What substances are used in a nuclear reactor. How the reactor is started. By the way of generating steam
: ... rather trivial, but nevertheless I never found the information in a digestible form - how an atomic reactor STARTS to work. About the principle and structure of work, everything has already been chewed 300 times and is understandable, but here is how the fuel is obtained and from what and why it is not so dangerous until it is in the reactor and why it does not react before being immersed in the reactor! - after all, it heats up only inside, nevertheless, before loading, the TVs are cold and everything is fine, so what is the reason for heating the elements is not entirely clear how they are affected, and so on, preferably not scientifically).
It is, of course, difficult to arrange such a topic not "scientifically", but I will try. Let's first figure out what these very fuel elements are.
Nuclear fuel is black tablets with a diameter of about 1 cm. And a height of about 1.5 cm. They contain 2% uranium dioxide 235, and 98% uranium 238, 236, 239. In all cases, for any amount nuclear fuel a nuclear explosion cannot develop, because for the avalanche-like rapid fission reaction characteristic of a nuclear explosion, a concentration of uranium 235 of more than 60% is required.
Two hundred pellets of nuclear fuel are loaded into a tube made of zirconium metal. The length of this tube is 3.5m. diameter 1.35 cm. This tube is called a fuel rod - fuel element. 36 fuel rods are assembled into a cassette (another name is "assembly").
Fuel element arrangement of RBMK reactor: 1 - plug; 2 - tablets of uranium dioxide; 3 - zirconium shell; 4 - spring; 5 - bushing; 6 - tip.
The transformation of a substance is accompanied by the release of free energy only if the substance has a reserve of energy. 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. A spontaneous transition is always hindered by an energy barrier, to overcome which a microparticle must receive from the outside a certain amount of energy - excitation energy. 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 colliding particles, or due to the binding energy of the joining particle.
If we bear in mind the macroscopic scales of energy release, then the kinetic energy necessary for the excitation of reactions must have all or, first, at least some fraction of the particles of the substance. This is achievable only when the temperature of the medium rises to a value at which the energy of the thermal motion approaches the value of the energy threshold, which limits the course of the process. In the case of molecular transformations, that is, chemical reactions, such an increase is usually hundreds of degrees Kelvin, while in the case of nuclear reactions it is a minimum of 107 K due to the very high height of the Coulomb barriers of colliding nuclei. Thermal excitation of nuclear reactions is realized in practice only in the synthesis of the lightest nuclei, for which the Coulomb barriers are minimal (thermonuclear fusion).
Excitation by attaching particles does not require 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 the forces of attraction. But on the other hand, the particles themselves are needed to excite the reactions. And if we again 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.
To control and protect the nuclear reactor, control rods are used, which can be moved along the entire height of the core. The rods are made of substances that strongly absorb neutrons, such as boron or cadmium. With deep insertion of the rods, a chain reaction becomes impossible, since neutrons are strongly absorbed and removed from the reaction zone.
The rods are moved remotely from the control panel. With a small movement of the rods, the chain process will either develop or damp. In this way, the power of the reactor is regulated.
Leningrad NPP, RBMK Reactor
Reactor start-up:
At the initial moment of time after the first loading with fuel, there is no fission chain reaction in the reactor, the reactor is in a subcritical state. The coolant temperature is much lower than the operating temperature.
As we have already mentioned here, for a chain reaction to start, fissile material must form a critical mass - a sufficient amount of spontaneously fissioning matter in a sufficiently small space, a condition under which the number of neutrons released during nuclear fission must be greater than the number of absorbed neutrons. This can be done by increasing the content of uranium-235 (the number of loaded fuel rods), or by slowing down the speed of neutrons so that they do not fly past uranium-235 nuclei.
The reactor is brought to power in several stages. With the help of reactivity control devices, the reactor is transferred to the supercritical state Kef> 1 and the reactor power grows to a level of 1-2% of the nominal. At this stage, the reactor is heated up to the operating parameters of the coolant, and the heating rate is limited. During the heating process, the regulators maintain the power at a constant level. Then the circulation pumps are started and the heat removal system is put into operation. After that, the reactor power can be increased to any level in the range from 2 to 100% of the rated power.
When the reactor heats up, the reactivity changes due to changes in the temperature and density of the core materials. Sometimes, during heating, the relative position of the core and the control elements that enter or leave the core change, causing the effect of reactivity in the absence of active movement of the control elements.
Regulation with solid, moving absorbing elements
In the overwhelming majority of cases, solid movable absorbers are used to quickly change the reactivity. In the RBMK reactor, the control rods contain boron carbide bushings enclosed in an aluminum alloy tube 50 or 70 mm in diameter. Each control rod is placed in a separate channel and cooled with water from the control and protection system (control and protection system) at an average temperature of 50 ° C. According to their purpose, the rods are divided into emergency protection rods, in RBMK there are 24 such rods. Automatic control rods - 12 pieces, Local automatic control rods - 12 pieces, manual control rods -131, and 32 shortened absorber rods (USP). There are 211 rods in total. Moreover, the shortened rods are introduced into the core from the bottom, the rest from the top.
Reactor VVER 1000. 1 - CPS drive; 2 - reactor cover; 3 - reactor vessel; 4 - block of protective pipes (BZT); 5 - mine; 6 - core baffle; 7 - fuel assemblies (FA) and control rods;
Burn-out absorbing elements.
To compensate for excess reactivity after loading fresh fuel, burnable absorbers are often used. The principle of operation of which is that they, like fuel, after capturing a neutron, in the future, cease to absorb neutrons (burn out). Moreover, the rate of loss as a result of absorption of neutrons, nuclei of absorbers, is less than or equal to the rate of loss, as a result of fission, of fuel nuclei. If we load the reactor core with fuel designed to operate for a year, then it is obvious that the number of fissile fuel nuclei at the beginning of operation will be greater than at the end, and we must compensate for the excess reactivity by placing absorbers in the core. If for this purpose control rods are used, then we must constantly move them as the number of fuel nuclei decreases. The use of burnable absorbers reduces the use of moving rods. Nowadays, burnable getters are often mixed directly into pellets when they are made.
Liquid regulation of reactivity.
Such regulation is applied, in particular, during operation of a VVER-type reactor, boric acid H3BO3, containing 10B absorbing neutron nuclei, is introduced into the coolant. By changing the concentration of boric acid in the coolant path, we thereby change the reactivity in the core. IN initial period When the reactor operates when there are many fuel nuclei, the acid concentration is maximum. As the fuel burns out, the acid concentration decreases.
Chain reaction mechanism
Nuclear reactor can work with a given power for a long time only if it has a reactivity margin at the beginning of work. The exception is subcritical reactors with an external source of thermal neutrons. The release of bound reactivity as it decreases due to natural reasons ensures the maintenance of the critical state of the reactor at every moment of its operation. The initial reactivity margin is created by building an active zone with dimensions significantly exceeding the critical ones. To prevent the reactor from becoming supercritical, the k0 of the breeding medium is artificially reduced at the same time. This is achieved by introducing neutron-absorbing substances into the core, which can be removed from the core later. As well as in the elements of regulation of a chain reaction, substances-absorbers are part of the material of the rods of one or another cross section moving along the corresponding channels in the core. But if one or two or several rods are sufficient for regulation, then to compensate for the initial excess of reactivity, the number of rods can reach hundreds. These rods are called compensating rods. Regulating and compensating rods do not necessarily represent different structural elements. A certain number of compensating rods can be control rods, but the functions of both are different. The control rods are designed to maintain a critical state at any time, to stop, start up the reactor, and move from one power level to another. All these operations require small changes in reactivity. Compensating rods are gradually removed from the reactor core, providing a critical state during the entire period of its operation.
Sometimes control rods are not made of absorbent materials, but of fissile material or scatterer material. In thermal reactors, these are mainly neutron absorbers, while there are no effective fast neutron absorbers. Such absorbers as cadmium, hafnium and others strongly absorb only thermal neutrons due to the proximity of the first resonance to the thermal region, and outside the latter they do not differ from other substances in their absorbing properties. An exception is boron, the neutron absorption cross section of which decreases with energy much more slowly than for the indicated substances, according to the l / v law. Therefore, boron absorbs fast neutrons, albeit weakly, but somewhat better than other substances. Only boron, if possible enriched in the isotope 10B, can serve as an absorbing material in a fast neutron reactor. In addition to boron, fast reactors also use fissile materials for control rods. The fissile material compensating rod performs the same function as the neutron absorber rod: it increases the reactivity of the reactor while it naturally decreases. However, unlike an absorber, such a rod is located outside the core at the beginning of the reactor operation, and then is introduced into the core.
Of the scatterer materials in fast reactors, nickel is used, which has a scattering cross section for fast neutrons that is slightly larger than the cross sections for other substances. The scatterer rods are located along the periphery of the core and their immersion into the corresponding channel causes a decrease in neutron leakage from the core and, consequently, an increase in reactivity. In some special cases, the purposes of controlling the chain reaction are the moving parts of the neutron reflectors, which, when moving, change the neutron leakage from the core. Regulating, compensating and emergency rods, together with all equipment ensuring their normal operation, form a reactor control and protection system (CPS).
Emergency protection:
Emergency protection of a nuclear reactor - a set of devices designed to quickly stop a nuclear chain reaction in the reactor core.
Active emergency protection is automatically triggered when one of the parameters of a nuclear reactor reaches a value that can lead to an accident. Such parameters can be: temperature, pressure and flow rate of the coolant, level and rate of power increase.
Actuating elements of emergency protection are, in most cases, rods with a substance that absorbs neutrons well (boron or cadmium). Sometimes, to shut down the reactor, a liquid absorber is injected into the coolant circuit.
In addition to active protection, many modern designs also include passive protection elements. For example, modern versions of VVER reactors include the Emergency Core Cooling System (ECCS) - special tanks with boric acid located above the reactor. In the event of a maximum design basis accident (rupture of the primary cooling loop of the reactor), the contents of these tanks by gravity flow inside the reactor core and the nuclear chain reaction is extinguished by a large amount of boron-containing substance that absorbs neutrons well.
According to the "Rules for Nuclear Safety of Reactor Installations nuclear power plants»At least one of the envisaged shutdown systems of the reactor must fulfill the function of emergency protection (EP). Emergency protection must have at least two independent groups of working bodies. On a signal from the AZ, the working bodies of the AZ should be activated from any working or intermediate positions.
AZ equipment should consist of at least two independent sets.
Each set of core protection equipment must be designed in such a way that in the range of variation of the neutron flux density from 7% to 120% of the nominal, protection is provided:
1. By the density of the neutron flux - at least three independent channels;
2. By the rate of increase of the neutron flux density - at least three independent channels.
Each set of reactor protection equipment must be designed in such a way that emergency protection is provided by at least three independent channels for each technological parameter for which protection is required over the entire range of changes in the technological parameters set in the design of the reactor plant (RP).
The control commands of each set for the AZ actuators must be transmitted through at least two channels. When deactivating one channel in one of the AZ equipment sets without deactivating this kit an alarm should be generated automatically from operation for this channel.
Emergency protection operation must occur at least in the following cases:
1. Upon reaching the core setpoint for the neutron flux density.
2. When the core setpoint is reached by the rate of increase in the neutron flux density.
3. When the voltage disappears in any set of AZ equipment and power supply buses of the CPS that is not taken out of operation.
4. In the event of failure of any two of the three channels of protection in terms of the neutron flux density or the rate of rise of the neutron flux in any set of core protection equipment not taken out of operation.
5. When the AZ settings are reached by the technological parameters, according to which it is necessary to carry out the protection.
6. When initiating the activation of the AZ from the key from the block control point (BPU) or reserve control point (RPU).
Maybe someone will be able to explain even less scientifically how the NPP power unit starts to work? :-)
Remember a topic like The original article is on the site InfoGlaz.rf The link to the article this copy was made from is
The importance of nuclear energy in the modern world
Nuclear power has taken a huge step forward over the past few decades, becoming one of the most important sources of electricity for many countries. At the same time, it should be remembered that behind the development of this branch of the national economy are the enormous efforts of tens of thousands of scientists, engineers and ordinary workers who are doing everything to prevent the "peaceful atom" from becoming a real threat to millions of people. The real core of any nuclear power plant is the nuclear reactor.
The history of the creation of a nuclear reactor
The first such device was built in the midst of World War II in the United States by the famous scientist and engineer E. Fermi. Because of its unusual appearance, which resembled a stack of stacked graphite blocks, this nuclear reactor was named "Chicago stack". It is worth noting that this device worked on uranium, which was placed just between the blocks.
Building a nuclear reactor in the Soviet Union
In our country, increased attention was also paid to nuclear issues. Despite the fact that the main efforts of scientists were concentrated on the military use of the atom, they actively used the results obtained for peaceful purposes. The first nuclear reactor, codenamed F-1, was built by a group of scientists led by the famous physicist I. Kurchatov at the end of December 1946. Its significant drawback was the absence of any kind of cooling system, so the power of the energy released by it was extremely insignificant. At the same time, Soviet researchers completed the work they had begun, resulting in the opening, just eight years later, of the world's first nuclear-fueled power plant in the city of Obninsk.
The principle of operation of the reactor
A nuclear reactor is an extremely complex and dangerous technical device. Its principle of operation is based on the fact that when uranium decays, several neutrons are emitted, which, in turn, knock out elementary particles from neighboring uranium atoms. As a result of this chain reaction, a significant amount of energy is released in the form of heat and gamma rays. At the same time, one should take into account the fact that if this reaction cannot be controlled in any way, then the fission of uranium atoms to the maximum short time can lead to a powerful explosion with undesirable consequences.
In order for the reaction to proceed within a strictly outlined framework, the design of a nuclear reactor is of great importance. Currently, each such structure is a kind of boiler through which the coolant flows. Water is usually used in this capacity, but there are nuclear power plants that use liquid graphite or heavy water. A modern nuclear reactor cannot be imagined without hundreds of special hexagonal cassettes. They contain fuel elements, through the channels of which the coolants flow. This cassette is covered with a special layer that is able to reflect neutrons and thereby slow down the chain reaction
Nuclear reactor and its protection
It has several levels of protection. In addition to the body itself, it is covered with special thermal insulation and biological protection from above. From an engineering point of view, this structure is a powerful reinforced concrete bunker, the doors to which are closed as tightly as possible.
Today we will take a short trip to the world of nuclear physics. The topic of our excursion will be a nuclear reactor. You will find out how it works, what physical principles are at the heart of its work and where this device is used.
The birth of nuclear power
The world's first nuclear reactor was created in 1942 in the USA an experimental group of physicists led by a laureate nobel prize Enrico Fermi. At the same time, they carried out a self-sustaining uranium fission reaction. The atomic genie was 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. Chief scientific supervisor of work in nuclear power 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 nuclear fuel fission takes place.
Let's see how this process takes place.
The nuclei of heavy elements, in particular Uranium-235 (U-235), under the influence of a small energy impulse, they are able to fall apart into 2 fragments of approximately equal mass. The causative agent of this process is a neutron.
The fragments are most often the nuclei of barium and krypton. Each of them carries a positive charge, so the forces of Coulomb repulsion force them to scatter into different sides at a speed of about 1/30 light speed. These fragments are carriers of colossal kinetic energy.
For the practical use of energy, it is necessary that its release be self-sustaining. Chain reaction, which we are talking about is so interesting that each fission act is accompanied by the emission of new neutrons. For one initial neutron, on average 2-3 new neutrons appear. The number of fissile uranium nuclei grows like an avalanche, causing the release of tremendous energy. If this process is not controlled, a nuclear explosion will occur. It takes place in.
To regulate 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 do you harness and harness the enormous 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 heats it 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 to a vapor state, high pressure is maintained in the core (up to 160 atm). For this reason, the walls of the reactor are made of special grades of ten-centimeter steel.
If neutrons fly out of the nuclear fuel, the chain reaction can be interrupted. Therefore, there is a critical mass of fissile matter, i.e. its minimum mass, at which the chain reaction will be maintained. It depends on various parameters, including the presence of a reflector surrounding the reactor core. It serves to prevent neutron leakage into environment... The most common material for this component 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 rods, is placed in the reactor core. They are tablets formed from fissile material and packed in 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 loop 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, converting it into steam. The resulting steam rotates the turbine generator. The electricity generated by this unit is transferred to the consumer. And the waste steam, cooled by water from the cooling pond, in the form of condensate, returns to the steam generator. The cycle is closed.
Such a double-circuit work nuclear facility excludes the penetration of radiation accompanying the processes occurring in the core beyond its limits.
So, a chain of energy transformations occurs in the reactor: the nuclear energy of the fissionable material → into the kinetic energy of the fragments → the thermal energy of the coolant → the kinetic energy of the turbine → and into the electric energy in the generator.
The inevitable loss of energy leads to the fact that Efficiency nuclear power plants relatively small 33-34%.
In addition to generating electrical energy at nuclear power plants, nuclear reactors are used to obtain various radioactive isotopes, for research in many areas of industry, for studying the permissible parameters of industrial reactors. Transport reactors, which provide power to vehicle engines, are becoming more and more widespread.
Types of nuclear reactors
Typically, nuclear reactors run on uranium U-235. However, its content in natural material is extremely small, only 0.7%. The bulk of natural uranium is the isotope U-238. Only slow neutrons can cause a chain reaction in U-235, and the U-238 isotope is split only by fast neutrons. As a result of nuclear fission, both slow and fast neutrons are born. Fast neutrons, undergoing deceleration in the coolant (water), become slow. But the amount of 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 unprofitable. In addition, the time of depletion of the natural resources of this isotope is estimated to be only 100-120 years.
Therefore, in the nuclear industry there is a gradual transition to fast reactors.
Their main difference is that liquid metals are used as a coolant, which do not slow down neutrons, and U-238 is used as a 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, the reproduction of nuclear fuel takes place, and in an amount exceeding its consumption.
According to experts reserves of the isotope Uranium-238 should be enough for 3000 years. This time is quite enough for humanity to have enough time to develop other technologies.
Problems of using nuclear power
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 one is disposal of radioactive waste and dismantled equipment nuclear energy. These elements have an active radiation background, which persists for long period... For the disposal of this waste, special lead containers are used. They are supposed to be buried in permafrost regions 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 no less difficult problem is ensuring safety in the course of NPP operation. Major accidents like the Chernobyl accident can claim many lives and take vast territories out of use.
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 power are so great that environmental problems fade into the background.
Today, humanity has no other way to satisfy the ever-growing energy hunger. The basis of the nuclear power industry of the future is likely to be "fast" reactors with the function of reproducing nuclear fuel.
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Nuclear (nuclear) reactor
Nuclear reactor
Nuclear (nuclear) reactor
- an installation in which a self-sustaining controlled chain nuclear fission reaction is carried out. Nuclear reactors are used for nuclear power and research purposes. The main part of the reactor is its active zone, where nuclear fission occurs and nuclear energy is released. The core, which is usually in the form of a cylinder with a volume from fractions of a liter to many cubic meters, contains fissile material (nuclear fuel) in an amount exceeding the critical mass. Nuclear fuel (uranium, plutonium) is placed, as a rule, inside fuel elements (fuel rods), the number of which in the core can reach tens of thousands. Fuel rods are grouped into packages of several tens or hundreds of pieces. The core in most cases is a set of fuel rods immersed in a moderating medium (moderator) - a substance, due to elastic collisions with the atoms of which the energy of neutrons, causing and accompanying fission, is reduced to energies of thermal equilibrium with the medium. Such "thermal" neutrons have an increased ability to induce fission. Water (including heavy water, D 2 O) and graphite are usually used as moderators. The reactor core is surrounded by a reflector made of materials capable of scattering neutrons well. This layer returns neutrons emitted from the core back to this zone, increasing the rate of the chain reaction and decreasing the critical mass. Radiation biological shielding made of concrete and other materials is placed around the reflector to reduce radiation outside the reactor to an acceptable level.
In the core, tremendous energy is released in the form of heat as a result of fission. It is removed from the core with the help of gas, water or other substance (coolant), which is constantly pumped through the core, washing the fuel rods. This heat can be used to create hot steam that turns the power plant's turbine.
To control the rate of the fission chain reaction, control rods made of materials that strongly absorb neutrons are used. Their introduction into the core reduces the speed of the chain reaction and, if necessary, completely stops it, despite the fact that the mass of nuclear fuel exceeds the critical one. As the control rods are removed from the core, the neutron absorption decreases and the chain reaction can be brought to a self-sustaining stage.
The first reactor was launched in the USA in 1942. In Europe, the first reactor was launched in 1946 in the USSR.
Especially the nuclei of the isotope and, most effectively capture slow neutrons. The probability of capturing slow neutrons with subsequent fission of nuclei is hundreds of times greater than that of fast ones. Therefore, in nuclear reactors fueled by natural uranium, neutron moderators are used to increase the neutron multiplication factor. The 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 power plant with a nuclear reactor.
The main elements of a nuclear reactor are: nuclear fuel, a neutron moderator (heavy or ordinary water, graphite, etc.), a coolant for removing the energy generated during the operation of the reactor (water, liquid sodium, etc.), and a device for regulating the reaction rate (introduced rods containing cadmium or boron - substances that absorb neutrons well from the neutrons) into the working space of the reactor. Outside, the reactor is surrounded by a protective shell that blocks γ-radiation and neutrons. The shell is made of iron-filled concrete.
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 United States. 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, the nuclei of which does 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 some critical values. The critical mass is the smallest mass of fissile matter at which a nuclear chain reaction can still proceed.
At small sizes, the neutron leakage through the surface of the reactor core (the volume in which the rods with uranium are located) is too large.
With an increase in the size of the system, the number of nuclei participating 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. The critical dimensions and, accordingly, the critical mass are determined by the type of nuclear fuel, moderator and design features of the reactor.
For pure (without moderator) uranium in the shape of a sphere, the critical mass is approximately 50 kg. In this case, the radius of the sphere is approximately 9 cm (uranium is a very heavy substance). By using neutron moderators and a neutron-reflecting beryllium shell, it was possible to reduce the critical mass to 250 g.
Kurchatov Igor Vasilievich (1903-1960)- Soviet physicist and organizer of scientific research, three times Hero of Socialist Labor. In 1943 he headed scientific work related to the atomic problem. Under his leadership, the first nuclear reactor in Europe (1946) and the first Soviet atomic bomb (1949). Early work relate to the study of ferroelectrics, nuclear reactions caused by neutrons, artificial radioactivity. Discovered the existence of excited states of nuclei with a relatively long "lifetime".
The reactor is controlled by rods containing cadmium or boron. When the rods are extended from the reactor core k> 1, and when the rods are fully extended, k< 1. Вдвигая стержни внутрь активной зоны, можно в любой момент времени приостановить развитие цепной реакции. Управление ядерными реакторами осуществляется дистанционно с помощью ЭВМ.
Fast reactors. Reactors operating without a moderator on fast neutrons have been built. Since the probability of fission caused by fast neutrons is small, such reactors cannot operate on natural uranium.
The reaction can be maintained only in an enriched mixture containing at least 15% of the isotope. The advantage of fast reactors is that they generate a significant amount of plutonium, which can then be used as nuclear fuel. These reactors are called breeder reactors because they reproduce fissile material. Reactors with a breeding ratio of up to 1.5 are under construction. This means that fission of 1 kg of the isotope in the reactor produces up to 1.5 kg of plutonium. In conventional reactors, the breeding ratio is 0.6-0.7.
First nuclear reactors. For the first time the valuable nuclear fission reaction of uranium was carried out in the USA 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 Vasil'evich Kurchatov. Currently, various types of reactors have been created, 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 heat carrier.
1. What is critical mass!
2. Why is a neutron moderator used in an atomic reactor!
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