Topic: Physical foundations of nuclear energy. Nuclear reactor.
Lesson Objectives: updating existing knowledge; continue the formation of concepts: fission of uranium nuclei, chain nuclear reaction, conditions of its flow, critical mass; introduce new concepts: nuclear reactor, basic elements nuclear reactor, the design of a nuclear reactor and the principle of its operation, the control of a nuclear reaction, the classification of nuclear reactors and their use; to continue the formation of skills to observe and draw conclusions, as well as to develop intellectual ability and curiosity of students; to continue the education of attitude towards physics as an experimental science; to cultivate a conscientious attitude to work, discipline, a positive attitude to knowledge.
Lesson type: learning new material.
During the classes
1. Organizational moment.
Today in the lesson we will repeat the fission of uranium nuclei, a nuclear chain reaction, the conditions for its occurrence, the critical mass, we will learn what a nuclear reactor is, the main elements of a nuclear reactor, the design of a nuclear reactor and the principle of its operation, control of a nuclear reaction, the classification of nuclear reactors and their use.
2. Checking the studied material.
Mechanism of fission of uranium nuclei.
Describe the mechanism of a nuclear chain reaction.
Give an example of a nuclear fission reaction of the uranium nucleus.
What is called critical mass?
How does a chain reaction proceed in uranium if its mass is less than critical, more than critical?
What is the critical mass of uranium 295, is it possible to reduce the critical mass?
How can you change the course of a nuclear chain reaction?
What is the purpose of slowing down fast neutrons?
What substances are used as moderators?
3. Explanation of new material.
: What is main part any nuclear power plant? ( nuclear reactor)
Well done. So, guys, now let's dwell on this issue in more detail.
History reference.
Igor Vasilyevich Kurchatov is an outstanding Soviet physicist, academician, founder and first director of the Institute of Atomic Energy from 1943 to 1960, chief scientific leader of the atomic problem in the USSR, one of the founders of the use of nuclear energy for peaceful purposes. Academician of the Academy of Sciences of the USSR (1943). The first Soviet atomic bomb was tested in 1949. Four years later, the world's first hydrogen bomb was successfully tested. And in 1949, Igor Vasilievich Kurchatov began work on the project of a nuclear power plant. The nuclear power plant is a messenger of the peaceful use of atomic energy. The project was successfully completed: on July 27, 1954, our nuclear power plant became the first in the world! Kurchatov rejoiced and had fun like a child!
Definition of a nuclear reactor.
A nuclear reactor is a device in which a controlled chain reaction of fission of some heavy nuclei is carried out and maintained.
The first nuclear reactor was built in 1942 in the USA under the leadership of E. Fermi. In our country, the first reactor was built in 1946 under the leadership of IV Kurchatov.
The main elements of a nuclear reactor are:
nuclear fuel (uranium 235, uranium 238, plutonium 239);
neutron moderator (heavy water, graphite, etc.);
coolant for the output of energy generated during the operation of the reactor (water, liquid sodium, etc.);
Control rods (boron, cadmium) - strongly absorbing neutrons
Protective shell that delays radiation (concrete with iron filler).
Operating principle nuclear reactor
Nuclear fuel is located in the active zone in the form of vertical rods called fuel elements (TVEL). Fuel rods are designed to control the power of the reactor.
The mass of each fuel rod is much less than the critical mass, so a chain reaction cannot occur in one rod. It begins after immersion in the active zone of all uranium rods.
The active zone is surrounded by a layer of a substance that reflects neutrons (reflector) and a protective shell of concrete that traps neutrons and other particles.
Heat removal from fuel cells. The coolant - water washes the rod, heated to 300 ° C at high pressure, enters the heat exchangers.
The role of the heat exchanger - water heated to 300 ° C, gives off heat to ordinary water, turns into steam.
Nuclear reaction control
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.
Reactor on slow neutrons.
The most efficient fission of uranium-235 nuclei occurs under the action of slow neutrons. Such reactors are called slow neutron reactors. The secondary neutrons produced in the fission reaction are fast. In order for their subsequent interaction with uranium-235 nuclei in a chain reaction to be most effective, they are slowed down by introducing a moderator into the core - a substance that reduces the kinetic energy of neutrons.
Fast neutron reactor.
Fast neutron reactors cannot operate on natural uranium. The reaction can only be maintained in an enriched mixture containing at least 15% of the uranium 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.
Homogeneous and heterogeneous reactors.
Nuclear reactors, depending on the mutual arrangement of fuel and moderator, are divided into homogeneous and heterogeneous. In a homogeneous reactor, the core is a homogeneous mass of fuel, moderator and coolant in the form of a solution, mixture or melt. A reactor is called heterogeneous, in which fuel in the form of blocks or fuel assemblies is placed in the moderator, forming a regular geometric lattice in it.
Converting the internal energy of atomic nuclei into electrical energy.
A nuclear reactor is the main element of a nuclear power plant (NPP), which converts thermal nuclear energy into electrical energy. Energy conversion occurs according to the following scheme:
internal energy of uranium nuclei -
kinetic energy of neutrons and fragments of nuclei -
internal energy of water -
steam internal energy -
steam kinetic energy -
kinetic energy of the turbine rotor and generator rotor -
Electric Energy.
Use of nuclear reactors.
Depending on the purpose, nuclear reactors are power, converters and breeders, research and multi-purpose, transport and industrial.
Nuclear power reactors are used to generate electricity at nuclear power plants, in ship power plants, nuclear combined heat and power plants, as well as at nuclear power plants.
Reactors designed to produce secondary nuclear fuel from natural uranium and thorium are called converters or breeders. In the reactor-converter secondary nuclear fuel is formed less than originally consumed.
In the breeder reactor, expanded reproduction of nuclear fuel is carried out, i.e. it turns out more than was spent.
Research reactors are used to study the processes of interaction of neutrons with matter, study the behavior of reactor materials in intense fields of neutron and gamma radiation, radiochemical and biological research, production of isotopes, experimental research in the physics of nuclear reactors.
Reactors have different power, stationary or pulsed mode of operation. Multi-purpose reactors are reactors that serve multiple purposes, such as power generation and nuclear fuel production.
Ecological disasters at nuclear power plants
1957 - accident in the UK
1966 - Partial core meltdown after reactor cooling failure near Detroit.
1971 - A lot of polluted water went into the US river
1979 - the largest accident in the USA
1982 - release of radioactive steam into the atmosphere
1983 - a terrible accident in Canada (radioactive water flowed out for 20 minutes - a ton per minute)
1986 - accident in the UK
1986 - accident in Germany
1986 - Chernobyl nuclear power plant
1988 - fire at a nuclear power plant in Japan
Modern nuclear power plants are equipped with a PC, and earlier, even after the accident, the reactors continued to operate, since there was no automatic system shutdowns.
4. Fixing the material.
What is a nuclear reactor?
What is nuclear fuel in a reactor?
What substance serves as a neutron moderator in a nuclear reactor?
What is the purpose of a neutron moderator?
What are control rods for? How are they used?
What is used as a coolant in nuclear reactors?
Why is it necessary that the mass of each uranium rod be less than the critical mass?
5. Test execution.
What particles are involved in the fission of uranium nuclei?
A. protons;
B. neutrons;
B. electrons;
G. helium nuclei.
What mass of uranium is critical?
A. the largest at which a chain reaction is possible;
B. any mass;
V. the smallest at which a chain reaction is possible;
D. the mass at which the reaction will stop.
What is the approximate critical mass of uranium 235?
A. 9 kg;
B. 20 kg;
B. 50 kg;
G. 90 kg.
Which of the following substances can be used in nuclear reactors as neutron moderators?
A. graphite;
B. cadmium;
B. heavy water;
G. bor.
For a nuclear chain reaction to occur at a nuclear power plant, it is necessary that the neutron multiplication factor be:
A. is equal to 1;
B. more than 1;
V. less than 1.
Regulation of the fission rate of nuclei of heavy atoms in nuclear reactors is carried out:
A. due to the absorption of neutrons when lowering the rods with an absorber;
B. due to an increase in heat removal with an increase in the speed of the coolant;
B. by increasing the supply of electricity to consumers;
G. by reducing the mass of nuclear fuel in the core when removing the fuel rods.
What energy transformations take place in a nuclear reactor?
A. the internal energy of atomic nuclei is converted into light energy;
B. the internal energy of atomic nuclei is converted into mechanical energy;
B. the internal energy of atomic nuclei is converted into electrical energy;
G. there is no correct answer among the answers.
In 1946, the first nuclear reactor was built in the Soviet Union. Who was the leader of this project?
A. S. Korolev;
B. I. Kurchatov;
V. D. Sakharov;
G. A. Prokhorov.
What way do you consider the most appropriate for increasing the reliability of nuclear power plants and preventing contamination of the external environment?
A. development of reactors capable of automatically cooling the reactor core, regardless of the will of the operator;
B. increasing the literacy of NPP operation, the level of professional training of NPP operators;
B. development of highly efficient technologies for dismantling nuclear power plants and processing radioactive waste;
D. the location of the reactors deep underground;
E. refusal to build and operate nuclear power plants.
What sources of pollution environment associated with the operation of nuclear power plants?
A. uranium industry;
B. nuclear reactors of various types;
B. radiochemical industry;
D. places of processing and disposal of radioactive waste;
E. the use of radionuclides in the national economy; E. nuclear explosions.
Answers: 1 B; 2 V; 3 V; 4 A, B; 5 A; 6 A; 7 V;. 8 B; 9 B. V; 10 A, B, C, D, F.
6. The results of the lesson.
What new did you learn at the lesson today?
What did you like about the lesson?
What are the questions?
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. The critical dimensions and, accordingly, the critical mass are determined by the type of nuclear fuel, the moderator, and the design features of the 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 European atomic reactor(1946) and the first Soviet atomic bomb (1949). 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. At present, 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 coolant.
1. What is critical mass!
2. Why is a neutron moderator used in a nuclear reactor!
For ordinary person modern high-tech devices are so mysterious and mysterious that it is just right to worship them, as the ancients worshiped lightning. School physics lessons, replete with mathematical calculations, do not solve the problem. But it’s interesting to tell even about a nuclear reactor, the principle of operation of which is clear even to a teenager.
How does a nuclear reactor work?
The principle of operation of this high-tech device is as follows:
- When a neutron is absorbed, nuclear fuel (most often this uranium-235 or plutonium-239) the division of the atomic nucleus occurs;
- Kinetic energy, gamma radiation and free neutrons are released;
- Kinetic energy is converted into thermal energy (when nuclei collide with surrounding atoms), gamma radiation is absorbed by the reactor itself and is also converted into heat;
- Some of the generated neutrons are absorbed by the fuel atoms, which causes a chain reaction. To control it, neutron absorbers and moderators are used;
- With the help of a coolant (water, gas or liquid sodium), heat is removed from the reaction site;
- Pressurized steam from heated water is used to drive steam turbines;
- With a generator mechanical energy turbine rotation is converted into alternating electric current.
Approaches to classification
There can be many reasons for the typology of reactors:
- By type of nuclear reaction. Fission (all commercial installations) or fusion (thermonuclear power, is widespread only in some research institutes);
- By coolant. In the vast majority of cases, water (boiling or heavy) is used for this purpose. Alternative solutions are sometimes used: liquid metal (sodium, lead-bismuth alloy, mercury), gas (helium, carbon dioxide or nitrogen), molten salt (fluoride salts);
- By generation. The first is the early prototypes, which didn't make any commercial sense. The second is the majority of currently used nuclear power plants that were built before 1996. The third generation differs from the previous one only in minor improvements. Work on the fourth generation is still underway;
- According to aggregate state fuel (gas still exists only on paper);
- By purpose of use(for the production of electricity, engine start, hydrogen production, desalination, transmutation of elements, obtaining neural radiation, theoretical and investigative purposes).
Nuclear reactor device
The main components of reactors in most power plants are:
- Nuclear fuel - a substance that is necessary for the production of heat for power turbines (usually low enriched uranium);
- The active zone of the nuclear reactor - this is where the nuclear reaction takes place;
- Neutron moderator - reduces the speed of fast neutrons, turning them into thermal neutrons;
- Starting neutron source - used for reliable and stable launch of a nuclear reaction;
- Neutron absorber - available in some power plants to reduce the high reactivity of fresh fuel;
- Neutron howitzer - used to re-initiate a reaction after being turned off;
- Coolant (purified water);
- Control rods - to control the rate of fission of uranium or plutonium nuclei;
- Water pump - pumps water to the steam boiler;
- Steam turbine - converts the thermal energy of steam into rotational mechanical energy;
- Cooling tower - a device for removing excess heat into the atmosphere;
- System for receiving and storing radioactive waste;
- Safety systems (emergency diesel generators, devices for emergency core cooling).
How the latest models work
The latest 4th generation of reactors will be available for commercial operation no earlier than 2030. Currently, the principle and arrangement of their work are at the development stage. According to current data, these modifications will differ from existing models in such benefits:
- Rapid gas cooling system. It is assumed that helium will be used as a coolant. According to project documentation, thus it is possible to cool reactors with a temperature of 850 °C. To work with such high temperatures specific raw materials will also be required: composite ceramic materials and actinide compounds;
- It is possible to use lead or a lead-bismuth alloy as a primary coolant. These materials have a low neutron absorption and a relatively low melting point;
- Also, a mixture of molten salts can be used as the main coolant. Thus, it will be possible to work at higher temperatures than modern water-cooled counterparts.
Natural analogues in nature
The nuclear reactor is perceived in the public mind solely as a product high technology. However, in fact the first the device is of natural origin. It was discovered in the Oklo region, in the Central African state of Gabon:
- The reactor was formed due to the flooding of uranium rocks by groundwater. They acted as neutron moderators;
- The thermal energy released during the decay of uranium turns water into steam, and the chain reaction stops;
- After the coolant temperature drops, everything repeats again;
- If the liquid had not boiled off and stopped the course of the reaction, humanity would have faced a new natural disaster;
- Self-sustaining nuclear fission began in this reactor about one and a half billion years ago. During this time, about 0.1 million watts of output power was allocated;
- Such a wonder of the world on Earth is the only one known. The appearance of new ones is impossible: the proportion of uranium-235 in natural raw materials is much lower than the level necessary to maintain a chain reaction.
How many nuclear reactors are in South Korea?
Poor in natural resources, but industrialized and overpopulated, the Republic of Korea is in dire need of energy. Against the backdrop of Germany's rejection of the peaceful atom, this country has high hopes for curbing nuclear technology:
- It is planned that by 2035 the share of electricity generated by nuclear power plants will reach 60%, and the total production - more than 40 gigawatts;
- The country does not have atomic weapons, but research in nuclear physics is ongoing. Korean scientists have developed designs for modern reactors: modular, hydrogen, with liquid metal, etc.;
- The success of local researchers allows you to sell technology abroad. It is expected that in the next 15-20 years the country will export 80 such units;
- But as of today, most of the nuclear power plants have been built with the assistance of American or French scientists;
- The number of operating stations is relatively small (only four), but each of them has a significant number of reactors - 40 in total, and this figure will grow.
When bombarded with neutrons, nuclear fuel enters a chain reaction, as a result of which great amount heat. The water in the system takes this heat and turns it into steam, which turns turbines that produce electricity. Here is a simple diagram of the operation of an atomic reactor, the most powerful source of energy on Earth.
Video: how nuclear reactors work
In this video, nuclear physicist Vladimir Chaikin will tell you how electricity is generated in nuclear reactors, their detailed structure:
Nuclear (atomic) reactor
nuclear reactor
Nuclear (atomic) reactor
- a facility in which a self-sustaining controlled nuclear fission chain reaction is carried out. Nuclear reactors are used in the nuclear power industry and for research purposes. The main part of the reactor is its active zone, where nuclear fission takes place and nuclear energy is released. The active zone, which usually has the shape 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 (FE elements), the number of which in the core can reach tens of thousands. TVELs are grouped into packages of several tens or hundreds of pieces. The core in most cases is a set of fuel elements immersed in a moderating medium (moderator) - a substance, due to elastic collisions with atoms of which the energy of neutrons that cause and accompany fission is reduced to the energies of thermal equilibrium with the medium. Such "thermal" neutrons have an increased ability to cause fission. As a moderator, water (including heavy, D 2 O) and graphite are usually used. The reactor core is surrounded by a reflector made of materials that can scatter neutrons well. This layer returns the neutrons emitted from the core back to this zone, increasing the rate of the chain reaction and reducing 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 active zone, as a result of fission, huge energy is released in the form of heat. It is removed from the core with the help of gas, water or another substance (coolant), which is constantly pumped through the core, washing the fuel elements. This heat can be used to create hot steam that turns a turbine in a power plant.
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 rate 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 absorption of neutrons decreases, and the chain reaction can be brought to the stage of self-sustaining.
The first reactor was launched in the USA in 1942. In Europe, the first reactor was launched in 1946 in the USSR.
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