Nuclear reactor

A nuclear reactor (including nuclear reactor or nuclear reactors, nuclear outdated burner ) is a facility in which a nuclear fission reaction continuously runs a chain reaction in the macroscopic scale.

Spread worldwide are power reactors, ie nuclear reactor plants by the cleavage (English fission ) of uranium or plutonium first heat and it usually electrical power (see Nuclear power plant ) win. In contrast, research reactors used for the production of free neutrons, for example for the purpose of research material, or for the production of certain radioactive nuclides, such as for medical purposes. In the Paleozoic, there was repeated to form natural nuclear reactors.

A nuclear power plant has often several reactors. Here, the term is often used inaccurately reactor. For example, with the statement " in Germany ran up to the nuclear phase-out 17 nuclear power plants " is meant that 17 nuclear reactors were running at fewer locations. Thus, for example, the nuclear power plant Gundremmingen of two reactors.

Most nuclear reactors are fixed installations. In addition, there are nuclear reactors in submarines, surface ships and spacecraft. for example

• The U.S. has some nuclear-powered aircraft carrier, and France one,
• Have six nuclear powers nuclear-powered submarines, for example, nuclear-powered hunting submarines,
• There were in 2011 ten nuclear-powered icebreaker and four nuclear-powered cargo ship.

In atomic euphoria of the late 1950s and early 1960s, the idea came up to nuclear-powered road vehicles, airplanes or spaceships.

• 2.1 light-water reactor
• 2.2 heavy water reactor
• 2.3 graphite reactor
• 2.4 breeder reactor
• 2.5 molten salt reactor
• 2.6 Special types
• 2.7 Natural nuclear reactor

Operation

The fission

Between the protons and neutrons of an atomic nucleus have a very strong attractive forces, however, have only a very limited range. Therefore, these nuclear power essentially acts on the nearest neighbors - more distant nucleons contribute to the attractive force with only a small extent. As long as the core force is greater than the repulsive Coulomb force between the positively charged protons, the nucleus holds together. Small nuclei are stable when they contain one neutron per proton: 40Ca is the heaviest stable nuclide with equal proton and neutron number. With increasing proton number for a growing surplus of neutrons for stability is required, because by the attractive nuclear force the extra neutrons, the repulsive Coulomb force of the protons is compensated.

Captures a very heavy nucleus, such as the uranium isotope 235U or plutonium isotope 239Pu, a neutron, it will be recovered by the binding energy of a highly excited unstable - 236U or 240Pu nucleus. Such highly excited heavy nuclei rain with extremely short half -lives from nuclear fission. Illustratively stated, the core unit by the neutron absorption as an activated water drops in motion while tearing into (usually) two fragments ( with a mass ratio of about 2 to 3 ), which fly apart at high kinetic energy; also about two to three fast neutrons are released. These new neutrons are available for further fissions available; this is the basis of the nuclear chain reaction.

Breeding reactions

When neutrons strike nuclear fuel, other nuclear reactions take place in addition to the inevitable nuclear fission. Of particular interest are reactions in which components of the nuclear fuel, which are not themselves be cleaved to be converted into fissile. Such reactions are called brood reactions, the process " brooding " or "conversion ". From a breeder reactor is referred to but only when more new fissionable material is produced than the reactor itself consumes at the same time, so the conversion rate is about 1.0.

The fuel of almost all nuclear reactors mainly contains uranium. Therefore, the incubation reaction at the non-fissile isotope of uranium 238 U is of particular importance. The 238U converts to by neutron capture in 239U; this goes without saying by two successive beta decays in the fissile plutonium isotope 239Pu on:

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

If the separated plutonium is intended for nuclear weapons purposes ( " weapons grade plutonium " ), it must be isotopically pure as possible, ie, it must not contain too much 240Pu. This nächstschwerere plutonium isotope occurs when the 239Pu - nucleus captures a neutron another. Therefore, we obtain weapons-grade plutonium only of such fuel elements that are removed from the reactor after a relatively short period of operation.

In a corresponding manner as Pu -239 from U -238 can also be the fissile U -233 from thorium Th- 232 are bred.

Energy release in nuclear fission

The newly formed intermediate-mass nuclei, the fission products as mentioned, have a greater binding energy per nucleon than the original heavy nucleus. The difference in the binding energies occurs mostly as kinetic energy of the fission fragments ( calculation). These give the energy from impact to the surrounding material as heat. The heat is removed by a coolant and for heating, for example, be used as process heat about seawater desalination or to generate electricity.

About 6 % of the total in a nuclear reactor released energy is released in the form of electron antineutrinos, which practically escape freely from the gap zone of the reactor and permeate all the material around. These particles do not exert any appreciable effects since they hardly react with matter.

Taken together have around 440 nuclear reactors currently 210 nuclear power plants, which are available in 30 countries worldwide, an electrical capacity of about 370 gigawatts. This is a share of 15 % of the total electrical energy worldwide (as of 2009).

Chain reaction, thermal neutrons, Moderator

The chain reaction is that neutron nuclei of the nuclear fuel column, wherein in addition to the high energy fission fragments and each of a number of new neutrons are free; they can more cores split etc. The cross section of the nuclei for cleavage increases with decreasing energy, thus decreasing velocity of the neutron to the slower is the neutron, the more likely it is that it is absorbed by a fissile nucleus and this is then split is. Therefore you brake in most reactors on fast neutrons from fission by a moderator. This is a material, such as graphite, heavy, or normal water containing light 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 the atomic nuclei, but rarely absorbed. They are therefore the chain reaction still available. The neutrons can be slowed down to the speed of the nuclei of the moderator; whose average velocity is given by the theory of Brownian motion by the temperature of the moderator. This means there is a thermalization. One speaks therefore not braked, but by thermal neutrons, because the neutrons then have a similar distribution as the thermal energy of the molecules of the moderator. A reactor used for the thermal neutron fission is accordingly referred to as " thermal reactor ." In contrast, a (hence the term " fast breeder " ) uses "fast " reactor are not braked, fast neutrons to fission.

Introduction and control of the chain reaction

In the off state, i.e., retracted control rods of the reactor is subcritical. Although some free neutrons are always present in the reactor - for example, released by spontaneous fission of atomic nuclei of the nuclear fuel - and solve some of cleavages, but the growth of a chain reaction is prevented in that most neutrons are absorbed by the material contained in the control rods, so the multiplication factor k is less than 1.

The " start " of the reactor, the control rods with continuous measurement of the neutron flux is more or less far removed from the reactor core that is easy to Überkritikalität achieved by delayed neutrons, that is, a self-sustaining chain reaction with gradually increasing the reaction rate. Neutron flux and thermal power of the reactor are proportional to the reaction rate and therefore rise with her. Means of the control rods of the neutron flux is adjusted to the respective desired flow or power level in the supercritical state, and even held constant; k is then equal to 1.0. Any changes of k by temperature increase or other influences be overcome by adjusting the control rods. This happens in virtually all reactors by automatic control means responsive to the measured neutron flux.

The multiplication factor of 1.0 means that an average of just one of the per fission neutrons released causes an additional fission. All remaining neutrons either absorbed - some unavoidable in structural material ( steel, etc. ) and non-fissile fuel components, partly in the absorber material of the control rods, usually boron or cadmium - or escape from the reactor to the outside ( " leakage ").

For reducing the power and for turning off the reactor, the control rods are moved in, whereby it is again subcritical, the multiplication factor is reduced to values ​​below 1.0; the reaction rate decreases, and the chain reaction ends.

A delayed supercritical reactor increases its power relatively "slow", over a period of several seconds. If the active control fails in water-moderated reactors, ie, the criticality is not regulated back to 1, the power increases beyond the face value. It warms the moderator and expands in sequence or evaporated. However, as moderating water is necessary to maintain the chain reaction, the reactor versa - if only the water evaporates, but the spatial arrangement of the fuel has been preserved - in the subcritical range back. This behavior is called intrinsically stable.

This behavior does not, for example for graphite-moderated reactors, arguing graphite with increasing temperature retains its moderating properties. Device, such a reactor by failure of control systems in the delayed supercritical region, the chain reaction does not come to a standstill and this leads to overheating and destruction of the reactor. Such a reactor is therefore not intrinsically stable.

In contrast to the delayed supercritical reactor, a prompt supercritical reactor is not adjustable, and it can lead to serious accidents. The neutron flux and thus the heat output of the reactor increases exponentially with a doubling time in the range of 10-4 sec. The achieved capacity, the rated power for a few milliseconds, more than 1000 times exceed until it is slowed down again in the so- heated fuel oil through the Doppler broadening. The fuel rods are heated by this power excursion abruptly at temperatures above 1000 ° C. Depending on type and exact circumstances of the accident, this can lead to serious damage to the reactor, especially by abruptly evaporating ( cooling ) water. Examples of prompt supercritical light water reactors, and the consequences show the BORAX experiments or accident in U.S. research reactor SL -1. The largest accident by an at least in some areas promptly supercritical reactor was the Chernobyl disaster, immediately after the power excursion suddenly have led to a wide-ranging distribution of the radioactive inventory in the vaporizing liquids, metals, and the subsequent graphite fire.

The automatic interruption of the chain reaction at water-moderated reactors, unlike sometimes claimed, no guarantee that it does not come to a meltdown because the decay heat is sufficient in case of failure of active cooling systems to bring about this. For this reason, the cooling systems are redundant and diversely. A meltdown is considered as a design basis accident since the accident at Three Mile Iceland in the planning of nuclear power plants and is in principle controllable. Because of the changed geometrical arrangement re- criticality is, however, not be excluded in principle.

Subcritical reactors operating

A chain reaction at a constant reaction rate can be achieved even in a sub-critical reactor, by feeding free neutrons from a neutron source independent. Such a system is sometimes referred to as driven reactor. If the neutron source based on a particle accelerator, so anytime switched off, the principle offers improved security against Reaktivitätsstörfälle. The decay heat (see below) occurs here, however, as well as on the critical operating reactor; Arrangements for the control of cooling loss incidents are here so just as necessary as in the conventional reactors. Driven reactors have occasionally been built and operated for experimental purposes and are designed as large-scale systems for transmutation of waste reactor (see Accelerator Driven System).

Decay heat

When a reactor is shut down, so heat is produced by the radioactive decay of the fission products continue. The performance of these so-called residual heat corresponding to about 5-10% of the initial thermal power of the reactor during normal operation, and sound in a period of several days from largely. It is often the term " residual heat " is used which is misleading, because it is not the current remaining heat of the reactor core, but additional energy supply, which is fed by the ongoing decay reactions.

To be able to remove the decay heat safely ( with a failed main cooling system ) in emergency situations, all nuclear power plants have a complex emergency and residual heat removal. However, if these systems fail, it can lead to a meltdown, melt parts of the nuclear fuel in the structural parts of the reactor core, and possibly by rising temperatures.

Meltdown

When the fuel rods and thereby produces a low melt agglomeration of fuel, the multiplication factor is increased, and there may be a rapid uncontrolled heating. In order to prevent this process, or at least delaying, in some reactors, the reactor core in the processed materials are chosen so that their neutron absorption capacity increases with increasing temperature, thus the reactivity is decreased. The case of core melt is considered as maximum credible accident ( MCA ), ie as the worst accident, which must be considered when designing the plant into consideration, and the withstand, without damage to the environment needs. Such an accident occurred for example in the nuclear power plant at Three Mile Iceland.

The worst case, for example, that the reactor building can not withstand and greater, the allowable limits far -border quantity of radioactive material leaking is called a worst-case scenario. This happened for example in 1986 at the Chernobyl disaster and 2011 in the Fukushima disaster.

As an inherently safe against meltdowns therefore only certain high-temperature reactors lower power and power density apply to the current state of the art; generally inherently safe is not this type of reactor, but also because of other potentially catastrophic accident types ( graphite fire, flood ). The power density is expressed in m³ MW /, ie in the megawatt thermal power per cubic meter of the reactor core. This specification allows a statement about which technical provisions must be taken to dissipate the resulting decay heat in the event of disturbances or scrams.

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

The European Pressurized Water Reactor (EPR ) has below the pressure vessel to safety in the event of a meltdown a specially shaped ceramic basin, the core catcher. In that the molten material of the reactor core is to be collected, and cooled by a special cooling.

Reactor types

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

Light water reactor

With normal light water moderated reactions take place in the light-water reactor ( LWR), which can be interpreted as a boiling water reactor or pressurized water reactor. A further development of pre-Convoy, Convoy and N4 is the European Pressurised Water Reactor (EPR ). A Russian pressurized water reactor, the VVER. Light water reactors require enriched uranium, plutonium or mixed oxides (MOX ) as fuel. A light-water reactor was also the Oklo natural reactor. The fuel of LWR are sensitive to thermodynamic and mechanical loads. To avoid this, sophisticated, technical and operational safeguards are needed that characterize 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 meltdown of the fuel elements and the bursting of the reactor pressure vessel have long been explained in the nuclear industry because of the improbability of its occurrence as irrelevant, for example by Heinrich Mandel.

Heavy water reactor

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

Graphite reactor

Gas -cooled graphite-moderated reactors were developed in the 1950s, initially primarily for military purposes ( plutonium production). They are the oldest commercially used nuclear reactors; the refrigerant is carbon dioxide in this case. In the United Kingdom (2011 ) is still a number of these plants are in operation. Because of the fuel rod cladding made ​​of a magnesium alloy called this type of reactor Magnox reactor. Similar systems were operated in France, but are now all off. On October 17, 1969 melted shortly after commissioning of the reactor 50 kg of fuel in gas-cooled graphite reactor of the French nuclear power station at Saint -Laurent A1 ( 450 MWe ). The reactor was then closed down in 1969 ( today's reactors at the nuclear power plant are pressurized water reactors ).

A successor of the Magnox reactors is developed in the UK Advanced Gas -cooled Reactor (AGR ). In contrast to the Magnox reactors use low-enriched uranium dioxide instead he uranium metal as fuel. This allows higher power densities, and coolant outlet temperature and thus better thermal efficiency. AGR have achieved with 42 %, the highest efficiency of all existing nuclear power plants.

High Temperature Reactor HTR also use graphite as a moderator; Helium as a coolant gas. A possible design of the high temperature reactor, the pebble-bed reactor by 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, since a meltdown due to the high melting point of graphite is impossible with a failure of the emergency and residual heat removal systems. However, there are a number of other serious types of accidents such as flooding or air ingress with graphite fire, which provide the alleged safety benefits in question, such as Rainer Moormann out, the price received for the whistleblower 2011. Also, a number of unresolved practical problems, the commercial implementation of the concept prevented. In addition, the equipment cost of the HTR are higher than that of the light water reactor. In Germany, research at the experimental nuclear power plant AVR ( Jülich) and built the prototype power plant THTR -300 at Hamm- Uentrop, the latter with a reactor pressure vessel made ​​of prestressed concrete. Both were shut down in 1989.

The Soviet RBMK also use graphite as a moderator, but light water as the coolant. Here, the graphite is present in blocks, by the hundreds to thousands ( by the power of the reactor dependent) channels are drilled, in which are pressure tubes with the fuel and the water cooling. This type of reactor is slow ( it takes much time to rules ) and secure than other types, because a loss of coolant does not mean loss moderator here (that does not reduce the reactivity) and since the amount of combustible graphite is very large. The wrecked reactor in Chernobyl was of this type.

Breeder reactor

Furthermore, there are breeder reactors ( fast breeder reactors ), in which so transformed in addition to the energy release 238U into 239Pu, so that more new fissionable material is produced than is consumed at the same time. This technology is (also safety ) more demanding than the other types. Their advantage is that with it, the uranium resources of the earth can be many times better utilized than when only the 235U "burned" is. Breeder reactors operate 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 ( MSR English for molten salt reactor or LFTR Liquid Fluoride Thorium Reactor for ) is a salt melt containing the nuclear fuel ( for example, thorium and uranium), circulated in a loop. The melt is the same fuel and the coolant. This type of reactor is not gone beyond the experimental stage.

In favor of molten salt reactors have been put forward various security arguments. The development was abandoned around 1975, mainly because of corrosion problems. It was only in the 2000s, the concept was taken up again, including in the Generation IV concepts.

Special types

There are also some special types for specific applications. So small reactors were constructed with highly enriched fuel for the power supply of spacecraft that do not require liquid coolant. These reactors are not to be confused with the nuclear batteries. Also, air-cooled reactors, which always require highly enriched fuel, were built, for example, for physical experiments in the BREN Tower in Nevada. It reactors were designed for spacecraft propulsion, in which liquid hydrogen is to cool the fuel used. However, this work came about soil tests does not exceed ( NERVA project, project Timber Wind ). Also do not have the experimental stage were reactors where the fuel is in gaseous form (gas nuclear reactor ). The molten salt reactor uses a molten uranium salt, usually uranium hexafluoride ( UF6) or uranium tetrafluoride ( UF4 ), as fuel and heat transfer and graphite as moderator. These reactors were developed, among others, in the United States in the 1960s for aircraft propulsion.

Work is currently underway worldwide actively in new reactor design, the Generation IV concepts, particularly in view of the expected growth in energy demand. These are to be used after the presentation of the U.S. Department of Energy from 2030.

Another, currently still in the experimental stage befindlicher, reactor type is the drive shaft reactor. This concept promises, provided that the reaction should succeed in a much more efficient use of nuclear fuel and the massive reduction of the problem of radioactive waste, since a run -wave reactor could be operated with radioactive waste and would use up this case systematically.

Nuclear reactor from the inside (active, lit )

Nuclear reactor from the inside ( without illumination)

View in a nuclear reactor. Clearly seen the bluish Cherenkov radiation

Natural nuclear reactor

A fission chain reaction can be achieved not only by complex technical systems, but came under some - albeit rare - circumstances in nature. 1972, French researchers discovered in the Oklo region of the West African country Gabon, the remains of the natural nuclear reactor Oklo, which was formed about two billion years ago, in the Proterozoic, by natural processes. Overall, have been found in Oklo uranium deposit and an adjacent evidence of previous fission reactions at 17 points.

A prerequisite for the realization of the past, of course, fission chain reactions was much higher in the Palaeozoic natural proportion of fissile 235U in the uranium, he at that time was about 3 %. Due to the shorter half-life of 235U compared to 238U, the natural content of 235U in the uranium is currently only about 0.7%. In this low content of fissile material new critical fission chain reactions on the earth can no longer occur naturally.

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 determined the amounts of various noble gas isotopes that were trapped in a material sample of the Oklo mine, with a mass spectrometer. From the distribution of the different resulting from the uranium fission xenon isotopes in the sample showed that the reaction has proceeded 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 released in the uranium rock heated the water in the columns until it finally evaporated and escaped like a geyser. As a result, could no longer act as a moderator, the water, so that the nuclear reaction came to a standstill ( resting phase ). Then the temperature dropped again so that fresh water seep and the columns could replenish. This created the conditions for re- criticality, and the cycle could start again. Calculations show that the active phase lasting about 30 minutes ( power generation ), followed by a rest period that lasted for more than two hours. In this way, the natural nuclear fission for about 500,000 years was kept going, with about 5 tons of uranium -235 were used. The performance of the reactor was in comparison to today's MW small reactors 100 kilowatts.

Importantly, the nature of Oklo reactor is also responsible for assessing the safety of Endlagerungen for radionuclides ( " nuclear "). The observed there is very little migration of some fission products and plutonium erbrüteten over billions of years were interpreted by nuclear power proponents so that atomic repository can exist that are sufficiently safe for long periods.

Applications

Most nuclear reactors are used to generate electrical ( rare: only thermal ) energy in nuclear power plants. In addition, nuclear reactors are also used for the production of radionuclides, for example, for use in radioisotope generators or in nuclear medicine. Here, the desired nuclides

• Either, unless they occur in the cleavage products are extracted from the spent fuel
• Or specifically produced by stable isotopes of the elements in the ruling in the nuclear reactor neutron radiation are exposed (see Neutronenanlagerung ).

Theoretically, one could in a reactor also produce gold ( gold synthesis), which would, however, very inefficient.

The main takes place in the reactor fuel conversion reaction ( in addition to the generation of cleavage products ) is the Incubation (see above) of plutonium -239 from uranium -238, the most common isotope of uranium. It takes place inevitably in each operated with uranium reactor. However, there are specially optimized military reactors are particularly adapted to the removal of the fuel after only brief operation, 239Pu so that with only a small content is available at 240Pu.

Nuclear reactors also serve as intense neutron sources controlled for physical examinations of all kinds Other applications include the use in vehicles ( nuclear-powered ) and the power supply of some spacecraft. The latter reactors are not to be confused with the nuclear batteries.

Security and policy

The light emanating from nuclear reactors as well as the potential danger as yet unresolved question of storage of radioactive waste have led after years of euphoria since the 1970s in many countries to protests by anti-nuclear activists and to a reassessment of nuclear energy. While in the 1990s, especially in Germany, the exit was propagated from nuclear power, took place about 2000 to 2010 against the backdrop of the fading memories of the risks ( the Chernobyl disaster is now more than 20 years back ) a try instead, the nuclear power to make socially acceptable again. The occasion is the required by international treaties reduction of CO2 emissions from the combustion of fossil fuels. This compares with a growing energy demand of emerging economies such as China.

For these reasons, some European countries to invest in new nuclear power plants determined. So the German group Siemens and the French group Areva to build a pressurized water reactor type EPR in Olkiluoto, Finland, which is to go on line in 2013. Russia wants to renew his old and partially ailing nuclear power plants and begin at least ten years per year a new reactor construction. In France is also negotiating the construction of a new reactor. Sweden stopped his plans to phase out nuclear power. Besides there are smaller and larger 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 Fukushima Daiichi brought in the wake of the magnitude -9 earthquake on 11 March 2011 this almost everywhere new ideas in motion.

The life of the nuclear reactor is not unlimited. Particularly, the reactor pressure vessel is exposed to neutron radiation constant, which leads to embrittlement of the material. How quickly this happens depends, among other things, on the fuel are placed in the reactor and what distance they have to the reactor pressure vessel. The nuclear power plants Stade Obrigheim and were therefore also taken first from the network because here this distance was less than in other, newer nuclear reactors. Currently, the operators of nuclear power plants try to reduce the neutron exposure of the reactor pressure vessel by a skillful fuel loading and additional moderator rods. Among other things, the Helmholtz -Zentrum Dresden -Rossendorf researched this problem.