The mechanism of nuclear fission of a uranium atom. Nuclear fission reactions
Nuclear reactions. The interaction of a particle with an atomic nucleus, leading to the transformation of this nucleus into a new nucleus with the release of secondary particles or gamma rays, is called a nuclear reaction.
First nuclear reaction was carried out by Rutherford in 1919. He discovered that collisions of alpha particles with the nuclei of nitrogen atoms produce rapidly moving protons. This meant that the nucleus of the nitrogen isotope, as a result of a collision with an alpha particle, was transformed into the nucleus of the oxygen isotope:
.
Nuclear reactions can occur with the release or absorption of energy. Using the law of the relationship between mass and energy, the energy output of a nuclear reaction can be determined by finding the difference in the masses of the particles entering the reaction and the reaction products:
Chain reaction fission of uranium nuclei. Among various nuclear reactions, it is especially important in the life of modern human society have chain reactions of fission of some heavy nuclei.
The fission reaction of uranium nuclei when bombarded with neutrons was discovered in 1939. As a result of experimental and theoretical studies carried out by E. Fermi, I. Joliot-Curie, O. Hahn, F. Strassmann, L. Meitner, O. Frisch, F. Joliot-Curie, it was found that when one neutron hits a uranium nucleus, the nucleus is divided into two or three parts.
The fission of one uranium nucleus releases about 200 MeV of energy. The kinetic energy of the movement of fragment nuclei accounts for approximately 165 MeV, the rest of the energy is carried away by gamma quanta.
Knowing the energy released during the fission of one uranium nucleus, it can be calculated that the energy output from the fission of all nuclei of 1 kg of uranium is 80 thousand billion joules. This is several million times more than what is released when burning 1 kg of coal or oil. Therefore, a search was made for ways to release nuclear energy in significant quantities for use for practical purposes.
The first suggestion about the possibility of chain nuclear reactions was made by F. Joliot-Curie in 1934. In 1939, he, together with H. Halban and L. Kowarski, experimentally discovered that during the fission of a uranium nucleus, in addition to nuclear fragments, 2 -3 free neutrons. At favorable conditions these neutrons can hit other uranium nuclei and cause them to fission. When three uranium nuclei fission, 6-9 new neutrons should be released, they will fall into new uranium nuclei, etc. A diagram of the development of a chain reaction of fission of uranium nuclei is presented in Figure 316.
Rice. 316
The practical implementation of chain reactions is not like that simple task how it looks on the diagram. Neutrons released during the fission of uranium nuclei are capable of causing the fission of only nuclei of the uranium isotope with a mass number of 235, but their energy is insufficient to destroy the nuclei of a uranium isotope with a mass number of 238. In natural uranium, the share of uranium with mass number 238 is 99.8%, and the share of uranium with mass number 235 is only 0.7%. Therefore the first possible way implementation of a fission chain reaction is associated with the separation of uranium isotopes and the production of pure form in sufficiently large quantities of the isotope. A necessary condition for a chain reaction to occur is the presence of sufficient large quantity uranium, since in a small sample most of the neutrons fly through the sample without hitting any nucleus. The minimum mass of uranium in which a chain reaction can occur is called the critical mass. The critical mass for uranium-235 is several tens of kilograms.
The simplest way to carry out a chain reaction in uranium-235 is the following: two pieces of uranium metal are made, each with a mass slightly less than the critical one. A chain reaction cannot occur in each of them separately. When these pieces are quickly connected, a chain reaction develops and colossal energy is released. The temperature of uranium reaches millions of degrees, the uranium itself and any other substances nearby turn into steam. The hot gaseous ball expands rapidly, burning and destroying everything in its path. This is how a nuclear explosion occurs.
It is very difficult to use the energy of a nuclear explosion for peaceful purposes, since the release of energy is uncontrollable. Controlled chain reactions of fission of uranium nuclei are carried out in nuclear reactors.
Nuclear reactor. The first nuclear reactors were slow neutron reactors (Fig. 317). Most of the neutrons released during the fission of uranium nuclei have an energy of 1-2 MeV. Their velocities are approximately 107 m/s, which is why they are called fast neutrons. At such energies, neutrons interact with uranium and uranium nuclei with approximately equal efficiency. And since there are 140 times more uranium nuclei in natural uranium than uranium nuclei, most of these neutrons are absorbed by uranium nuclei and a chain reaction does not develop. Neutrons moving at speeds close to the speed of thermal motion (about 2·10 3 m/s) are called slow or thermal. Slow neutrons interact well with uranium-235 nuclei and are absorbed by them 500 times more efficiently than fast neutrons. Therefore, when natural uranium is irradiated with slow neutrons, most of them are absorbed not in the nuclei of uranium-238, but in the nuclei of uranium-235 and cause their fission. Consequently, for a chain reaction to develop in natural uranium, neutron velocities must be reduced to thermal ones.
Rice. 317
Neutrons slow down as a result of collisions with atomic nuclei of the medium in which they move. To slow down neutrons in a reactor, a special substance called a moderator is used. The nuclei of atoms of the moderator substance must have a relatively small mass, since when colliding with a light nucleus, a neutron loses more energy than when colliding with a heavy one. The most common moderators are ordinary water and graphite.
The space in which the chain reaction occurs is called the reactor core. To reduce neutron leakage, the reactor core is surrounded by a neutron reflector, which rejects a significant portion of the escaping neutrons into the core. The same substance that serves as a moderator is usually used as a reflector.
The energy released during reactor operation is removed using a coolant. Only liquids and gases that do not have the ability to absorb neutrons can be used as a coolant. Ordinary water is widely used as a coolant; carbon dioxide and even liquid metallic sodium are sometimes used.
The reactor is controlled using special control (or control) rods inserted into the reactor core. Control rods are made of boron or cadmium compounds, which absorb thermal neutrons with very high efficiency. Before the reactor starts operating, they are completely introduced into its core. By absorbing a significant portion of neutrons, they make it impossible for a chain reaction to develop. To start the reactor, the control rods are gradually removed from the core until the energy release reaches a predetermined level. When the power increases above the set level, automatic machines are switched on, plunging the control rods deep into the core.
Nuclear energy. Nuclear energy was put to the service of peace for the first time in our country. The first organizer and leader of work on atomic science and technology in the USSR was Academician Igor Vasilyevich Kurchatov (1903-1960).
Currently, the largest in the USSR and Europe is the Leningrad NPP named after. IN AND. Lenin has a capacity of 4000 MW, i.e. 800 times the power of the first nuclear power plant.
The cost of electricity generated at large nuclear power plants is lower than the cost of electricity generated at thermal power plants. Therefore, nuclear energy is developing at an accelerated pace.
Nuclear reactors are used as power plants on naval ships. The world's first peaceful ship with a nuclear power plant, the nuclear-powered icebreaker Lenin, was built in the Soviet Union in 1959.
The Soviet nuclear-powered icebreaker Arktika, built in 1975, became the world's first surface ship to reach the North Pole.
Thermonuclear reaction. Nuclear energy is released not only in nuclear reactions of fission of heavy nuclei, but also in reactions of combination of light atomic nuclei.
To connect like-charged protons, it is necessary to overcome the Coulomb repulsive forces, which is possible at sufficiently high velocities of colliding particles. The necessary conditions for the synthesis of helium nuclei from protons exist in the interior of stars. On Earth, thermonuclear fusion reaction was carried out during experimental thermonuclear explosions.
The synthesis of helium from the light isotope of hydrogen occurs at a temperature of about 108 K, and for the synthesis of helium from the heavy isotopes of hydrogen - deuterium and tritium - according to the scheme
requires heating to approximately 5 10 7 K.
When 1 g of helium is synthesized from deuterium and tritium, the energy released is 4.2·10 11 J. This energy is released when 10 tons of diesel fuel are burned.
Hydrogen reserves on Earth are practically inexhaustible, so the use of thermonuclear fusion energy for peaceful purposes is one of the most important tasks modern science and technology.
The controlled thermonuclear reaction of synthesis of helium from heavy isotopes of hydrogen by heating is supposed to be carried out by passing electric current through plasma. A magnetic field is used to keep the heated plasma from contacting the chamber walls. At the Tokamak-10 experimental installation, Soviet physicists managed to heat the plasma to a temperature of 13 million degrees. Up to more high temperatures hydrogen can be heated using laser radiation. To do this, light beams from several lasers must be focused on a glass ball containing a mixture of heavy isotopes of deuterium and tritium. In experiments on laser installations, plasma with a temperature of several tens of millions of degrees has already been obtained.
Uranium nuclei fission occurs in the following way: First, a neutron hits the nucleus, like a bullet hitting an apple. In the case of an apple, a bullet would either make a hole in it or blow it into pieces. When a neutron enters the nucleus, it is captured by nuclear forces. The neutron is known to be neutral, so it is not repelled by electrostatic forces.
How does a uranium nucleus fission occur?
So, having entered the nucleus, the neutron disturbs the equilibrium, and the nucleus is excited. It stretches out to the sides like a dumbbell or an infinity sign: ∞ . Nuclear forces, as is known, act at a distance commensurate with the size of the particles. When the nucleus is stretched, the effect of nuclear forces becomes insignificant for the outer particles of the “dumbbell,” while electrical forces act very powerfully at such a distance, and the nucleus is simply torn into two parts. In this case, two or three more neutrons are emitted.
Nuclear fragments and released neutrons scatter at great speed in different sides. The fragments are slowed down quite quickly by the environment, but their kinetic energy is enormous. It is converted into internal energy of the environment, which heats up. In this case, the amount of energy released is enormous. The energy obtained from the complete fission of one gram of uranium is approximately equal to the energy obtained from burning 2.5 tons of oil.
Chain reaction of fission of several nuclei
We looked at the fission of one uranium nucleus. During fission, several (usually two or three) neutrons are released. They fly apart at great speed and can easily get into the nuclei of other atoms, causing a fission reaction in them. This is a chain reaction.
That is, the neutrons obtained as a result of nuclear fission excite and force other nuclei to fission, which in turn themselves emit neutrons, which continue to stimulate further fission. And so on until fission of all uranium nuclei in the immediate vicinity occurs.
In this case, a chain reaction can occur avalanche-like, for example, in the event of an atomic bomb explosion. The number of nuclear fissions increases in geometric progression in a short period of time. However, a chain reaction can also occur with attenuation.
The fact is that not all neutrons meet nuclei on their way, which they induce to fission. As we remember, inside a substance the main volume is occupied by the void between the particles. Therefore, some neutrons fly through all matter without colliding with anything along the way. And if the number of nuclear fissions decreases over time, then the reaction gradually fades.
Nuclear reactions and critical mass of uranium
What determines the type of reaction? From the mass of uranium. The greater the mass, the more particles the flying neutron will meet on its path and the greater the chance of getting into the nucleus. Therefore, a “critical mass” of uranium is distinguished - this is the minimum mass at which a chain reaction is possible.
The number of neutrons produced will be equal to the number of neutrons that fly out. And the reaction will proceed at approximately the same speed until the entire volume of the substance is produced. This is used in practice in nuclear power plants and is called a controlled nuclear reaction.
Nuclear chain reaction. As a result of experiments on neutron irradiation of uranium, it was found that under the influence of neutrons, uranium nuclei are divided into two nuclei (fragments) of approximately half the mass and charge; this process is accompanied by the emission of several (two or three) neutrons (Fig. 402). In addition to uranium, some other elements from among the last elements are capable of fission periodic table Mendeleev. These elements, like uranium, fission not only under the influence of neutrons, but also without external influences (spontaneously). Spontaneous fission was established experimentally by Soviet physicists K. A. Petrzhak and Georgiy Nikolaevich Flerov (b. 1913) in 1940. It is a very rare process. Thus, in 1g of uranium, only about 20 spontaneous fissions occur per hour.
Rice. 402. Fission of a uranium nucleus under the influence of neutrons: a) the nucleus captures a neutron; b) the impact of a neutron on a nucleus causes the latter to oscillate; c) the core is divided into two fragments; at the same time several more neutrons are emitted
Due to mutual electrostatic repulsion, fission fragments scatter in opposite directions, acquiring enormous kinetic energy (about ). The fission reaction thus occurs with a significant release of energy. Fast-moving fragments intensively ionize the atoms of the medium. This property of fragments is used to detect fission processes using an ionization chamber or cloud chamber. A photograph of fission fragment traces in a cloud chamber is shown in Fig. 403. It is extremely significant that neutrons emitted during the fission of a uranium nucleus (the so-called secondary fission neutrons) are capable of causing the fission of new uranium nuclei. Thanks to this, it is possible to carry out a fission chain reaction: once occurring, the reaction can, in principle, continue on its own, covering all larger number cores. The development diagram of such an increasing cellon reaction is shown in Fig. 404.
Rice. 403. Photograph of traces of uranium fission fragments in a cloud chamber: fragments () fly in opposite directions from a thin layer of uranium deposited on a plate blocking the chamber. The image also shows many thinner traces belonging to protons knocked out by neutrons from the molecules of the water car contained in the chamber
Carrying out a fission chain reaction in practice is not easy; experience shows that in the mass of natural uranium a chain reaction does not occur. The reason for this lies in the loss of secondary neutrons; in natural uranium, most neutrons escape without causing fission. As studies have revealed, the loss of neutrons occurs in the most common isotope of uranium - uranium - 238 (). This isotope easily absorbs neutrons by a reaction similar to the reaction of silver with neutrons (see § 222); this produces an artificially radioactive isotope. It divides with difficulty and only under the influence of fast neutrons.
The isotope that is contained in natural uranium in quantities has more favorable properties for a chain reaction. It is divided under the influence of neutrons of any energy - fast and slow, and the lower the neutron energy, the better. The process competing with fission - simple absorption of neutrons - is unlikely, unlike. Therefore, in pure uranium-235 a fission chain reaction is possible, provided, however, that the mass of uranium-235 is large enough. In low-mass uranium, the fission reaction is terminated due to the emission of secondary neutrons outside its substance.
Rice. 404. Development of a valuable fission reaction: it is conventionally accepted that when a nucleus fissions, two neutrons are emitted and there is no loss of neutrons, i.e. each neutron causes a new fission; circles - fission fragments, arrows - fission neutrons
In fact, due to the tiny size of atomic nuclei, a neutron travels a considerable distance (measured in centimeters) through matter before accidentally colliding with a nucleus. If the size of the body is small, then the probability of a collision on the way to the exit is small. Almost all secondary fission neutrons are emitted through the surface of the body without causing new fissions, i.e., without continuing the reaction.
From a large body, mainly neutrons formed in the surface layer fly out. Neutrons formed inside the body have a sufficient thickness of uranium in front of them and, for the most part, cause new fissions, continuing the reaction (Fig. 405). The greater the mass of uranium, the smaller the proportion of its volume is the surface layer, from which many neutrons are lost, and the more favorable the conditions for the development of a chain reaction.
Rice. 405. Development of a fission chain reaction in. a) At low mass, most fission neutrons fly out. b) In a large mass of uranium, many fission neutrons cause the fission of new nuclei; the number of divisions increases from generation to generation. Circles - fission fragments, arrows - fission neutrons
By gradually increasing the amount of , we will reach a critical mass, i.e. the smallest mass, starting from which an undamped chain reaction of fission in . With a further increase in mass, the reaction will begin to develop rapidly (it will begin with spontaneous fissions). When the mass decreases below the critical value, the reaction dies out.
So, a fission chain reaction can be carried out. If you have a sufficient amount of clean, separated from.
As we saw in §202, the separation of isotopes, although complex and expensive, is still a feasible operation. Indeed, extraction from natural uranium was one of the ways in which the fission chain reaction was put into practice.
Along with this, the chain reaction was achieved in another way that did not require the separation of uranium isotopes. This method is somewhat more complicated in principle, but easier to implement. It uses the slowing down of fast secondary fission neutrons to thermal motion velocities. We have seen that in natural uranium the immediate secondary neutrons are absorbed mainly by the isotope. Since absorption in does not lead to fission, the reaction terminates. As measurements show, when neutrons are slowed down to thermal speeds, the absorption capacity increases more than the absorption capacity. The absorption of neutrons by the isotope, leading to fission, takes precedence. Therefore, if fission neutrons are slowed down, preventing them from being absorbed into , a chain reaction will become possible with natural uranium.
Rice. 406. A system of natural uranium and a moderator in which a fission chain reaction can develop
In practice, this result is achieved by placing hot rods of natural uranium in the form of a rare lattice in the moderator (Fig. 406). Substances that have low atomic mass and weakly absorb neutrons are used as moderators. Good moderators are graphite, heavy water, and beryllium.
Let a uranium nucleus fission occur in one of the rods. Since the rod is relatively thin, almost all of the fast secondary neutrons will escape into the moderator. The rods are located quite sparsely in the lattice. The emitted neutron, before hitting the new rod, experiences many collisions with moderator nuclei and slows down to the speed of thermal motion (Fig. 407). Having then hit the uranium rod, the neutron will most likely be absorbed into and cause a new fission, thereby continuing the reaction. The fission chain reaction was first carried out in the USA in 1942. a group of scientists led by the Italian physicist Enrico Fermi (1901-1954) in a system with natural uranium. This process was independently implemented in the USSR in 1946. Academician Igor Vasilievich Kurchatov (1903-1960) and his staff.
Rice. 407. Development of a valuable fission reaction in a system of natural uranium and a moderator. A fast neutron, escaping from a thin rod, enters the moderator and is slowed down. Once back into uranium, the slowed-down neutron is most likely absorbed into , causing fission (symbol: two white circles). Some neutrons are absorbed into , without causing fission (symbol: black circle)
>> Fission of uranium nuclei
§ 107 FISSION OF URANIUM NUCLEI
Only the nuclei of some heavy elements can be divided into parts. When nuclei fission, two or three neutrons and -rays are emitted. At the same time, a lot of energy is released.
Discovery of uranium fission. The fission of uranium nuclei was discovered in 1938 by German scientists O. Hahn iF. Strassmann. They established that when uranium is bombarded with neutrons, elements of the middle part of the periodic table arise: barium, krypton, etc. However, the correct interpretation of this fact as the fission of a uranium nucleus that captured a neutron was given at the beginning of 1939. English physicist O. Frisch together with the Austrian physicist L. Meitner.
Neutron capture disrupts the stability of the nucleus. The nucleus becomes excited and becomes unstable, which leads to its division into fragments. Nuclear fission is possible because the rest mass of a heavy nucleus is greater than the sum of the rest masses of the fragments resulting from fission. Therefore, there is a release of energy equivalent to the decrease in rest mass that accompanies fission.
The possibility of fission of heavy nuclei can also be explained using a graph of the specific binding energy versus mass number A (see Fig. 13.11). The specific binding energy of the nuclei of atoms of elements occupying the last places in the periodic table (A 200) is approximately 1 MeV less than the specific binding energy in the nuclei of elements located in the middle of the periodic system (A 100). Therefore, the process of fission of heavy nuclei into nuclei of elements in the middle part of the periodic table is energetically favorable. After fission, the system enters a state with minimal internal energy. After all, the greater the binding energy of the nucleus, the greater the energy that should be released upon the emergence of the nucleus and, consequently, the less the internal energy of the newly formed system.
During nuclear fission, the binding energy per nucleon increases by 1 MeV and the total energy released must be enormous - on the order of 200 MeV. No other nuclear reaction (not related to fission) releases such large energies.
Direct measurements of the energy released during the fission of a uranium nucleus confirmed the above considerations and gave a value of 200 MeV. Moreover, most of this energy (168 MeV) falls on the kinetic energy of the fragments. In Figure 13.13 you see the tracks of fissile uranium fragments in a cloud chamber.
The energy released during nuclear fission is of electrostatic rather than nuclear origin. The large kinetic energy that the fragments have arises due to their Coulomb repulsion.
Mechanism of nuclear fission. The process of fission of the atomic nucleus can be explained based on the droplet model of the nucleus. According to this model, a bunch of nucleons resembles a droplet of charged liquid (Fig. 13.14, a). Nuclear forces between nucleons are short-range, like the forces acting between liquid molecules. Along with the large forces of electrostatic repulsion between the protons, which tend to tear the nucleus into pieces, there are even greater nuclear forces of attraction. These forces keep the nucleus from disintegrating.
The uranium-235 nucleus is spherical in shape. Having absorbed an extra neutron, it becomes excited and begins to deform, acquiring an elongated shape (Fig. 13.14, b). The core will stretch until the repulsive forces between the halves of the elongated core begin to prevail over the attractive forces acting in the isthmus (Fig. 13.14, c). After this, it breaks into two parts (Fig. 13.14, d).
Under the influence of Coulomb repulsive forces, these fragments fly away at a speed equal to 1/30 of the speed of light.
Emission of neutrons during fission. A fundamental fact of nuclear fission is the emission of two to three neutrons during the fission process. It was thanks to this that the practical use of intranuclear energy became possible.
It is possible to understand why free neutrons are emitted based on the following considerations. It is known that the ratio of the number of neutrons to the number of protons in stable nuclei increases with increasing atomic number. Therefore, the relative number of neutrons in fragments arising during fission is greater than is permissible for the nuclei of atoms located in the middle of the periodic table. As a result, several neutrons are released during the fission process. Their energy has different meanings- from several million electron volts to very small ones, close to zero.
Fission usually occurs into fragments, the masses of which differ by approximately 1.5 times. These fragments are highly radioactive, as they contain an excess amount of neutrons. As a result of a series of successive -decays, stable isotopes are eventually obtained.
In conclusion, we note that there is also spontaneous fission of uranium nuclei. It was discovered by Soviet physicists G.N. Flerov and K.A. Petrzhak in 1940. The half-life for spontaneous fission is 10 16 years. This is two million times longer than the half-life of uranium.
The reaction of nuclear fission is accompanied by the release of energy.
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“Fission of uranium nuclei. Chain reaction"
The purpose of the lesson: to familiarize students with the process of fission of uranium atomic nuclei and the mechanism of the chain reaction.
Tasks:
educational:
study the mechanism of fission of uranium-235 nuclei; introduce the concept of critical mass; determine the factors that determine the occurrence of a chain reaction.
educational:
lead students to understand the significance of scientific discoveries and the danger that may come from scientific achievements with a thoughtless, illiterate or immoral attitude towards them.
developing:
development of logical thinking; development of monologue and dialogic speech; development of mental operations in students: analysis, comparison, learning. Formation of an idea of the integrity of the picture of the world
Lesson type: lesson in learning new knowledge.
Competencies that the lesson aims to develop:
value-semantic - the ability to see and understand the world around us,
general cultural - mastery by the student scientific picture peace,
educational and cognitive - the ability to distinguish facts from speculation,
Communication - group work skills, mastery of various social roles in a team,
competencies of personal self-improvement - culture of thinking and behavior
Lesson progress: 1. Organizational moment.
A new lesson has arrived. I will smile at you, and you will smile at each other. And you will think: how good it is that we are all here together today. We are modest and kind, friendly and affectionate. We are all healthy. - Take a deep breath and exhale. Exhale yesterday's resentment, anger and anxiety. I wish all of us good lesson .
2. Checking homework.
Test.
1. What charge does the nucleus have?
1) positive 2) negative 3) the nucleus has no charge
2. What is an alpha particle?
1) electron 2) nucleus helium atom
3. How many protons and neutrons does the nucleus of a berylliumBe atom contain?
1) Z =9, N =4 2) Z =5, N =4 3) Z =4, N =5
4. What core chemical element is formed during α – decay of radium?
Ra → ? +He.
1) radon 2) uranium 3) fermium
5. The mass of a nucleus is always ... the sum of the masses of the nucleons of which it consists.
1) greater than 2) equal to 3) less
6. A neutron is a particle
1) having a charge of +1, atomic mass 1;
2) having a charge – 1, atomic mass 0;
3) having charge 0, atomic mass 1.
7.Indicate the second product of the nuclear reaction
Answers: Option 1. 1)1; 2)2; 3)3; 4)1; 5)3; 6)3; 7)3.
8. How do protons in the nucleus interact with each other electrically?
9. What is a mass defect? Write down the formula.
10. What is binding energy? Write down the formula.
Learning new material.
We recently learned that some chemical elements transform into other chemical elements during radioactive decay. What do you think will happen if you send some particle into the nucleus of an atom of some chemical element, for example, a neutron into the nucleus of uranium?
In 1939, German scientists Otto Hahn and Fritz Strassmann discovered the fission of uranium nuclei. They found that when uranium is bombarded with neutrons, elements of the middle part of the periodic table appear - radioactive isotopes of barium (Z = 56), krypton (Z = 36), etc.
Let us consider in more detail the process of fission of a uranium nucleus during bombardment with a neutron according to the figure. A neutron entering a uranium nucleus is absorbed by it. The core gets excited and begins to deform like a liquid drop.
The nucleus becomes excited and begins to deform. Why does the nucleus break into two parts? Under what forces does the rupture occur?
What forces act inside the nucleus?
– Electrostatic and nuclear.
Okay, but how do electrostatic forces manifest themselves?
– Electrostatic forces act between charged particles. The charged particle in the nucleus is the proton. Since the proton is positively charged, it means that repulsive forces act between them.
True, but how do nuclear forces manifest themselves?
– Nuclear forces are the forces of attraction between all nucleons.
So, under the influence of what forces does the nucleus rupture?
– (If difficulties arise, I ask guiding questions and lead students to the correct conclusion) Under the influence of electrostatic repulsive forces, the nucleus breaks into two parts, which fly apart in different directions and emit 2-3 neutrons.
It stretches until the electrical repulsive forces begin to prevail over the nuclear ones. The nucleus breaks into two fragments, releasing two or three neutrons. This is the technology of fission of a uranium nucleus.
The fragments fly away at very high speed. It turns out that part of the internal energy of the nucleus is converted into the kinetic energy of flying fragments and particles. The fragments fall into environment. What do you think is happening to them?
– The fragments are slowed down in the environment.
In order not to violate the law of conservation of energy, we must say what will happen to the kinetic energy?
– The kinetic energy of the fragments is converted into internal energy of the environment.
Can you notice that the internal energy of the medium has changed?
– Yes, the environment is heating up.
Will the change in internal energy be influenced by the fact that different numbers of uranium nuclei will participate in fission?
– Of course, with the simultaneous fission of a large number of uranium nuclei, the internal energy of the environment surrounding the uranium increases.
From your chemistry course, you know that reactions can occur both with the absorption of energy and the release. What can we say about the course of the fission reaction of uranium nuclei?
– The fission reaction of uranium nuclei releases energy into the environment.
(Slide 13)
Uranium occurs in nature in the form of two isotopes: U (99.3%) and U (0.7%). In this case, the fission reaction of U occurs most intensively with slow neutrons, while U nuclei simply absorb a neutron, and fission does not occur. Therefore, the main interest is in the fission reaction of the U nucleus. Currently, about 100 different isotopes with mass numbers from about 90 to 145 are known that arise during the fission of this nucleus. Two typical reactions divisions of this nucleus have the form:
Let us note that the energy released during the fission of uranium nuclei is enormous. For example, the complete fission of all nuclei contained in 1 kg of uranium releases the same energy as the combustion of 3000 tons of coal. Moreover, this energy can be released instantly.
(Slide 14)
We found out what will happen to the fragments, how will neutrons behave?
When a uranium-235 nucleus fissions, which is caused by a collision with a neutron, 2 or 3 neutrons are released. Under favorable conditions, these neutrons can hit other uranium nuclei and cause them to fission. At this stage, from 4 to 9 neutrons will appear, capable of causing new decays of uranium nuclei, etc. This avalanche-like process is called chain reaction. (Write in notebook: Nuclear chain reaction- a sequence of nuclear reactions, each of which is caused by a particle that appeared as a reaction product at the previous step of the sequence). We will consider the development diagram of the chain reaction of fission of uranium nuclei in more detail using a video fragment in slow motion for a more detailed consideration
We see that the total number of free neutrons in a piece of uranium increases like an avalanche over time. What could this lead to?
- To the explosion.
Why?
– The number of nuclear fissions increases and, accordingly, the energy released per unit time.
But another option is also possible, in which the number of free neutrons decreases with time, and the neutron does not meet the nucleus on its way. In this case what will happen to the chain reaction?
- It will stop.
Is it possible to use the energy of such reactions for peaceful purposes?
How should the reaction proceed?
– The reaction must proceed in such a way that the number of neutrons remains constant over time.
How can we ensure that the number of neutrons remains constant all the time?
– (guys' suggestions)
To solve this problem, you need to know what factors influence the increase and decrease total number neutrons are free in a piece of uranium in which a chain reaction occurs.
(Slide 15)
One of these factors is mass of uranium . The fact is that not every neutron emitted during nuclear fission causes the fission of other nuclei. If the mass (and, accordingly, the dimensions) of a piece of uranium is too small, then many neutrons will fly out of it, not having time to meet the nucleus on their way, causing its fission and thus generating a new generation of neutrons necessary to continue the reaction. In this case, the chain reaction will stop. In order for the reaction to continue, it is necessary to increase the mass of uranium to a certain value, called critical.
Why does a chain reaction become possible as mass increases?
For a chain reaction to occur, it is necessary that the so-called reproduction rate neutrons were greater than one. In other words, in each subsequent generation there should be more neutrons than in the previous one. The multiplication coefficient is determined not only by the number of neutrons produced in each elementary act, but also by the conditions under which the reaction occurs - some of the neutrons can be absorbed by other nuclei or leave the reaction zone. Neutrons released during the fission of uranium-235 nuclei are capable of causing the fission of only the nuclei of the same uranium, which accounts for only 0.7% of natural uranium. This concentration is insufficient to start a chain reaction. The U isotope can also absorb neutrons, but this does not cause a chain reaction.
( Write in your notebook: Neutron multiplication factork - the ratio of the number of neutrons of the subsequent generation to the number in the previous generation in the entire volume of the neutron-multiplying medium)
Chain reaction in uranium with increased content Uranium-235 can only develop when the mass of uranium exceeds the so-called critical mass. In small pieces of uranium, most neutrons fly out without hitting any nucleus. For pure uranium-235, the critical mass is about 50 kg.
( Write in your notebook: Critical mass- the minimum amount of fissile material required to start a self-sustaining fission chain reaction).
(Slide 16)
The critical mass of uranium can be reduced many times by using so-called neutron moderators. The fact is that neutrons produced during the decay of uranium nuclei have too high speeds, and the probability of capturing slow neutrons by uranium-235 nuclei is hundreds of times greater than fast ones. The best neutron moderator is heavy water H 2 O. When interacting with neutrons, ordinary water itself turns into heavy water.
Graphite, whose nuclei do not absorb neutrons, is also a good moderator. During elastic interaction with deuterium or carbon nuclei, neutrons slow down their movement.
The use of neutron moderators and a special beryllium shell, which reflects neutrons, makes it possible to reduce the critical mass to 250 g (0.25 kg).
Write in your notebook:
Critical mass can be reduced if:
Use moderators (graphite, ordinary and heavy water)
Reflective shell (beryllium)).
And in atomic bombs, an uncontrolled nuclear chain reaction occurs when two pieces of uranium-235 quickly combine, each of which has a mass slightly below the critical one.
The atomic bomb is a terrible weapon. The damaging factors of which are: 1) Light radiation (including X-ray and thermal radiation); 2) Shock wave; 3) radiation contamination of the area. But the fission of uranium nuclei is also used for peaceful purposes - in nuclear reactors at nuclear power plants. We will consider the processes occurring in these cases in the next lesson.
The middle of the 20th century is defined by the acceleration of science: fantastic acceleration, the introduction of scientific achievements into production and into our lives. All this makes us think - what will science give us tomorrow?
To alleviate all the hardships of human existence is the main goal of truly progressive science. To make humanity happier - not just one, not two, but humanity. And this is very important, because, as you know, science can also act against a person. The atomic explosion in the Japanese cities of Hiroshima and Nagasaki is a tragic example of this.
So, 1945, August. Second World War is coming to its end.
(Slide 2)
On August 6 at 1:45 a.m., an American B-29 bomber under the command of Colonel Paul Tibbetts took off from the island, which was approximately 6 hours flight time from Hiroshima.
(Slide 3)
Hiroshima after the atomic explosion.
Whose shadow wanders there unseen,
Are you blind from trouble?
This is Hiroshima crying
In clouds of ash.
Whose voice is there in the hot darkness?
Can you hear the frenzy?
It's Nagasaki crying
On the burned land
In this crying and sobbing
There is no falsehood
The whole world froze in anticipation -
Who will cry next?
(Slide 4)
The number of deaths from the direct impact of the explosion ranged from 70 to 80 thousand people. By the end of 1945, due to radioactive contamination and other post-effects of the explosion, the total number of deaths ranged from 90 to 166 thousand people. After 5 years, the total number of deaths reached 200,000 people.
(Slide 5)
On August 6, after receiving news of the successful atomic bombing of Hiroshima, US President Truman announced that
“We are now ready to destroy, even faster and more completely than before, all land-based production facilities of the Japanese in any city. We will destroy their docks, their factories, and their communications. Let there be no misunderstanding - we will completely destroy Japan's ability to wage war."
(Slide 6)
At 2:47 on August 9, an American B-29 bomber under the command of a major, carrying an atomic bomb on board, took off from the island. At 10:56 B-29 arrived at Nagasaki. The explosion occurred at 11:02 local time.
(Slide 7)
The number of deaths by the end of 1945 ranged from 60 to 80 thousand people. After 5 years, the total death toll, including deaths from cancer and other long-term effects of the explosion, may have reached or even exceeded 140,000.
This is the story, sad and warning
Every person is not an island,
every person is part of a large continent.
And never ask for whom the bell tolls.
He's calling for you...
Consolidation.
What did we learn about in class today? (with a mechanism of fission of uranium nuclei, with a chain reaction)
What are the conditions for a chain reaction to occur?
What is critical mass?
What is the reproduction rate?
What serves as a neutron moderator?
Reflection.
How do you feel when you leave class?
Assessment.
Homework: paragraphs 74,75, questions pp. 252-253