as the kinetic energy of the fissure fragments ( 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, to generate electricity, heating or as process heat, for example for seawater desalination.

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 fissure zone of the reactor and penetrate all of the surrounding material. These particles have no noticeable effects because they hardly react with matter . Your energy can therefore not be used technically. The remaining, usable energy from the fission of 1 gram of U-235 is about 0.91 MWd (megawatt days) or 21,500 kilowatt hours. This corresponds to around 9.5 tons of lignite or 1.8 tons of heating oil.

Taken together, the approximately 440 nuclear reactors of the 210 nuclear power plants currently in operation in 30 countries generate an electrical output of approximately 370 gigawatts . This is a share of 15% of the total electrical energy worldwide (status: 2009).

### Chain reaction, thermal neutrons, moderator

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

The chain reaction consists in the fact that neutrons split atomic nuclei of the nuclear fuel , whereby in addition to the high-energy fission fragments, some new neutrons are also released each time; these can split further nuclei etc. The cross-section of the nuclei for the most common fuels increases with decreasing energy, i.e. decreasing speed of the neutron: the slower the neutron is, the more likely it is that it will be absorbed by a fissile core subsequently split. Therefore, in most reactors, the fast neutrons from nuclear fission are slowed down using a moderator . This is a material like graphite , heavy or normal water , which contains light atomic nuclei (smaller mass number ) and has a very low absorption cross-section for neutrons. In this material, the neutrons are strongly slowed down by collisions with its atomic nuclei, but are only rarely absorbed. So they are 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 . Instead of decelerated, one usually speaks of thermal neutrons , because the neutrons then have a similar thermal energy distribution as the molecules of the moderator. A reactor that uses thermal neutrons for nuclear fission is called a thermal reactor . In contrast, a fast reactor uses the fast neutrons that are not slowed down for fission (hence the name fast breeder ).

### Initiation and control of the chain reaction

In the switched-off state, i.e. H. when the control rods are retracted , the reactor is sub-critical . Some free neutrons are always present in the reactor - for example released by spontaneous fission of atomic nuclei of the nuclear fuel - and sometimes trigger fission, but the growth of a chain reaction is prevented by the fact that most of the neutrons are removed from the material contained in the control rods (e .g . Boron ) are absorbed so that the multiplication factor k is less than 1.

To start up the reactor, the control rods are pulled out of the reactor core to a greater or lesser extent 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 nuclear reaction rate . The 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 - in the case of pressurized water reactors also via the concentration of boric acid in the water - the neutron flux is regulated to the required flux and thus power level in the critical state and kept constant; k is then equal to 1.0. Any changes in k due to a rise in 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 shut down the reactor, the control rods are retracted, making it subcritical again. The multiplication factor drops below 1, the reaction rate decreases, and the chain reaction ends.

A delayed supercritical reactor increases its output slowly enough that the control equipment can follow the process. If the active control fails with 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 evaporates. However, since moderating water is necessary to maintain the chain reaction, the reactor returns to the subcritical area - provided that only the water evaporates but the spatial arrangement of the fuel has been retained. This behavior is called inherently stable.

This behavior does not apply, for example, to graphite-moderated reactor types , since graphite retains its moderating properties even with increasing temperature. If such a reactor enters the delayed supercritical range due to failure of the control system, the chain reaction does not come to a standstill, and this can lead to overheating and possibly destruction of the reactor. Such a reactor is therefore not inherently stable. The Chernobyl reactors were part of this type of construction, which only exists in Russia.

In contrast to the delayed supercritical reactor, a promptly supercritical reactor can no longer be regulated 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. The power achieved can exceed the nominal power for a few milliseconds by more than a thousand times until it is reduced again by the Doppler broadening in the fuel heated in this way. The fuel rods can be quickly heated to temperatures above 1000 ° C through this power excursion. Depending on the design and the exact circumstances of the accident, this can lead to serious damage to the reactor, especially due to suddenly evaporating (cooling) water. The BORAX experiments or the accident in the US research reactor SL-1 show examples of promptly supercritical light water reactors and the consequences . The biggest accident to date, caused by a reactor that was promptly supercritical at least in some areas, was the Chernobyl nuclear disaster , in which immediately after the power excursion, liquids, metals and the subsequent graphite fire suddenly evaporated and led to a widespread distribution of the radioactive inventory.

Contrary to what is sometimes claimed, the automatic interruption of the chain reaction during a power excursion of a water-moderated reactor is no guarantee that a core meltdown will not occur, because in the event of an additional failure of all active cooling devices, the decay heat is sufficient to cause it . 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 accident in Three Mile Island when planning nuclear power plants and is in principle controllable. However, due to the possibly changed geometrical arrangement of the reactor core due to the power excursion, renewed criticality cannot fundamentally be ruled out.

#### Sub-critical working 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 referred to as a driven 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 as in the critical working reactor; Precautions to control loss of cooling accidents are just as necessary here as with conventional reactors.

Powered reactors have occasionally been built and operated for experimental purposes. They are also designed as large-scale plants for energy generation and simultaneous transmutation of reactor waste (see Accelerator Driven System ) and in this case sometimes referred to as hybrid reactors . The heavier actinides produced in reactors , the generation factor of which is too small for a critical chain reaction, could also be used as nuclear fuel in them.

### Emissions

Radioactive contaminants ( tritium , radioactive iodine, etc., etc.) that constantly arise during normal operation are discharged into the environment through an exhaust air chimney and the waste water . In this regard, it is assumed that alleged accumulations of cancer case numbers are causally related to these emissions.

### Decay heat

If a reactor is shut down, the radioactive decay of the fission products continues to produce heat. The output of this so-called decay heat initially corresponds to around 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 is often used for this , but it is misleading because it is not about the remaining current heat of the reactor core, but about additional heat production that is caused by the ongoing decomposition 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 after-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. This was the case with the core meltdown in Fukushima , as all active cooling systems came to a standstill due to a complete failure of the power supply.

### Meltdown

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. In the case of light water reactors , which supply almost 90% of the total nuclear power, a core meltdown during operation is not possible because the nuclear fission chain reaction only takes place in the presence of water. However, if there is insufficient cooling in the switched-off reactor, a core meltdown is possible due to the decay heat , even if over longer periods of time. The case of a core meltdown is considered to be the worst possible accident ( GAU ), i.e. the most serious accident that must be taken into account when planning the plant and which it must withstand without damage to the environment. One such 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 exceed 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 .

In the current state of the art, only certain high-temperature reactors with lower power and power density are considered inherently safe against core meltdowns ; In general, however, this type of reactor is not inherently safe either, since accidents such as graphite fire or water ingress could have catastrophic consequences.

The power density in MW / m³ (megawatts of thermal power per cubic meter of reactor core) determines which technical precautions have to be taken in order to dissipate the decay heat generated after an emergency shutdown. Typical power densities are 6 MW / m³ for gas-cooled high-temperature reactors, 50 MW / m³ for boiling water reactors and 100 MW / m³ for pressurized water reactors.

The European pressurized water reactor (EPR) has a specially shaped ceramic basin, the core catcher , underneath the pressure vessel for safety in the event of a core meltdown . In this the melted material of the reactor core is to be caught, but prevented from agglomeration and cooled by a special cooling system.

## Reactor types

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

### Light water reactor

Reactions moderated with normal light water take place in the light water reactor (LWR), which can be designed as a boiling water reactor (BWR) or a pressurized water reactor (PWR). Light water reactors generate almost 90% of nuclear energy worldwide (68% PWR, 20% BWR) and 100% in Germany. A further development of the Vor-Konvoi , Konvoi (the German PWR) and the N4 is the European pressurized water reactor (EPR). A Russian pressurized water reactor is the VVER . Light water reactors require enriched uranium, plutonium or mixed oxides ( MOX ) as fuel. The natural Oklo reactor was also a light water reactor .

The essential feature of the light water reactor is the negative vapor bubble coefficient : Since water is both a coolant and a moderator, no chain reaction is possible without water, i.e. no core meltdown during reactor operation.

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 as a whole. The same applies to the reactor pressure vessel with its risk of bursting . The remaining residual risks of the core meltdown of the fuel assemblies due to the decay heat and the bursting of the reactor pressure vessel were long declared irrelevant in the nuclear energy industry because of the improbability of their occurrence, for example by Heinrich Mandel .

### Heavy water reactor

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

### Graphite reactor types

Gas-cooled, graphite-moderated reactors were already developed in 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 plants are still in operation in Great Britain (2011). Because of the fuel rod cladding made from a magnesium alloy , this type of reactor is called the 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 MW el ). 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 Advanced Gas-cooled Reactor (AGR) developed in Great Britain . 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 a better thermal efficiency . With 42%, EGR achieved the highest efficiency of all previous nuclear power plants.

High temperature reactors (HTR) also use graphite as a moderator; helium gas is usedas the coolant. One possible design of the high-temperature reactor is the pebble bed reactor according to Farrington Daniels and Rudolf Schulten , in which the fuel is completely enclosed in graphite. This type of reactor has long been considered one of the safest, because if the emergency and after-cooling systems fail, a core meltdown is impossible due to the high melting point of the graphite. However, there are a number of other serious types of accidents, such as water ingress or air ingress with graphite fire, which call into question the alleged safety advantages, as Rainer Moormann , who received the2011 Whistleblower Prize, pointed out. A number of unsolved practical problems have also prevented the concept from being implemented commercially. In addition, the plant costs of the HTR are higher than those of the light water reactor. In Germany, research was carried out at the AVR experimental nuclear power plant (Jülich) and the THTR-300 prototype power plant was builtin Schmehausen , the latter with a reactor pressure vessel made of prestressed concrete . Both were shut down in 1989.

The Soviet reactors of the RBMK type also use graphite as a moderator, but light water as a coolant. Here the graphite is in blocks, through which numerous channels have been drilled, 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 because the vapor bubble coefficient is positive: unlike in light water reactors, a loss of coolant does not mean loss of moderator, but it does reduce the neutron absorption by the coolant; so it increases the reactivity instead of reducing it. The resulting increased heat output without sufficient cooling can quickly lead to a core meltdown. The wrecked reactor in Chernobyl was of this type. Nowadays, reactors of this type can only be found in Russia.

### Breeder

There are also breeder reactors ( fast breeders ) in which, in addition to the release of energy, 238 U is converted into 239 Pu, so that more new fissile material is created than is consumed at the same time. This technology is also more demanding in terms of safety than that of the other types. Its advantage is that with it the uranium reserves of the earth can be used up to 50-100 times better than if only the 235 U is "burned". Breeder reactors work with fast neutrons and use liquid metal such as sodium as a coolant.

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

### Molten salt reactor

(In a molten salt reactor English MSR for molten salt reactor or LFTR for Liquid Fluoride Thorium Reactor ) is circulated a salt melt which contains the nuclear fuel (such as thorium and uranium) in a circuit. The melt is both fuel and coolant. However, this type of reactor has not progressed beyond the experimental stage.

Various safety and sustainability arguments have been put forward in favor of molten salt reactors: The fluoride salts used are not water-soluble, which makes it difficult to contaminate the environment in the event of an accident. As breeder reactors, the molten salt reactors can use the fuel very efficiently and can be operated with a wide range of fuels. These reactors were researched in the USA in the 1960s for propulsion for aircraft. Development was abandoned around 1975, mainly due to corrosion problems. It was not until the 2000s that the concept was taken up again. a. also in the Generation IV concepts .

### Special types

There are also some special types for special applications. Small reactors with highly enriched fuel for the power supply of spacecraft have been constructed 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 core reactor ) did not get beyond the experimental stage .

Work is currently being carried out on new reactor concepts around the world, the Generation IV concepts , especially with a view to the expected growing energy demand. These should meet special criteria of sustainability, safety and economy. In particular, breeder reactors achieve a significantly higher efficiency in the utilization of fuel and a lower amount of radioactive waste. The risk of core meltdown due to decay heat is reduced to zero with sufficiently strong passive cooling. The first Gen IV reactors are to 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.

### Natural nuclear reactor

A nuclear fission chain reaction does not necessarily require complex technical systems. Under certain - albeit rare - circumstances it can also develop in nature. 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 in 17 locations in Oklo and a neighboring uranium deposit.

A prerequisite for the naturally occurring fission chain reactions to occur was the much higher natural proportion of fissile 235 U in uranium in ancient times . At that time it was around 3%. Due to the shorter half-life of 235 U compared to 238 U, the natural content of 235 U 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 quantities 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 led to the moderator effect of water present in the columns of the uranium rock water for criticality. The resulting heat 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 five tons of uranium-235. The output of the reactor was (compared to today's megawatt reactors) at a low 100 kilowatts.

The Oklo natural reactor is important for assessing the safety of final disposal for radionuclides ( nuclear waste ). The low migration of some fission products and of the breeding plutonium observed there over billions of years was interpreted by proponents of nuclear energy in such a way that nuclear repositories can exist that are sufficiently safe for long periods of time.

## Applications

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 searched nuclides

Theoretically, one could also produce gold in a reactor, but that would be very uneconomical.

The most important material conversion reaction taking place in the reactor (besides the generation of fission products) is the breeding (see above) of plutonium-239 from uranium-238, the most common uranium isotope. It inevitably occurs in any uranium-powered reactor. However, there are military reactors specially optimized for this purpose, which are set up in particular to remove the fuel after only a short period of operation, so that 239 Pu with only a low content of 240 Pu is available.

Nuclear reactors also serve as intensively controllable neutron sources for physical investigations of all kinds. Further applications are the propulsion of vehicles ( nuclear energy propulsion ) and the energy supply of some spacecraft.

## Security and Politics

After years of euphoria since the 1970s, the potential danger posed by nuclear reactors and the hitherto unsolved 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, an attempt was made to make nuclear power socially acceptable again between 2000 and 2010, against the background of fading memories of the risks (the Chernobyl disaster was 20 years ago) close. The reason for this is the reduction in CO 2 emissions required by international treaties when burning fossil fuels. This contrasts with the growing energy demand of emerging economies like 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 EPR type in Olkiluoto , Finland , which is scheduled to go online in 2021. 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 ). In addition, many countries in the Generation IV International Forum research association are developing six new reactor types that are supposed to guarantee greater sustainability, safety and economic efficiency.

The nuclear accidents in the Japanese power plant Fukushima-Daiichi in the wake of the magnitude 9 earthquake and the subsequent tsunami of March 11, 2011 triggered new considerations almost everywhere. Unlike the accident in Chernobyl, in which a completely outdated and known dangerous type of reactor was used, the accidents in Fukushima showed a weakness of light water reactors, the most common type.

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 how far 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 by skillfully loading fuel elements and additional moderator rods. The Helmholtz Center Dresden-Rossendorf , among others, is researching this problem.

## literature

• S. Glasstone, MC Edlund: The Elements of Nuclear Reactor Theory. Van Nostrand, New York 1966.
• A. Ziegler, H.-J. Allelein (Hrsg.): Reaktortechnik : Physical-technical basics . 2nd edition, Springer-Vieweg, Berlin, Heidelberg 2013, ISBN 978-3-642-33845-8 .
• Dieter Smidt: Reactor technology . 2 volumes, Karlsruhe 1976, ISBN 3-7650-2018-4
• Dieter Emendörfer, Karl-Heinz Höcker: Theory of nuclear reactors . BI-Wiss-Verlag, Mannheim / Vienna / Zurich 1982, ISBN 3-411-01599-3 .
• Julia Mareike Neles, Christoph Pistner (Ed.): Nuclear energy: a technology for the future? Springer Vieweg, Berlin, Heidelberg 2012, ISBN 978-3-642-24328-8 , doi : 10.1007 / 978-3-642-24329-5 .
• Günther Kessler (* 1934): Sustainable and Safe Nuclear Fission Energy: Technology and Safety of Fast and Thermal Nuclear Reactors . Springer, Berlin 2012, ISBN 978-3-642-11990-3 , doi : 10.1007 / 978-3-642-11990-3 .
• Ulrich Goetz, Georg Fischer (photos): Uranium: The element that moves the world . In: Geo-Magazin . No. 6 . Gruner & Jahr, June 1979, ISSN  0342-8311 , p. 8–42 (informative report with overviews: "Circulatory disorders", arguments for and against nuclear energy, as well as "Anatomy of an immersion heater").

Wiktionary: nuclear reactor  - explanations of meanings, word origins, synonyms, translations
Commons : Nuclear Reactors  - Collection of pictures, videos and audio files

## Individual evidence

1. WT Hering: Applied nuclear physics . Stuttgart / Leipzig: Teubner, 1999, p. 272, ISBN 3-519-03244-9
2. The technical term in physics and nuclear engineering is 'split', not 'split'.
3. R. Zahoransky (Ed.): Energy technology . 5th edition, Vieweg / Teubner, 2010, ISBN 978-3-8348-1207-0 , p. 81
4. Brockhaus Encyclopedia, 21st edition, under nuclear energy
5. ^ Gerstner, E .: Nuclear energy: The hybrid returns . In: Nature . 460, 2009, p. 25. doi : 10.1038 / 460025a .
6. H. Borgwaldt et al .: SUAK, a fast subcritical facility for pulsed neutron measurements . (1965)
7. Y. Rugama et al .: Experimental results from noise measurements in a source driven subcritical fast reactor. Progress in Nuclear Energy Vol. 44 (2004) pp. 1-12
8. WT Hering: Applied nuclear physics: introduction and overview . Teubner, 1999, ISBN 978-3-519-03244-1 , p. 303
9. Emissions from nuclear power plants and radiation exposure . German Atomic Forum e. V. 2008. Archived from the original on December 15, 2017. Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Retrieved February 23, 2017.
10. ^ Nuclear power plants, world-wide, reactor types; European Nuclear Society, 2015
11. Mandel, Heinrich: Location issues with nuclear power plants , atw atomwirtschaft 1/1971, pp. 22-26
12. News from February 10, 2011
13. Accidents: 1960's. In: Nuclear Age Peace Foundation. March 14, 2011, accessed on March 14, 2011 . As well as Nuclear Power in Switzerland. In: World Nuclear Association. March 14, 2011, accessed on March 14, 2011 .
14. Technology Roadmap Update of Gen IV , from the Nuclear Energy Agency of the OECD , January 2014. Accessed July 10, 2015 (English)
15. AP 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)
16. ^ Press release from the FZD from August 9, 2010