What is a reactor

Nuclear reactor

Lexicon> Letter K> Nuclear reactor

Definition: a facility in which a nuclear reaction is carried out and is often part of a nuclear power plant

Alternative term: nuclear reactor

More general term: heat generator

More specific terms: nuclear fission reactor, nuclear fusion reactor, pressurized water reactor, boiling water reactor, breeder reactor, traveling wave reactor

English: nuclear reactor

Categories: Basic Concepts, Nuclear Energy

Author: Dr. Rüdiger Paschotta

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Original creation: 04/03/2011; last change: 03/30/2020

URL: https://www.energie-lexikon.info/kernreaktor.html

A Nuclear reactor (or Nuclear reactor) is a facility in which certain nuclear reactions (usually nuclear fission processes) are carried out with a high intensity (i.e. with a high power density). Different applications are possible for this:

  • In a nuclear power plant, the nuclear reactions produce heat that is used to generate hot water vapor. (A boiling water reactor generates steam directly, while a pressurized water reactor works with liquid water and the steam is then produced in a separate steam generator.) The steam is used to drive a steam turbine and ultimately a generator for generating electrical energy. A nuclear power plant can have several Reactor blocks which can usually be operated independently of one another.
  • The generated radioactive radiation is primarily used in research reactors. For example, the intensive neutron radiation is suitable for studying the atomic structure of solids.
  • Still other reactors are mainly used for the production of radioactive isotopes (radionuclides), which z. B. for medicine (diagnostics and radiation therapy), various branches of scientific research or in industry (for example for the detection of material defects).
  • A number of reactors were and are mainly operated for the breeding of weapons-grade plutonium. Reactors used for commercial power generation can also be used to produce plutonium at the same time and were often used to camouflage nuclear weapons projects. (So ​​far, no state has achieved nuclear weapons capability without first establishing a civilian nuclear program.)
  • Certain nuclear reactors can also be used for transmutation in order to reduce the hazard potential of radioactive waste. Here, the intense neutron radiation in the reactor is used to transform long-lived emitters into shorter-lived ones.

The term Nuclear reactor is synonymous with Nuclear reactor, just a little less precise: it is not clear that the crucial processes in atomcores take place and not, as in chemical processes, in the electron shells. In the early days of the nuclear reactors one also spoke of Uranium machines.

Nuclear fission reactions always take place in today's nuclear reactors. Nuclear fission releases a great deal of energy in the form of heat - orders of magnitude more than would be possible with the same amount of chemical fuel. In addition, a considerable amount of heat is also generated by the radioactive decay of the fission products. These Decay heat represents one of the major safety problems of reactor operation.

It is conceivable that future nuclear reactors will be based on Nuclear fusion (instead of nuclear fission) will be based. However, a nuclear fusion reactor is extremely difficult to implement; Despite decades of efforts at great expense, it has not yet been possible to maintain a nuclear fusion chain reaction for more than a few seconds. The rest of the article therefore deals exclusively with nuclear fission reactors, excluding exotic types that will remain of no technical importance for the foreseeable future.

Safety issues are dealt with in a separate article on reactor safety.

Basic principle of the nuclear fission reactor

A nuclear chain reaction is critical to the operation of a nuclear reactor.

In order to be able to carry out nuclear fission at a high rate (i.e. with a much higher output than, for example, in an isotope battery), a nuclear chain reaction is required. This can be achieved relatively easily in the nuclear fission of materials such as uranium or plutonium, because on the one hand, bombarding fissile atomic nuclei can trigger their fission and, on the other hand, the fission process itself releases several neutrons, which can then trigger further fission.

The prerequisite for the chain reaction is that the so-called Criticality is achieved: For each nuclear fission that takes place, the released neutrons must on average trigger at least one further nuclear fission. (The Multiplication factor is then at least 1.) This requires that a sufficient amount of fissile material is concentrated in a sufficiently narrow space, i.e. the so-called critical mass is achieved. Otherwise too many neutrons escape unused to the outside. Operation with a multiplication factor slightly below 1 is possible in principle if a strong external neutron source is available; however, this option has not yet been used.

The chain reaction can be started without any problems with the help of neutrons released spontaneously by radioactive decay as soon as criticality is reached. Therefore, no special neutron source is required to start a reactor.

It has a supporting effect in most reactors (only not in the so-called “Fast” reactors) a Moderator, d. H. a material that slows down the neutrons released during fission (but if possible without absorbing neutrons). Slow (“thermal”) neutrons can trigger further fission more effectively. Ordinary water is used in light water reactors and heavy water reactors (H.2O) or heavy water (D2O) as a moderator (and also to remove the generated heat). Heavy water favors the construction of smaller reactors, since it allows the critical mass to be kept smaller, but is not used in large-scale technical applications, if only because of its high production costs. Another disadvantage is that much more tritium is formed in heavy water - a highly radiotoxic element (which, however, is specifically generated in certain reactors). In some reactors, the moderator consists of other substances such as B. graphite.

How do you manage to maintain the delicate balance for a nuclear chain reaction with roughly constant power?

In order to achieve operation over a long period of time with constant power, a delicate balance of the neutron flux must be maintained by a fast automatic control: For each nuclear fission that takes place, the released neutrons must on average trigger exactly one further nuclear fission. If it were even slightly more, the output would increase exponentially and in a short time lead to the destruction of the reactor (with possibly dramatic consequences). (An extremely intense explosion like an atomic bomb is not possible, however, since the chain reaction would be slowed down as a result of overheating before a substantial part of the fissile material can be converted.) Conversely, nuclear fission could quickly come to a standstill if a little too few neutrons Spark divisions. The power is regulated with the help of Control rods, which absorb neutrons and can be driven into the reactor core. You can e.g. B. contain cadmium. The more the nuclear fuel is consumed ("burned down"), the more the control rods need to be pulled out. To shut down the reactor, they are driven far in.

The criticality of the reactor is also influenced by the accumulation of fission products and their decay products. For example, during prolonged partial load operation or after a shutdown, the strongly neutron-absorbing xenon 135 can accumulate so much that the criticality can no longer be achieved; "Xenon poisoning" occurs, which only subsides over the course of hours due to the disintegration of this isotope. For such reasons, the criticality properties of a reactor depend significantly on the precise operating conditions.

Construction of a light water reactor

This section explains how a typical Light water reactor is constructed, d. H. a reactor cooled with ordinary (light) water. This type is by far the most common in the world. Other types of reactors are discussed below.

Fissile material

Every nuclear fission reactor must contain enough fissile material to achieve the critical mass for nuclear fission. In most cases, uranium 235 (235U) is used, sometimes also plutonium 239 or uranium 233, which can be bred from thorium 232, or a mixture of different fissile isotopes. The fissile material (the Nuclear fuel) is contained in fuel rods, which in turn are bundled into fuel assemblies. These fuel elements are installed in the reactor core with a relatively small spacing and can be replaced individually.

The fuel elements should safely enclose the fissile material and, above all, the highly radioactive fission products - if possible even in the event of severe accidents. However, their resilience is z. B. limited for cases with a sharp rise in temperature. Even in normal operation, fuel element damage with leaks occasionally occurs, which can lead to severe contamination of the entire primary cooling circuit.

Control of performance

There are several neutron-absorbing control rods (also Control rods or Control rods), which are used to regulate the power (see above) and to switch off the reactor.

Removal of heat

The heat generated is removed from ordinary (light) water in the light water reactor. This is chemically prepared and additional substances (e.g. boron salts) can be added.

The cooling water is transported through the reactor by powerful coolant pumps at high pressure in a closed cooling circuit and flows through between the fuel rods. Depending on the type of reactor, the water is evaporated in the reactor and then enters a steam turbine, or it remains liquid due to the high pressure and reaches a steam generator as a heat exchanger (see below).

The cooling water not only removes the heat gained, but also has a moderating effect.

Basically, the heat transport through the cooling water has two important functions: on the one hand, the heat is used in this way, and on the other hand, the reactor is protected from overheating. However, the cooling water has another function: it moderates the neutrons (i.e. it slows them down) and thus enables criticality to be reached with a significantly lower amount of fuel or a lower degree of enrichment than without a moderator. This also has a beneficial safety-related side effect: in the event of a sharp increase in temperature, which leads to the evaporation of the water, its moderating function is greatly reduced, which can weaken or even stop the nuclear chain reaction.

Even after the reactor has been switched off (ie after the chain reaction has stopped), effective cooling must be guaranteed, since the so-called decay heat of the radioactive fission products has to be dissipated. Otherwise, a failure of the cooling system can lead to serious reactor accidents, up to and including a core meltdown.

Security container

Most reactors contain an additional one around the reactor pressure vessel Reactor containment, which is also called Containment referred to as. If the reactor pressure vessel fails, it should prevent radioactive substances from escaping as far as possible. In its lower part, cooling water can also be collected, which can escape in the event of leaks.

Boiling water and pressurized water reactors

There is another distinction between light water reactors:

  • Boiling water reactors are under moderate pressure, usually below 100 bar, and the cooling water is converted into steam in the reactor, which can directly drive a turbine.
  • At Pressurized water reactors (see Figure 1), despite the high temperature of several hundred degrees Celsius, evaporation is prevented by putting the reactor under very high pressure (well over 100 bar). The steam generation for a turbine then takes place in a heat exchanger, the Steam generator.

Systems with boiling water reactors are of a simpler design, as they do not require a separate steam generator, and can achieve a somewhat higher degree of electricity generation efficiency. One of the disadvantages, however, is that if fuel elements are defective, radioactive substances contaminate (contaminate) the entire turbine system, whereas in a pressurized water reactor only the primary circuit (up to the steam generator) is contaminated.

Other reactor types

Various other reactor types are briefly described below, all of which are far less common than the light water reactors described above. Since nuclear reactors are categorized according to different aspects (e.g. type of coolant, temperature level, use of fast or slow neutrons), there are significant overlaps here.

Heavy water reactors

Some research reactors cooperate heavy Water (D2O), because it absorbs fewer neutrons and therefore reduces the critical mass. The reactor can then mostly be operated with natural uranium or less enriched uranium and, if necessary, also be more compact, i. H. contain less nuclear fuel. Since heavy water is very expensive, this concept can only be used for relatively small reactors.

Gas-cooled reactors

There are gas-cooled reactors in which, for example, helium gas or carbon dioxide transports the heat away under high pressure. This enables very high temperatures, i.e. the construction of high-temperature reactors (see below). The gas does not have a significant neutron-decelerating (moderating) effect, which is why either an additional moderator (e.g. graphite) is used or a moderator is dispensed with (in the case of “fast reactors”).

Cooling with liquid metals or salts

Various types of reactors are cooled with molten metals such as sodium or lead. For example, sodium is used in certain fast breeder reactors. This enables the heat to be effectively removed at high temperatures without the neutrons being severely slowed down or absorbed; Sodium is therefore suitable for breeder reactors (see below).

Unfortunately, metallic sodium is quite dangerous; in particular, it burns on contact with air with strong smoke and heat generation, and contact with water can lead to hydrogen explosions. In addition, the reactor must never be cooled down completely, otherwise the sodium would solidify. This also applies to cooling with other metals such as B. lead.

Breeder reactors

Breeder reactors are those reactors that are specially optimized to “breed” as much fissile material as possible from originally non-fissile material by bombarding them with neutrons. The most important type of breeder reactor is the "fast breeders”Which breeds from uranium 238 plutonium 239. For various reasons, the reactor design here is significantly different from that of light water reactors. Water cannot be used as a coolant because it would slow down the neutrons, but fast neutrons are needed for the breeding process. Another coolant is therefore required, such as helium or liquid sodium (see above). The actual brooding takes place mainly in a special one Breeding zonewho are around the Cleavage zone is arranged around.

The advantages and disadvantages of this concept are explained in the article on breeder reactors.

High temperature reactors

High-temperature reactors (or very high-temperature reactors) are reactors that can deliver heat at particularly high temperatures (e.g. 750 ° C). This enables very high temperatures and thus a higher degree of efficiency in power generation as well as the use of high-temperature process heat, for example to generate hydrogen.

High temperature reactors have been implemented in very different ways. One example is that Pebble bed reactor (PBMR = pebble bed modular reactor), in which the nuclear fuel is embedded in spheres made of graphite, for example. These fuel balls can be cooled with helium, for example. At such temperatures, water cannot be used as a coolant.

Generation IV nuclear reactors

The development of nuclear reactors is roughly divided into generations. The nuclear reactors used today and those newly built today practically all belong to generations II and III; many Generation I reactors have already been decommissioned. For Generation IV there are various very different plans with which the following goals are primarily pursued:

A large number of improvements would be desirable for new reactor types - of course, if possible, without introducing serious disadvantages at the same time.
  • The nuclear fuel should be used more efficiently than with the previous light water reactors, which cannot use most of the uranium (even with reprocessing of the used fuel).
  • In general, the profitability is to be improved, since the previous cost development has been very negative, in stark contrast to the original expectations.
  • A higher level of reactor safety should be achieved - for example, after shutdown, passive cooling by convection should be sufficient, so that a core meltdown is avoided even without the operation of cooling water pumps. Such properties belong to inherent security. This is most likely to be achieved with smaller reactors.
  • Higher coolant temperatures are intended to increase the efficiency of electricity generation and / or to use it as process heat e.g. B. for the chemical industry, hydrogen production or coal refining.
  • The risk of proliferation (further spread of nuclear weapons) should be reduced by appropriate design of the fuel cycle.
  • In some cases, a use for transmutation in order to reduce the dangers of radioactive waste is sought.
For many decades, no reactor type has been found that could seriously replace the light water reactors that have been largely preferred up to now.

Unfortunately, these goals are hardly attainable at the same time, and there is no consensus among experts as to what kind of new reactor types would be preferred. For example, inherent safety is more likely to be achieved with small reactors, which, however, generally have higher specific costs. A significantly increased utilization of uranium usually requires additional expensive and risky process steps such as reprocessing or handling of plutonium, which under certain circumstances also increase the risk of proliferation. The costs of novel concepts are at least less reliable and often significantly higher than with conventional reactor types.

Generation IV reactors will not be available for a few decades at the earliest and are therefore only relevant for the long-term use of nuclear energy.

Replacement of nuclear fuel

Since nuclear fuels have an enormous energy content (a very high energy density), a nuclear reactor can be operated at high output for a relatively long time (months or even years) without replacing the fuel. However, it is necessary to replace at least some of the fuel assemblies if the burnup becomes too high. Then the concentration of fissile material decreases, while the concentration of neutron-absorbing substances increases. At some point the criticality would no longer be reached.

To replace fuel assemblies, the reactor pressure vessel usually has to be opened. As a rule, after the excess pressure has been relieved, a cover is removed and the fuel assemblies are pulled out with a crane and transported to a spent fuel pool. There they have to be stored in constantly chilled water for a few years until their residual heat has subsided sufficiently to enable them to be removed without constant water cooling.

In the pebble bed reactor, the fuel balls can also be exchanged during operation.

Concepts for small nuclear reactors have already been developed, in which the entire reactor would be completely replaced if the fuel elements were used up too much. It would then be the responsibility of the manufacturer and not the operator to use any recyclable materials that may still be present and to safely dispose of the remaining material.

Spent nuclear fuel still contains substantial amounts of fissile material. The usual uranium fuel rods are residual uranium 235 and spawned plutonium 239. The reprocessing of nuclear fuel, which is sometimes practiced, involves the separation and reuse of these substances in new fuel elements. The processes required for this are, however, very expensive (definitely not economical compared to the use of natural uranium) and dangerous, and they also increase the risk of misuse for nuclear weapons, in particular through the extraction of plutonium, even if this reactor plutonium is less suitable for nuclear weapons due to its isotopic composition is well suited as plutonium which is specially bred from uranium 238.

With or without reprocessing, used nuclear fuel results in highly hazardous radioactive waste that must be safely stored for a long time, i.e. it must not be allowed to enter the biosphere.

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See also: nuclear fission, breeder reactor, reactor safety, radioactive waste, transmutation
as well as other articles in the categories Basic Concepts, Nuclear Energy