The main elements of a nuclear reactor table. How to start a reactor
The nuclear reactor works smoothly and accurately. Otherwise, as you know, there will be trouble. But what's going on inside? Let's try to formulate the principle of operation of a nuclear (atomic) reactor briefly, clearly, with stops.
In fact, the same process is going on there as in a nuclear explosion. Only now the explosion occurs very quickly, and in the reactor all this stretches for a long time. In the end, everything remains safe and sound, and we get energy. Not so much that everything around immediately smashed, but quite enough to provide electricity to the city.
how a reactor worksNPP cooling towers
Before you understand how a controlled nuclear reaction works, you need to know what a nuclear reaction is in general.
A nuclear reaction is a process of transformation (fission) of atomic nuclei during their interaction with elementary particles and gamma quanta.
Nuclear reactions can take place both with absorption and with the release of energy. Second reactions are used in the reactor.
A nuclear reactor is a device whose purpose is to maintain a controlled nuclear reaction with the release of energy.
Often a nuclear reactor is also called a nuclear reactor. Note that there is no fundamental difference here, but from the point of view of science, it is more correct to use the word "nuclear". There are now many types of nuclear reactors. These are huge industrial reactors designed to generate energy at power plants, nuclear submarine reactors, small experimental reactors used in scientific experiments. There are even reactors used to desalinate seawater.
The history of the creation of a nuclear reactor
The first nuclear reactor was launched in the not so distant 1942. It happened in the USA under the leadership of Fermi. This reactor was called the "Chicago woodpile".
In 1946, the first Soviet reactor started up under the leadership of Kurchatov. The body of this reactor was a ball seven meters in diameter. The first reactors did not have a cooling system, and their power was minimal. By the way, the Soviet reactor had an average power of 20 watts, while the American one had only 1 watt. For comparison: the average power of modern power reactors is 5 Gigawatts. Less than ten years after the launch of the first reactor, the world's first industrial nuclear power plant was opened in the city of Obninsk.
The principle of operation of a nuclear (atomic) reactor
Any nuclear reactor has several parts: core with fuel and moderator, neutron reflector, coolant, control and protection system. The isotopes of uranium (235, 238, 233), plutonium (239) and thorium (232) are most often used as fuel in reactors. The active zone is a boiler through which ordinary water (coolant) flows. Among other coolants, “heavy water” and liquid graphite are less commonly used. If we talk about the operation of a nuclear power plant, then a nuclear reactor is used to generate heat. Electricity itself is generated by the same method as in other types of power plants - steam rotates a turbine, and the energy of movement is converted into electrical energy.
Below is a diagram of the operation of a nuclear reactor.
scheme of operation of a nuclear reactorScheme of a nuclear reactor at a nuclear power plant
As we have already said, the decay of a heavy uranium nucleus produces lighter elements and a few neutrons. The resulting neutrons collide with other nuclei, also causing them to fission. In this case, the number of neutrons grows like an avalanche.
Here it is necessary to mention the neutron multiplication factor. So, if this coefficient exceeds a value equal to one, a nuclear explosion occurs. If the value is less than one, there are too few neutrons and the reaction dies out. But if you maintain the value of the coefficient equal to one, the reaction will proceed for a long time and stably.
The question is how to do it? In the reactor, the fuel is in the so-called fuel elements (TVELs). These are rods that contain nuclear fuel in the form of small pellets. The fuel rods are connected into hexagonal cassettes, of which there can be hundreds in the reactor. Cassettes with fuel rods are located vertically, while each fuel rod has a system that allows you to adjust the depth of its immersion in the core. In addition to the cassettes themselves, there are control rods and emergency protection rods among them. The rods are made of a material that absorbs neutrons well. Thus, the control rods can be lowered to different depths in the core, thereby adjusting the neutron multiplication factor. The emergency rods are designed to shut down the reactor in the event of an emergency.
How is a nuclear reactor started?
We figured out the very principle of operation, but how to start and make the reactor function? Roughly speaking, here it is - a piece of uranium, but after all, a chain reaction does not start in it by itself. The fact is that in nuclear physics there is the concept of critical mass.
Nuclear fuelNuclear fuel
Critical mass is the mass of fissile material necessary to start a nuclear chain reaction.
With the help of fuel elements and control rods, a critical mass of nuclear fuel is first created in the reactor, and then the reactor is brought to the optimal power level in several stages.
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In this article, we have tried to give you general idea on the design and principle of operation of a nuclear (atomic) reactor. If you still have questions on the topic or the university asked a problem in nuclear physics - please contact the specialists of our company. We, as usual, are ready to help you solve any pressing issue of your studies. In the meantime, we are doing this, your attention is another educational video!
blog/kak-rabotaet-yadernyj-reaktor/
The nuclear reactor works smoothly and accurately. Otherwise, as you know, there will be trouble. But what's going on inside? Let's try to formulate the principle of operation of a nuclear (atomic) reactor briefly, clearly, with stops.
In fact, the same process is going on there as in a nuclear explosion. Only now the explosion occurs very quickly, and in the reactor all this stretches for a long time. In the end, everything remains safe and sound, and we get energy. Not so much that everything around immediately smashed, but quite enough to provide electricity to the city.
Before you can understand how a controlled nuclear reaction works, you need to know what nuclear reaction generally.
nuclear reaction - this is the process of transformation (fission) of atomic nuclei during their interaction with elementary particles and gamma quanta.
Nuclear reactions can take place both with absorption and with the release of energy. Second reactions are used in the reactor.
Nuclear reactor - This is a device whose purpose is to maintain a controlled nuclear reaction with the release of energy.
Often a nuclear reactor is also called a nuclear reactor. Note that there is no fundamental difference here, but from the point of view of science, it is more correct to use the word "nuclear". There are now many types of nuclear reactors. These are huge industrial reactors designed to generate energy at power plants, nuclear submarine reactors, small experimental reactors used in scientific experiments. There are even reactors used to desalinate seawater.
The history of the creation of a nuclear reactor
The first nuclear reactor was launched in the not so distant 1942. It happened in the USA under the leadership of Fermi. This reactor was called the "Chicago woodpile".
In 1946, the first Soviet reactor started up under the leadership of Kurchatov. The body of this reactor was a ball seven meters in diameter. The first reactors did not have a cooling system, and their power was minimal. By the way, the Soviet reactor had an average power of 20 watts, while the American one had only 1 watt. For comparison: the average power of modern power reactors is 5 Gigawatts. Less than ten years after the launch of the first reactor, the world's first industrial nuclear power plant was opened in the city of Obninsk.
The principle of operation of a nuclear (atomic) reactor
Any nuclear reactor has several parts: core with fuel and moderator , neutron reflector , coolant , control and protection system . Isotopes are the most commonly used fuel in reactors. uranium (235, 238, 233), plutonium (239) and thorium (232). The active zone is a boiler through which ordinary water (coolant) flows. Among other coolants, “heavy water” and liquid graphite are less commonly used. If we talk about the operation of a nuclear power plant, then a nuclear reactor is used to generate heat. The electricity itself is generated by the same method as in other types of power plants - steam rotates the turbine, and the energy of movement is converted into electrical energy.
Below is a diagram of the operation of a nuclear reactor.
As we have already said, the decay of a heavy uranium nucleus produces lighter elements and a few neutrons. The resulting neutrons collide with other nuclei, also causing them to fission. In this case, the number of neutrons grows like an avalanche.
It needs to be mentioned here neutron multiplication factor . So, if this coefficient exceeds a value equal to one, a nuclear explosion occurs. If the value is less than one, there are too few neutrons and the reaction dies out. But if you maintain the value of the coefficient equal to one, the reaction will proceed for a long time and stably.
The question is how to do it? In the reactor, the fuel is in the so-called fuel elements (TVELah). These are rods in which, in the form of small tablets, nuclear fuel . The fuel rods are connected into hexagonal cassettes, of which there can be hundreds in the reactor. Cassettes with fuel rods are located vertically, while each fuel rod has a system that allows you to adjust the depth of its immersion in the core. In addition to the cassettes themselves, among them are control rods and emergency protection rods . The rods are made of a material that absorbs neutrons well. Thus, the control rods can be lowered to different depths in the core, thereby adjusting the neutron multiplication factor. The emergency rods are designed to shut down the reactor in the event of an emergency.
How is a nuclear reactor started?
We figured out the very principle of operation, but how to start and make the reactor function? Roughly speaking, here it is - a piece of uranium, but after all, a chain reaction does not start in it by itself. The fact is that in nuclear physics there is a concept critical mass .
Critical mass is the mass of fissile material necessary to start a nuclear chain reaction.
With the help of fuel elements and control rods, a critical mass of nuclear fuel is first created in the reactor, and then the reactor is brought to the optimal power level in several stages.
In this article, we have tried to give you a general idea of the structure and principle of operation of a nuclear (atomic) reactor. If you have any questions on the topic or the university asked a problem in nuclear physics, please contact specialists of our company. We, as usual, are ready to help you solve any pressing issue of your studies. In the meantime, we are doing this, your attention is another educational video!
Today we will make a short journey into the world of nuclear physics. The theme of our excursion will be a nuclear reactor. You will learn how it works, what physical principles underlie its operation and where this device is used.
The birth of nuclear energy
The world's first nuclear reactor was built in 1942 in the USA. experimental group of physicists led by Nobel laureate Enrico Fermi. At the same time, they carried out a self-sustaining uranium fission reaction. The atomic genie has been released.
The first Soviet nuclear reactor was launched in 1946, and 8 years later, the world's first nuclear power plant in the city of Obninsk gave current. The chief scientific supervisor of work in the nuclear power industry of the USSR was an outstanding physicist Igor Vasilievich Kurchatov.
Since then, several generations of nuclear reactors have changed, but the main elements of its design have remained unchanged.
Anatomy of a nuclear reactor
This nuclear facility is a thick-walled steel tank with a cylindrical capacity ranging from a few cubic centimeters to many cubic meters.
Inside this cylinder is the holy of holies - reactor core. It is here that the chain reaction of fission of nuclear fuel takes place.
Let's see how this process takes place.
Nuclei heavy elements, in particular Uranium-235 (U-235), under the influence of a small energy push, they are able to fall apart into 2 fragments of approximately equal mass. The causative agent of this process is the neutron.
Fragments are most often barium and krypton nuclei. Each of them carries a positive charge, so the forces of Coulomb repulsion force them to scatter in different directions at a speed of about 1/30 of the speed of light. These fragments are carriers of colossal kinetic energy.
For practical use energy, it is necessary that its release be self-sustaining. Chain reaction, which is in question is all the more interesting because each fission event is accompanied by the emission of new neutrons. For one initial neutron, on average, 2-3 new neutrons arise. The number of fissile uranium nuclei is growing like an avalanche, causing the release of enormous energy. If this process is not controlled, a nuclear explosion will occur. It takes place in .
To control the number of neutrons materials that absorb neutrons are introduced into the system, providing a smooth release of energy. Cadmium or boron are used as neutron absorbers.
How to curb and use the huge kinetic energy of the fragments? For these purposes, a coolant is used, i.e. a special medium, moving in which the fragments are decelerated and heat it up to extremely high temperatures. Such a medium can be ordinary or heavy water, liquid metals (sodium), as well as some gases. In order not to cause the transition of the coolant into a vapor state, supported in the core high pressure(up to 160 atm). For this reason, the walls of the reactor are made of ten-centimeter steel of special grades.
If the neutrons fly out of the nuclear fuel, then the chain reaction can be interrupted. Therefore, there is a critical mass of fissile material, i.e. its minimum mass at which a chain reaction will be maintained. It depends on various parameters, including the presence of a reflector surrounding the reactor core. It serves to prevent leakage of neutrons into environment. The most common material for this structural element is graphite.
The processes taking place in the reactor are accompanied by the release of the most dangerous type of radiation - gamma radiation. To minimize this danger, it provides anti-radiation protection.
How a nuclear reactor works
Nuclear fuel, called fuel elements, is placed in the reactor core. They are tablets formed from a fissile material and packed into thin tubes about 3.5 m long and 10 mm in diameter.
Hundreds of fuel assemblies of the same type are placed in the core, and they become sources of thermal energy released during the chain reaction. The coolant washing the fuel rods forms the first circuit of the reactor.
Heated to high parameters, it is pumped to the steam generator, where it transfers its energy to the water of the secondary circuit, turning it into steam. The resulting steam rotates the turbine generator. The electricity generated by this unit is transferred to the consumer. And the exhaust steam, cooled by water from the cooling pond, in the form of condensate, is returned to the steam generator. The cycle closes.
Such a two-circuit operation nuclear installation excludes the penetration of radiation accompanying the processes occurring in the active zone beyond its limits.
So, a chain of energy transformations takes place in the reactor: the nuclear energy of the fissile material → into the kinetic energy of the fragments → thermal energy coolant → kinetic energy of the turbine → and into electrical energy in the generator.
The inevitable loss of energy leads to the fact that The efficiency of nuclear power plants is relatively low, 33-34%.
In addition to generating electrical energy at nuclear power plants, nuclear reactors are used to produce various radioactive isotopes, for research in many areas of industry, and to study the permissible parameters of industrial reactors. Transport reactors are becoming more and more widespread, providing energy to engines. Vehicle.
Types of nuclear reactors
Typically, nuclear reactors run on uranium U-235. However, its content in natural material is extremely low, only 0.7%. The main mass of natural uranium is the U-238 isotope. A chain reaction in U-235 can only be caused by slow neutrons, and the U-238 isotope is only fissioned by fast neutrons. As a result of nuclear fission, both slow and fast neutrons are born. Fast neutrons, experiencing deceleration in the coolant (water), become slow. But the amount of the U-235 isotope in natural uranium is so small that it is necessary to resort to its enrichment, bringing its concentration to 3-5%. This process is very expensive and economically disadvantageous. In addition, the time of exhaustion of the natural resources of this isotope is estimated at only 100-120 years.
Therefore, in nuclear industry there is a gradual transition to reactors operating on fast neutrons.
Their main difference is that liquid metals are used as a coolant, which do not slow down neutrons, and U-238 is used as nuclear fuel. The nuclei of this isotope pass through a chain of nuclear transformations into Plutonium-239, which is subject to a chain reaction in the same way as U-235. That is, there is a reproduction of nuclear fuel, and in an amount exceeding its consumption.
According to experts Uranium-238 isotope reserves should last for 3,000 years. This time is quite enough for humanity to have enough time to develop other technologies.
Problems in the use of nuclear energy
Along with the obvious advantages of nuclear power, the scale of the problems associated with the operation of nuclear facilities cannot be underestimated.
The first of these is disposal of radioactive waste and dismantled equipment nuclear energy. These elements have an active radiation background, which persists for a long period. For the disposal of these wastes, special lead containers are used. They are supposed to be buried in permafrost areas at a depth of up to 600 meters. Therefore, work is constantly underway to find a way to process radioactive waste, which should solve the problem of disposal and help preserve the ecology of our planet.
The second major problem is ensuring safety during NPP operation. Major accidents like Chernobyl can take away a lot of human lives and decommission vast areas.
The accident at the Japanese nuclear power plant "Fukushima-1" only confirmed the potential danger that manifests itself in the event of an emergency situation at nuclear facilities.
However, the possibilities of nuclear energy are so great that ecological problems fade into the background.
Today, humanity has no other way to satisfy the ever-increasing energy hunger. The basis of the nuclear power industry of the future will probably be "fast" reactors with the function of breeding nuclear fuel.
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Nuclear reactor
A nuclear reactor is a device in which a controlled nuclear chain reaction is carried out, accompanied by the release of energy. The first nuclear reactor was built and launched in December 1942 in the USA under the leadership of E. Fermi. The first reactor built outside the United States was ZEEP, launched in Canada in September 1945. In Europe, the first nuclear reactor was the F-1 installation, which was launched on December 25, 1946 in Moscow under the leadership of I. V. Kurchatov.
By 1978, about a hundred nuclear reactors of various types were already operating in the world. Components of any nuclear reactor are: a core with nuclear fuel, usually surrounded by a neutron reflector, coolant, a chain reaction control system, radiation protection, a system remote control. The reactor vessel is subject to wear (especially under the influence of ionizing radiation). The main characteristic of a nuclear reactor is its power. A power of 1 MW corresponds to a chain reaction in which 3·10 16 fission events occur in 1 sec.
Story
Nazi Germany's "Uranium Project" theoretical group, working in the Kaiser Wilhelm Society, was headed by Weizsäcker, but only formally. Heisenberg, who developed the theoretical foundations of the chain reaction, became the actual leader, while Weizsacker, with a group of participants, focused on creating the "uranium machine" - the first reactor. In the late spring of 1940, one of the scientists of the group - Harteck - conducted the first experiment with an attempt to create a chain reaction using uranium oxide and a solid graphite moderator. However, the available fissile material was not enough to achieve this goal. In 1941, at the University of Leipzig, Döpel, a member of the Heisenberg group, built a stand with a heavy water moderator, in experiments on which, by May 1942, it was possible to achieve the production of neutrons in an amount exceeding their absorption. A full-fledged chain reaction was achieved by German scientists in February 1945 in an experiment conducted in a mine working near Haigerloch. However, a few weeks later, Germany's nuclear program ceased to exist.
The nuclear fission chain reaction (short chain reaction) was first carried out in December 1942. A group of physicists at the University of Chicago, headed by E. Fermi, created the world's first nuclear reactor, called the Chicago Pile-1, CP-1. It consisted of graphite blocks, between which were located balls of natural uranium and its dioxide. Fast neutrons that appear after the fission of 235U nuclei were slowed down by graphite to thermal energies, and then caused new nuclear fissions. Reactors like SR-1, in which the main share of fissions occurs under the action of thermal neutrons, are called thermal neutron reactors. They contain a lot of moderator compared to nuclear fuel.
In the USSR, theoretical and experimental studies of the features of the start-up, operation and control of reactors were carried out by a group of physicists and engineers led by Academician I. V. Kurchatov. The first Soviet F-1 reactor was built at Laboratory No. 2 of the USSR Academy of Sciences (Moscow). This reactor was put into critical condition on December 25, 1946. The F-1 reactor was assembled from graphite blocks and had the shape of a ball with a diameter of about 7.5 m. In the central part of the ball with a diameter of 6 m, uranium rods were placed through holes in the graphite blocks. The F-1 reactor, like the CP-1 reactor, did not have a cooling system, so it operated at very low power levels (fractions of a watt, rarely a few watts). The results of research at the F-1 reactor became the basis for projects of more complex industrial reactors. In 1948, the I-1 reactor (according to other sources it was called A-1) was put into operation for the production of plutonium, and on June 27, 1954, the world's first nuclear power plant with an electric power of 5 MW was put into operation in the city of Obninsk.
Device and principle of operation
Power release mechanism The transformation of a substance is accompanied by the release of free energy only if the substance has a reserve of energies. The latter means that the microparticles of the substance are in a state with a rest energy greater than in another possible state, the transition to which exists. Spontaneous transition is always hindered by an energy barrier, to overcome which the microparticle must receive some amount of energy from the outside - the energy of excitation. The exoenergetic reaction consists in the fact that in the transformation following the excitation, more energy is released than is required to excite the process. There are two ways to overcome the energy barrier: either due to the kinetic energy of the colliding particles, or due to the binding energy of the acceding particle.
If we keep in mind the macroscopic scales of the energy release, then the kinetic energy necessary for the excitation of reactions must have all or at first at least some of the particles of the substance. This can only be achieved by increasing the temperature of the medium to a value at which the energy of thermal motion approaches the value of the energy threshold that limits the course of the process. In the case of molecular transformations, that is, chemical reactions, such an increase is usually hundreds of kelvins, while in the case of nuclear reactions it is at least 107 K due to the very high height of the Coulomb barriers of colliding nuclei. Thermal excitation of nuclear reactions has been carried out in practice only in the synthesis of the lightest nuclei, in which the Coulomb barriers are minimal (thermonuclear fusion).
Excitation by the joining particles does not require a large kinetic energy, and, therefore, does not depend on the temperature of the medium, since it occurs due to unused bonds inherent in the particles of attractive forces. But on the other hand, the particles themselves are necessary to excite the reactions. And if again we have in mind not a separate act of reaction, but the production of energy on a macroscopic scale, then this is possible only when a chain reaction occurs. The latter arises when the particles that excite the reaction reappear as products of an exoenergetic reaction.
Design
Any nuclear reactor consists of the following parts:
- Core with nuclear fuel and moderator;
- Neutron reflector surrounding the core;
- Coolant;
- Chain reaction control system, including emergency protection;
- Radiation protection;
- Remote control system.
iodine pit
Iodine pit - the state of a nuclear reactor after it is turned off, characterized by the accumulation of the short-lived xenon isotope 135Xe. This process leads to the temporary appearance of a significant negative reactivity, which, in turn, makes it impossible to bring the reactor to its design capacity for a certain period (about 1-2 days).
Classification
By appointment
According to the nature of the use of nuclear reactors are divided into:
- Power reactors designed to produce electrical and thermal energy used in the energy sector, as well as for seawater desalination (desalination reactors are also classified as industrial ones). Such reactors are mainly used in nuclear power plants. The thermal power of modern power reactors reaches 5 GW. In a separate group allocate:
- Transport reactors designed to supply energy to vehicle engines. The widest application groups are marine transport reactors used on submarines and various surface vessels, as well as reactors used in space technology.
- Experimental reactors designed to study various physical quantities, the value of which is necessary for the design and operation of nuclear reactors; the power of such reactors does not exceed a few kW.
- Research reactors in which neutron and gamma-ray fluxes generated in the core are used for research in the field of nuclear physics, solid state physics, radiation chemistry, biology, for testing materials intended for operation in intense neutron fluxes (including . parts of nuclear reactors), for the production of isotopes. The power of research reactors does not exceed 100 MW. The released energy is usually not used.
- Industrial (weapons, isotope) reactors used to produce isotopes used in various fields. The most widely used for the production of nuclear weapons materials, such as 239Pu. Also, industrial reactors include reactors used for desalination of sea water.
Often reactors are used to solve two or more various tasks, in which case they are called multipurpose. For example, some power reactors, especially at the dawn of nuclear energy, were intended mainly for experiments. Fast neutron reactors can be both power-generating and producing isotopes at the same time. Industrial reactors, in addition to their main task, often generate electrical and thermal energy.
According to the neutron spectrum
- Thermal (slow) neutron reactor ("thermal reactor")
- Fast neutron reactor ("fast reactor")
- Reactor on intermediate neutrons
- Mixed Spectrum Reactor
By fuel placement
- Heterogeneous reactors, where the fuel is placed in the core discretely in the form of blocks, between which there is a moderator;
- Homogeneous reactors, where the fuel and moderator are a homogeneous mixture (homogeneous system).
In a heterogeneous reactor, the fuel and the moderator can be spaced apart, in particular, in a cavity reactor, the moderator-reflector surrounds the cavity with fuel that does not contain a moderator. From a nuclear-physical point of view, the criterion of homogeneity/heterogeneity is not the design, but the placement of fuel blocks at a distance exceeding the neutron moderation length in a given moderator. For example, so-called “close-lattice” reactors are designed to be homogeneous, although the fuel is usually separated from the moderator in them.
Blocks of nuclear fuel in a heterogeneous reactor are called fuel assemblies (FA), which are placed in the core at the nodes of a regular lattice, forming cells.
By type of fuel
- uranium isotopes 235, 238, 233 (235U, 238U, 233U)
- plutonium isotope 239 (239Pu), also isotopes 239-242Pu as a mixture with 238U (MOX fuel)
- thorium isotope 232 (232Th) (via conversion to 233U)
According to the degree of enrichment:
- natural uranium
- low enriched uranium
- highly enriched uranium
By chemical composition:
- metal U
- UO2 (uranium dioxide)
- UC (uranium carbide), etc.
By type of coolant
- H2O (pressure water reactor)
- Gas, (Graphite-gas reactor)
- Reactor with organic coolant
- Reactor with liquid metal coolant
- Molten salt reactor
- Solid cooled reactor
By type of moderator
- C (Graphite-gas reactor, Graphite-water reactor)
- H2O (Light water reactor, Pressurized water reactor, VVER)
- D2O (Heavy Water Nuclear Reactor, CANDU)
- Be, BeO
- Metal hydrides
- Without moderator (Fast neutron reactor)
By design
- Tank reactors
- Channel reactors
steam generation method
- Reactor with external steam generator (PWR, VVER)
- Boiling reactor
IAEA classification
- PWR (pressurized water reactors) - pressurized water reactor (pressurized water reactor);
- BWR (boiling water reactor) - boiling water reactor;
- FBR (fast breeder reactor) - fast breeder reactor;
- GCR (gas-cooled reactor) - gas-cooled reactor;
- LWGR (light water graphite reactor) - graphite-water reactor
- PHWR (pressurised heavy water reactor) - heavy water reactor
The most common in the world are pressurized water (about 62%) and boiling water (20%) reactors.
Nuclear reactor control
The control of a nuclear reactor is only possible due to the fact that during fission some of the neutrons fly out of the fragments with a delay that can range from several milliseconds to several minutes.
To control the reactor, absorbing rods are used, introduced into the core, made of materials that strongly absorb neutrons (mainly B, Cd, and some others) and / or a solution of boric acid added to the coolant in a certain concentration (boron control). The movement of the rods is controlled by special mechanisms, drives, operating on signals from the operator or equipment automatic regulation neutron flux.
In case of various emergencies in each reactor, an emergency termination of the chain reaction is provided, carried out by dropping all absorbing rods into the core - an emergency protection system.
Residual heat
An important issue directly related to nuclear safety is decay heat. This is specific feature nuclear fuel, which lies in the fact that, after the termination of the fission chain reaction and the thermal inertia common to any energy source, the release of heat in the reactor continues for a long time, which creates a number of technically complex problems.
Decay heat is a consequence of β- and γ-decay of fission products that have accumulated in the fuel during the operation of the reactor. The nuclei of fission products, as a result of decay, pass into a more stable or completely stable state with the release of significant energy.
Although the decay heat release rate rapidly drops to values that are small compared to stationary values, in high-power power reactors it is significant in absolute values. For this reason, decay heat release requires a long time to provide heat removal from the reactor core after it has been shut down. This task requires the presence of cooling systems with reliable power supply in the design of the reactor facility, and also necessitates long-term (within 3-4 years) storage of spent nuclear fuel in storage facilities with a special temperature regime- spent fuel pools, which are usually located in the immediate vicinity of the reactor.
Especially the nuclei of the isotope and most effectively capture slow neutrons. The probability of capture of slow neutrons with subsequent nuclear fission is hundreds of times greater than that of fast ones. Therefore, nuclear reactors fueled by natural uranium use neutron moderators to increase the neutron multiplication factor. Processes in a nuclear reactor are shown schematically in Figure 13.15.
The main elements of a nuclear reactor. Figure 13.16 shows a diagram of a power plant with a nuclear reactor.
The main elements of a nuclear reactor are: nuclear fuel, neutron moderator (heavy or ordinary water, graphite, etc.), coolant for removing energy generated during reactor operation (water, liquid sodium, etc.), and a device for controlling the reaction rate (introduced rods containing cadmium or boron - substances that absorb neutrons well) into the working space of the reactor). From the outside, the reactor is surrounded by a protective shell that traps β-radiation and neutrons. The shell is made of concrete with iron filler.
Fermi Enrico (1901 - 1954)- the great Italian physicist who made a great contribution to the development of modern theoretical and experimental physics. In 1938 he emigrated to the USA. Simultaneously with Dirac, he created the quantum statistical theory of electrons and other particles (Fermi-Dirac statistics). Developed a quantitative theory of p-decay - a prototype of the modern quantum theory of the interaction of elementary particles. He made a number of fundamental discoveries in neutron physics. Under his leadership, in 1942, a controlled nuclear reaction was carried out for the first time.
The best moderator is heavy water (see § 102). Ordinary water itself captures neutrons and turns into heavy water. Graphite, whose nuclei do not absorb neutrons, is also considered a good moderator.
Critical mass. The multiplication factor k can become equal to unity only if the dimensions of the reactor and, accordingly, the mass of uranium exceed certain critical values. Critical mass is the smallest mass of fissile material at which a nuclear chain reaction can still proceed.
At small sizes, the leakage of neutrons through the surface of the reactor core (the volume in which the uranium rods are located) is too great.
With an increase in the size of the system, the number of nuclei involved in fission increases in proportion to the volume, and the number of neutrons lost due to leakage increases in proportion to the surface area. Therefore, by increasing the size of the system, it is possible to achieve the value of the multiplication factor k 1. The system will have critical dimensions if the number of neutrons lost due to capture and leakage is equal to the number of neutrons obtained in the fission process. Critical dimensions and, accordingly, critical mass are determined by the type of nuclear fuel, moderator and design features reactor.
For pure (without moderator) uranium, having the shape of a ball, the critical mass is approximately equal to 50 kg. In this case, the radius of the ball is approximately 9 cm (uranium is a very heavy substance). Using neutron moderators and a beryllium shell reflecting neutrons, it was possible to reduce the critical mass to 250 g.
Kurchatov Igor Vasilyevich (1903-1960)- Soviet physicist and organizer scientific research, three times Hero of Socialist Labor. In 1943 he headed scientific work related to the atomic problem. Under his leadership, the first atomic reactor in Europe (1946) and the first Soviet atomic bomb (1949) were created. Early work relates to the study of ferroelectrics, nuclear reactions caused by neutrons, artificial radioactivity. He discovered the existence of excited states of nuclei with a relatively long "lifetime".
The reactor is controlled by rods containing cadmium or boron. With the rods extended from the reactor core, k > 1, and with the rods fully retracted, k< 1. Вдвигая стержни внутрь активной зоны, можно в любой момент времени приостановить развитие цепной реакции. Управление ядерными реакторами осуществляется дистанционно с помощью ЭВМ.
Reactors on fast neutrons. Reactors have been built that operate without a moderator on fast neutrons. Since the probability of fission caused by fast neutrons is small, such reactors cannot operate on natural uranium.
The reaction can only be maintained in an enriched mixture containing at least 15% of the isotope . The advantage of fast neutron reactors is that their operation produces a significant amount of plutonium, which can then be used as nuclear fuel. These reactors are called breeder reactors because they breed fissile material. Reactors with a breeding ratio of up to 1.5 are being built. This means that up to 1.5 kg of plutonium is obtained in the reactor during the fission of 1 kg of the isotope. In conventional reactors, the breeding ratio is 0.6-0.7.
The first nuclear reactors. For the first time, a valuable nuclear fission reaction of uranium was carried out in the United States by a team of scientists led by Enrico Fermi in December 1942.
In our country, the first nuclear editor was launched on December 25, 1946 by a team of physicists headed by our remarkable scientist Igor Vasilievich Kurchatov. Currently created Various types reactors differing from each other both in power and in their purpose.
In nuclear reactors, in addition to nuclear fuel, there is a neutron moderator and control rods. The released energy is removed by the coolant.
1. What is critical mass!
2. Why in nuclear reactor using a neutron moderator!
