Give an example of a nuclear fission reaction of uranium nuclei. Lesson summary "Fission of uranium nuclei
The energy E released during fission increases with increasing Z 2 /A. The value of Z 2 /A = 17 for 89 Y (yttrium). Those. fission is energetically favorable for all nuclei heavier than yttrium. Why are most nuclei resistant to spontaneous fission? To answer this question, it is necessary to consider the mechanism of division.
During fission, the shape of the nucleus changes. The nucleus sequentially passes through the following stages (Fig. 7.1): a ball, an ellipsoid, a dumbbell, two pear-shaped fragments, two spherical fragments. How does the potential energy of the nucleus change at different stages of fission?
Initial core with magnification r takes the form of an increasingly elongated ellipsoid of revolution. In this case, due to the evolution of the shape of the nucleus, the change in its potential energy is determined by the change in the sum of the surface and Coulomb energies E p + E k. In this case, the surface energy increases, since the surface area of the nucleus increases. The Coulomb energy decreases as the average distance between protons increases. If, with a slight deformation, characterized by a small parameter , the initial core takes the form of an axially symmetric ellipsoid, the surface energy E" p and the Coulomb energy E" k as functions of the deformation parameter change as follows:
In ratios (7.4–7.5) E n and E k are the surface and Coulomb energies of the initial spherically symmetric nucleus.
In the region of heavy nuclei, 2E n > Ek, and the sum of the surface and Coulomb energies increases with increasing . It follows from (7.4) and (7.5) that at small deformations, an increase in the surface energy prevents a further change in the shape of the nucleus and, consequently, fission.
Relation (7.5) is valid for small strains . If the deformation is so great that the nucleus takes the form of a dumbbell, then the surface and Coulomb forces tend to separate the nucleus and give the fragments a spherical shape. Thus, with a gradual increase in the deformation of the nucleus, its potential energy passes through a maximum. The plot of surface and Coulomb energies of the nucleus as a function of r is shown in fig. 7.2.
The presence of a potential barrier prevents instantaneous spontaneous nuclear fission. In order for the nucleus to split, it needs to be given energy Q that exceeds the height of the fission barrier H. The maximum potential energy of a fissile nucleus E + H (for example, gold) into two identical fragments is ≈ 173 MeV, and the energy E released during fission is 132 MeV . Thus, during the fission of the gold nucleus, it is necessary to overcome a potential barrier with a height of about 40 MeV.
The height of the fission barrier H is the greater, the smaller the ratio of the Coulomb and surface energies E to /E p in the initial nucleus. This ratio, in turn, increases with an increase in the division parameter Z 2 /A (7.3). The heavier the core, the less height fission barrier H, since the fission parameter, assuming that Z is proportional to A, increases with increasing mass number:
| E k / E p \u003d (a 3 Z 2) / (a 2 A) ~ A. | (7.6) |
Therefore, heavier nuclei generally need to be supplied with less energy in order to cause nuclear fission.
The height of the fission barrier vanishes at 2E p – Ec = 0 (7.5). In this case
2E p / E k \u003d 2 (a 2 A) / (a 3 Z 2),
Z 2 /A \u003d 2a 2 / (a 3 Z 2) ≈ 49.
Thus, according to the drop model, nuclei with Z 2 /A > 49 cannot exist in nature, since they should spontaneously split into two fragments almost instantaneously in a characteristic nuclear time of the order of 10–22 s. The dependences of the shape and height of the potential barrier H, as well as the fission energy, on the value of the parameter Z 2 /A are shown in Figs. 7.3.
Rice. 7.3. Radial dependence of the shape and height of the potential barrier and the fission energy E at various values of the parameter Z 2 /A. The value of E p + E k is plotted on the vertical axis.
Spontaneous nuclear fission with Z 2 /A< 49,
для которых высота барьера H
не равна нулю, с точки зрения классической физики невозможно. Однако в квантовой
механике такое деление возможно за счет туннельного эффекта – прохождения
осколков деления через потенциальный барьер. Оно носит название спонтанного
деления. Вероятность спонтанного деления растет с увеличением параметра деления
Z 2 /A,
т. е. с уменьшением высоты барьера деления. В целом период спонтанного деления
уменьшается при переходе от менее тяжелых ядер к более тяжелым от
T 1/2 >10 21 years for 232 Th to 0.3 s for 260 Rf.
Forced nuclear fission with Z 2 /A< 49
может быть вызвано их возбуждением фотонами, нейтронами, протонами, дейтронами,
a частицами и другими частицами, если вносимая в
ядро энергия достаточна для преодоления барьера деления.
The minimum value of the excitation energy of the compound nucleus E* formed during the capture of a neutron is equal to the binding energy of the neutron in this nucleus ε n . Table 7.1 compares the barrier height H and the neutron binding energy ε n for Th, U, Pu isotopes formed after neutron capture. The binding energy of a neutron depends on the number of neutrons in the nucleus. Due to the pairing energy, the binding energy of an even neutron is greater than the binding energy of an odd neutron.
Table 7.1
Fission barrier height H, neutron binding energy ε n
| Isotope | Fission barrier height H, MeV | Isotope | Neutron binding energy ε n |
|---|---|---|---|
| 232Th | 5.9 | 233Th | 4.79 |
| 233 U | 5.5 | 234 U | 6.84 |
| 235 U | 5.75 | 236 U | 6.55 |
| 238 U | 5.85 | 239 U | 4.80 |
| 239 Pu | 5.5 | 240 Pu | 6.53 |
A characteristic feature of fission is that the fragments, as a rule, have different masses. In the case of the most probable fission of 235 U, the fragment mass ratio is on average ~1.5. The mass distribution of 235 U fission fragments by thermal neutrons is shown in Fig. . 7.4. For the most probable fission, a heavy fragment has a mass number of 139, a light one - 95. Among the fission products there are fragments with A = 72 - 161 and Z = 30 - 65. The probability of fission into two fragments of equal mass is not equal to zero. In fission of 235 U by thermal neutrons, the probability of symmetrical fission is approximately three orders of magnitude lower than in the case of the most probable fission into fragments with A = 139 and 95.
Asymmetric fission is explained by the shell structure of the nucleus. The nucleus tends to split in such a way that the main part of the nucleons of each fragment forms the most stable magical core.
The ratio of the number of neutrons to the number of protons in the 235 U nucleus N/Z = 1.55, while for stable isotopes with a mass number close to the mass number of fragments, this ratio is 1.25 − 1.45. Consequently, fission fragments turn out to be heavily overloaded with neutrons and must be
β - radioactive. Therefore, fission fragments experience successive β - decays, and the charge of the primary fragment can change by 4 - 6 units. Below is a characteristic chain of radioactive decays of 97 Kr - one of the fragments formed during the fission of 235 U:
The excitation of fragments, caused by a violation of the ratio of the number of protons and neutrons, which is characteristic of stable nuclei, is also removed due to the emission of prompt fission neutrons. These neutrons are emitted by moving fragments in a time less than ~ 10 -14 s. On average, 2 − 3 prompt neutrons are emitted in each fission event. Their energy spectrum is continuous with a maximum around 1 MeV. The average energy of a prompt neutron is close to 2 MeV. The emission of more than one neutron in each fission event makes it possible to obtain energy through a nuclear fission chain reaction.
In the most probable fission of 235 U by thermal neutrons, a light fragment (A = 95) acquires a kinetic energy of ≈ 100 MeV, and a heavy one (A = 139) acquires about 67 MeV. Thus, the total kinetic energy of fragments is ≈ 167 MeV. The total fission energy in this case is 200 MeV. Thus, the remaining energy (33 MeV) is distributed among other fission products (neutrons, electrons and antineutrinos of β - decay of fragments, γ-radiation of fragments and their decay products). The distribution of fission energy between different products during the fission of 235 U by thermal neutrons is given in Table 7.2.
Table 7.2
Fission energy distribution 235 U thermal neutrons
Nuclear fission products (NFs) are a complex mixture of more than 200 radioactive isotopes of 36 elements (from zinc to gadolinium). Most of the activity is made up of short-lived radionuclides. Thus, after 7, 49, and 343 days after the explosion, the activity of PNDs decreases by 10, 100, and 1000 times, respectively, compared with the activity one hour after the explosion. The yield of the most biologically significant radionuclides is given in Table 7.3. In addition to PND, radioactive contamination is caused by radionuclides of induced activity (3 H, 14 C, 28 Al, 24 Na, 56 Mn, 59 Fe, 60 Co, etc.) and the undivided part of uranium and plutonium. The role of induced activity in thermonuclear explosions is especially great.
Table 7.3
Release of some fission products in a nuclear explosion
| Radionuclide | Half life | Output per division, % | Activity per 1 Mt, 10 15 Bq |
|---|---|---|---|
| 89Sr | 50.5 days | 2.56 | 590 |
| 90Sr | 29.12 years old | 3.5 | 3.9 |
| 95 Zr | 65 days | 5.07 | 920 |
| 103 Ru | 41 days | 5.2 | 1500 |
| 106 Ru | 365 days | 2.44 | 78 |
| 131 I | 8.05 days | 2.9 | 4200 |
| 136Cs | 13.2 days | 0.036 | 32 |
| 137Cs | 30 years | 5.57 | 5.9 |
| 140 Ba | 12.8 days | 5.18 | 4700 |
| 141Cs | 32.5 days | 4.58 | 1600 |
| 144Cs | 288 days | 4.69 | 190 |
| 3H | 12.3 years old | 0.01 | 2.6 10 -2 |
During nuclear explosions in the atmosphere, a significant part of the precipitation (up to 50% in ground explosions) falls near the test area. Part of the radioactive substances is retained in the lower part of the atmosphere and, under the influence of the wind, moves over long distances, remaining approximately at the same latitude. Being in the air for about a month, radioactive substances during this movement gradually fall to the Earth. Most of the radionuclides are released into the stratosphere (to a height of 10÷15 km), where they are globally dispersed and largely decay.
Various elements of the design of nuclear reactors have high activity for decades (Table 7.4)
Table 7.4
Specific activity values (Bq/t uranium) of the main fission products in fuel elements removed from the reactor after three years of operation
| Radionuclide | 0 | 1 day | 120 days | 1 year | 10 years | |
|---|---|---|---|---|---|---|
| 85 kr | 5. 78· 10 14 | 5. 78· 10 14 | 5. 66· 10 14 | 5. 42· 10 14 |
4. 7· 10 14 |
3. 03· 10 14 |
| 89Sr | 4. 04· 10 16 | 3. 98· 10 16 | 5. 78· 10 15 | 2. 7· 10 14 |
1. 2· 10 10 |
|
| 90Sr | 3. 51· 10 15 | 3. 51· 10 15 | 3. 48· 10 15 | 3. 43· 10 15 |
3. 26· 10 15 |
2. 75· 10 15 |
| 95 Zr | 7. 29· 10 16 | 7. 21· 10 16 | 1. 99· 10 16 | 1. 4· 10 15 | 5. 14· 10 11 | |
| 95Nb | 7. 23· 10 16 | 7. 23· 10 16 | 3. 57· 10 16 | 3. 03· 10 15 | 1. 14· 10 12 | |
| 103 Ru | 7. 08· 10 16 | 6. 95· 10 16 | 8. 55· 10 15 | 1. 14· 10 14 | 2. 97· 10 8 | |
| 106 Ru | 2. 37· 10 16 | 2. 37· 10 16 | 1. 89· 10 16 | 1. 19· 10 16 | 3. 02· 10 15 | 2. 46· 10 13 |
| 131 I | 4. 49· 10 16 | 4. 19· 10 16 | 1. 5· 10 12 | 1. 01· 10 3 | ||
| 134Cs | 7. 50· 10 15 | 7. 50· 10 15 | 6. 71· 10 15 | 5. 36· 10 15 | 2. 73· 10 15 | 2. 6· 10 14 |
| 137Cs | 4. 69· 10 15 | 4. 69· 10 15 | 4. 65· 10 15 | 4. 58· 10 15 | 4. 38· 10 15 | 3. 73· 10 15 |
| 140 Ba | 7. 93· 10 16 | 7. 51· 10 16 | 1. 19· 10 14 | 2. 03· 10 8 | ||
| 140la | 8. 19· 10 16 | 8. 05· 10 16 | 1. 37· 10 14 | 2. 34· 10 8 | ||
| 141 Ce | 7. 36· 10 16 | 7. 25· 10 16 | 5. 73· 10 15 | 3. 08· 10 13 | 5. 33· 10 6 | |
| 144 Ce | 5. 44· 10 16 | 5. 44· 10 16 | 4. 06· 10 16 | 2. 24· 10 16 | 3. 77· 10 15 | 7. 43· 10 12 |
| 143 Pm | 6. 77· 10 16 | 6. 70· 10 16 | 1. 65· 10 14 | 6. 11· 10 8 | ||
| 147 Pm | 7. 05 10 15 | 7. 05· 10 15 | 6. 78· 10 15 | 5. 68· 10 15 |
3. 35· 10 14 |
Fission of uranium nuclei was discovered in 1938 by German scientists O. Hahn and F. Strassmann. They managed to establish that when bombarding uranium nuclei with neutrons, elements of the middle part are formed periodic system: barium, krypton, etc. Correct interpretation this fact was given by the Austrian physicist L. Meitner and the English physicist O. Frisch. They explained the appearance of these elements by the decay of uranium nuclei, which captured a neutron, into two approximately equal parts. This phenomenon is called nuclear fission, and the resulting nuclei are called fission fragments.
see also
- Vasiliev, A. Fission of uranium: from Klaproth to Gan, Kvant. - 2001. - No. 4. - S. 20-21.30.
Drop model of the nucleus
This fission reaction can be explained based on the drop model of the nucleus. In this model, the nucleus is considered as a drop of an electrically charged incompressible liquid. In addition to the nuclear forces acting between all the nucleons of the nucleus, protons experience an additional electrostatic repulsion, due to which they are located on the periphery of the nucleus. In the unexcited state, the electrostatic repulsion forces are compensated, so the nucleus has a spherical shape (Fig. 1a).
After the capture by the nucleus \(~^(235)_(92)U\) of a neutron, an intermediate nucleus \(~(^(236)_(92)U)^*\) is formed, which is in an excited state. In this case, the neutron energy is evenly distributed among all nucleons, and the intermediate nucleus itself is deformed and begins to oscillate. If the excitation is small, then the nucleus (Fig. 1, b), freeing itself from excess energy by emitting γ -quantum or neutron, returns to a stable state. If the excitation energy is sufficiently high, then the deformation of the core during vibrations can be so large that a constriction is formed in it (Fig. 1c), similar to the constriction between two parts of a splitting liquid drop. Nuclear forces acting in a narrow waist can no longer resist the significant Coulomb force of repulsion of parts of the nucleus. The constriction breaks, and the nucleus breaks up into two "fragments" (Fig. 1d), which scatter in opposite directions.
Currently, about 100 different isotopes with mass numbers from about 90 to 145 are known, arising from the fission of this nucleus. Two typical fission reactions of this nucleus have the form:
\(~^(235)_(92)U + \ ^1_0n \ ^(\nearrow)_(\searrow) \ \begin(matrix) ^(144)_(56)Ba + \ ^(89)_( 36)Kr + \ 3^1_0n \\ ^(140)_(54)Xe + \ ^(94)_(38)Sr + \ 2^1_0n \end(matrix)\) .
Note that as a result of nuclear fission initiated by a neutron, new neutrons are produced that can cause fission reactions in other nuclei. The fission products of uranium-235 nuclei can also be other isotopes of barium, xenon, strontium, rubidium, etc.
During the fission of nuclei of heavy atoms (\(~^(235)_(92)U\)) a very large energy is released - about 200 MeV during the fission of each nucleus. About 80% of this energy is released in the form of fragment kinetic energy; the remaining 20% is accounted for by the energy of the radioactive radiation of fragments and the kinetic energy of prompt neutrons.
The energy released during nuclear fission can be estimated using the specific binding energy of nucleons in the nucleus. The specific binding energy of nucleons in nuclei with a mass number A≈ 240 of the order of 7.6 MeV/nucleon, while in nuclei with mass numbers A= 90 – 145 specific energy is approximately equal to 8.5 MeV/nucleon. Therefore, the fission of a uranium nucleus releases an energy of the order of 0.9 MeV/nucleon, or approximately 210 MeV per uranium atom. With the complete fission of all the nuclei contained in 1 g of uranium, the same energy is released as during the combustion of 3 tons of coal or 2.5 tons of oil.
see also
- Varlamov A.A. Drop model of the nucleus // Kvant. - 1986. - No. 5. - S. 23-24
Chain reaction
Chain reaction- a nuclear reaction in which the particles causing the reaction are formed as products of this reaction.
In the fission of a uranium-235 nucleus, 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 already appear, capable of causing new decays of uranium nuclei, etc. Such an avalanche-like process is called a chain reaction. The scheme for the development of a chain reaction of fission of uranium nuclei is shown in fig. 3.
Uranium occurs in nature in the form of two isotopes \[~^(238)_(92)U\] (99.3%) and \(~^(235)_(92)U\) (0.7%). When bombarded by neutrons, the nuclei of both isotopes can split into two fragments. In this case, the fission reaction \(~^(235)_(92)U\) proceeds most intensively on slow (thermal) neutrons, while the nuclei \(~^(238)_(92)U\) enter into the reaction fission only with fast neutrons with an energy of the order of 1 MeV. Otherwise, the excitation energy of the formed nuclei \(~^(239)_(92)U\) is insufficient for fission, and then instead of fission, nuclear reactions occur:
\(~^(238)_(92)U + \ ^1_0n \to \ ^(239)_(92)U \to \ ^(239)_(93)Np + \ ^0_(-1)e\ ) .
Uranium isotope \(~^(238)_(92)U\) β -radioactive, half-life 23 min. The neptunium isotope \(~^(239)_(93)Np\) is also radioactive, with a half-life of about 2 days.
\(~^(239)_(93)Np \to \ ^(239)_(94)Pu + \ ^0_(-1)e\) .
The plutonium isotope \(~^(239)_(94)Np\) is relatively stable, with a half-life of 24,000 years. The most important property of plutonium is that it is fissile under the influence of neutrons in the same way as \(~^(235)_(92)U\). Therefore, with the help of \(~^(239)_(94)Np\) a chain reaction can be carried out.
The chain reaction scheme discussed above is an ideal case. In real conditions, not all neutrons produced during fission participate in the fission of other nuclei. Some of them are captured by non-fissile nuclei of foreign atoms, others fly out of uranium (neutron leakage).
Therefore, the chain reaction of fission of heavy nuclei does not always occur and not for any mass of uranium.
Neutron multiplication factor
The development of a chain reaction is characterized by the so-called neutron multiplication factor TO, which is measured by the ratio of the number N i neutrons that cause nuclear fission of matter at one of the stages of the reaction, to the number N i-1 neutrons that caused fission at the previous stage of the reaction:
\(~K = \dfrac(N_i)(N_(i - 1))\) .
The multiplication factor depends on a number of factors, in particular, on the nature and amount of fissile material, on geometric shape the volume it occupies. The same amount of a given substance has different meaning TO. TO maximum if the substance has a spherical shape, since in this case the loss of prompt neutrons through the surface will be the smallest.
The mass of fissile material in which the chain reaction proceeds with the multiplication factor TO= 1 is called the critical mass. In small pieces of uranium, most of the neutrons, without hitting any nucleus, fly out.
The value of the critical mass is determined by the geometry of the physical system, its structure and the external environment. So, for a ball of pure uranium \(~^(235)_(92)U\) the critical mass is 47 kg (a ball with a diameter of 17 cm). The critical mass of uranium can be reduced many times over 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 capture of slow neutrons by uranium-235 nuclei is hundreds of times greater than that of fast ones. The best moderator of neutrons is heavy water D 2 O. When interacting with neutrons, ordinary water itself turns into heavy water.
A good moderator is also graphite, whose nuclei do not absorb neutrons. Upon elastic interaction with deuterium or carbon nuclei, neutrons are slowed down to thermal velocities.
The use of neutron moderators and a special beryllium shell that reflects neutrons makes it possible to reduce the critical mass to 250 g.
With a multiplication factor TO= 1 the number of fissile nuclei is maintained at a constant level. This mode is provided in nuclear reactors.
If the mass of nuclear fuel is less than the critical mass, then the multiplication factor TO < 1; каждое новое поколение вызывает все меньшее и меньшее число делений, и реакция без внешнего источника нейтронов быстро затухает.
If the mass of nuclear fuel is greater than the critical one, then the multiplication factor TO> 1 and each new generation of neutrons causes all more divisions. The chain reaction grows like an avalanche and has the character of an explosion, accompanied by a huge release of energy and an increase in the ambient temperature to several million degrees. A chain reaction of this kind occurs when an atomic bomb explodes.
Nuclear bomb
In the normal state, a nuclear bomb does not explode because the nuclear charge in it is divided into several small parts by partitions that absorb the decay products of uranium - neutrons. The nuclear chain reaction that causes a nuclear explosion cannot be sustained under such conditions. However, if the fragments of the nuclear charge are connected together, then their total mass will be sufficient for the chain reaction of uranium fission to begin to develop. The result is a nuclear explosion. At the same time, the explosion power developed by a nuclear bomb is relatively small sizes, is equivalent to the power released during the explosion of millions and billions of tons of TNT.
Rice. 5. Atomic bomb
Purpose: to form students' understanding of the fission of uranium nuclei.
- check previously studied material;
- consider the mechanism of fission of the uranium nucleus;
- consider the condition for the occurrence of a chain reaction;
- find out the factors influencing the course of a chain reaction;
- develop the speech and thinking of students;
- develop the ability to analyze, control and adjust their own activities within a given time.
Equipment: computer, projection system, didactic material (test “Composition of the core”), disks “Interactive course. Physics 7-11kl ”(Fizikon) and“ 1C-repeater. Physics” (1C).
Lesson progress
I. Organizational moment (2 ').
Greetings, lesson plan announcement.
II. Repetition of previously studied material (8’).
Independent work of students - performing a test ( Annex 1 ). In the test, you must indicate one correct answer.
III. Learning new material (25’). As the lesson progresses, we make a summary(application 2 ).
We recently learned that some chemical elements are converted into other chemical elements during radioactive decay. And what do you think will happen if some particle is directed into the nucleus of an atom of a certain chemical element, well, for example, a neutron into the nucleus of uranium? (listen to student suggestions)
Let's check your assumptions (work with the interactive model “Nuclear Fission”“Interactive course. Physics 7-11kl” ).
What was the result?
- When a neutron hits the uranium nucleus, we see that as a result 2 fragments and 2-3 neutrons are formed.
The same effect was obtained in 1939 by the German scientists Otto Hahn and Fritz Strassmann. They found that as a result of the interaction of neutrons with uranium nuclei, radioactive fragment nuclei appear, the masses and charges of which are approximately half the corresponding characteristics of uranium nuclei. Nuclear fission occurring in this way is called forced fission, in contrast to spontaneous fission, which occurs during natural radioactive transformations.
The nucleus enters a state of excitation and begins to deform. Why does the core break into 2 parts? What forces cause the break?
What forces act inside the nucleus?
– Electrostatic and nuclear.
Okay, so 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.
Right, but how do nuclear forces manifest themselves?
– Nuclear forces are forces of attraction between all nucleons.
So, under the action of what forces does the nucleus break?
- (If there are any difficulties, I ask leading questions and lead students to the correct conclusion) Under the influence of electrostatic repulsive forces, the nucleus is torn into two parts, which scatter in different directions and emit 2-3 neutrons.
The fragments scatter at a 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?
– Fragments are decelerated 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 environment.
Is it possible to notice that the internal energy of the medium has changed?
Yes, the environment is warming up.
But will the factor that will participate in fission affect the change in internal energy different amount uranium nuclei?
- Of course, with the simultaneous fission of a large number of uranium nuclei, the internal energy of the environment surrounding uranium increases.
From the course of chemistry, you know that reactions can occur both with the absorption of energy and with the release. What can we say about the course of the uranium fission reaction?
- The reaction of fission of uranium nuclei goes with the release of energy into the environment.
The energy contained in the nuclei of atoms is colossal. For example, with the complete fission of all the nuclei present in 1 g of uranium, the same amount of energy would be released as is released during the combustion of 2.5 tons of oil. Figured out what's going to happen to the shards How will neutrons behave?
– (I listen to the assumptions of students, check the assumptions, working with the interactive model “Chain Reaction”“1C repeater. Physics" ).
True, neutrons on their way can meet uranium nuclei and cause fission. Such a reaction is called a chain reaction.
So, what is the condition for a chain reaction to occur?
- A chain reaction is possible due to the fact that during the fission of each nucleus, 2-3 neutrons are formed, which can take part in the fission of other nuclei.
We see that the total number of free neutrons in a piece of uranium increases like an avalanche with time. What can this lead to?
- To the explosion.
- The number of nuclear fission increases and, accordingly, the energy released per unit of time.
But after all, another option is also possible, in which the number of free neutrons decreases with time, the nucleus did not meet the neutron on its way. In this case what happens to the chain reaction?
- It will stop.
Can the energy of such reactions be used 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 is it possible to ensure that the number of neutrons remains constant all the time?
- (children's suggestions)
To solve this problem, it is necessary to know what factors influence the increase and decrease in the total number of free neutrons in a piece of uranium in which a chain reaction takes place.
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 size) 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, cause its fission and thus generate 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 with an increase in mass?
– The larger the mass of the piece, the greater the probability of neutrons meeting with nuclei. Accordingly, the number of nuclear fissions and the number of emitted neutrons increase.
At a certain so-called critical mass of uranium, the number of neutrons that appeared during the fission of nuclei becomes equal to the number of neutrons lost (that is, captured by nuclei without fission and flown out of the piece).
Therefore, their total number remains unchanged. In this case, the chain reaction can go on for a long time, without stopping and without acquiring an explosive character.
The smallest mass of uranium at which a chain reaction is possible is called the critical mass.
How will the reaction proceed if the mass of uranium is greater than the critical mass?
– As a result of a sharp increase in the number of free neutrons, a chain reaction leads to an explosion.
What if it's less critical?
The reaction does not proceed due to the lack of free neutrons.
It is possible to reduce the loss of neutrons (which fly out of uranium without reacting with nuclei) not only by increasing the mass of uranium, but also by using a special reflective shell . To do this, a piece of uranium is placed in a shell made of a substance that reflects neutrons well (for example, beryllium). Reflected from this shell, neutrons return to uranium and can take part in nuclear fission.
In addition to the mass and the presence of a reflective shell, there are several other factors on which the possibility of a chain reaction depends. For example, if a piece of uranium contains too much impurities other chemical elements, they absorb most of the neutrons and the reaction stops.
Another factor that influences the course of the reaction is Availability in the so-called uranium neutron moderator . The fact is that the nuclei of uranium-235 are most likely to fission under the action of slow neutrons. Nuclear fission produces fast neutrons. If fast neutrons are slowed down, then most of them will be captured by uranium-235 nuclei with subsequent fission of these nuclei; substances such as graphite, hearth, heavy water and some others are used as moderators. These substances only slow down neutrons, almost without absorbing them.
So, what are the main factors that can influence the course of a chain reaction?
- The possibility of a chain reaction is determined by the mass of uranium, the amount of impurities in it, the presence of a shell and a moderator.
The critical mass of a spherical piece of uranium-235 is approximately 50 kg. At the same time, its radius is only 9 cm, since uranium has a very high density.
By using a moderator and a reflective shell, and by reducing the amount of impurities, it is possible to reduce the critical mass of uranium to 0.8 kg.
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 quanta, is called a nuclear reaction.
The first nuclear reaction was carried out by Rutherford in 1919. He discovered that when alpha particles collide with the nuclei of nitrogen atoms, fast moving protons are formed. This meant that the nucleus of the nitrogen isotope, as a result of a collision with an alpha particle, turned into the nucleus of an oxygen isotope:
.
Nuclear reactions can proceed with the release or absorption of energy. Using the law of the relationship between mass and energy, the energy yield of a nuclear reaction can be determined by finding the difference between the masses of the particles entering into the reaction and the reaction products:
Chain reaction of fission of uranium nuclei. Among the various nuclear reactions, chain reactions of fission of some heavy nuclei are of particular importance in the life of modern human society.
The fission reaction of uranium nuclei during their bombardment 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 enters the 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, we can calculate that the energy yield 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 1 kg is burned. hard coal or oil. Therefore, searches were made for ways to release nuclear energy in significant quantities for its use for practical purposes.
F. Joliot-Curie was the first to suggest the possibility of nuclear chain reactions in 1934. In 1939, together with H. Halban and L. Kovarsky, he experimentally discovered that during the fission of a uranium nucleus, in addition to fragments-nuclei, 2 -3 free neutrons. Under favorable conditions, these neutrons can hit other uranium nuclei and cause them to fission. During the fission of three uranium nuclei, 6-9 new neutrons should be released, they will fall into new uranium nuclei, etc. The scheme for the development of a chain reaction of fission of uranium nuclei is shown in Figure 316.
Rice. 316
The practical implementation of chain reactions is not such simple task how it looks on the diagram. The neutrons released during the fission of uranium nuclei are capable of causing fission only of the nuclei of the uranium isotope with a mass number of 235, while their energy is insufficient to destroy the nuclei of the uranium isotope with a mass number of 238. In natural uranium, uranium with a mass number of 238 accounts for 99.8%, while uranium with a mass number of 235 accounts for only 0.7%. Therefore, the first possible path the implementation of a fission chain reaction is associated with the separation of isotopes of uranium and obtaining in pure form in sufficient large quantities isotope . A necessary condition for the implementation of a chain reaction is the presence of a sufficiently large amount of uranium, since in a small sample, most 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 as follows: two pieces of uranium metal are made, each with a mass slightly less than the critical one. A chain reaction in each of them separately cannot go. At fast connection These pieces develop a chain reaction and a colossal energy is released. The temperature of uranium reaches millions of degrees, the uranium itself and any other substances that are nearby turn into steam. The hot gaseous ball expands rapidly, burning and destroying everything in its path. This is how a nuclear explosion happens.
It is very difficult to use the energy of a nuclear explosion for peaceful purposes, since the release of energy in this case cannot be controlled. 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. At the same time, their speeds are equal to approximately 107 m / s, therefore they are called fast neutrons. At such energies, neutrons interact with the nuclei of uranium and uranium with approximately the same 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 the 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 ones. Therefore, when natural uranium is irradiated with slow neutrons, most of them are absorbed not in uranium-238 nuclei, but in uranium-235 nuclei and cause their fission. Consequently, for the development of a chain reaction in natural uranium, the neutron velocities must be reduced to thermal.
Rice. 317
Neutrons are slowed down as a result of collisions with the 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 should have a relatively small mass, since in a collision with a light nucleus, a neutron loses more energy than in a collision with a heavy one. The most common moderators are ordinary water and graphite.
The space in which the chain reaction takes place is called the reactor core. To reduce the leakage of neutrons, the reactor core is surrounded by a neutron reflector, which throws a significant part of the emitted neutrons into the core. The reflector is usually the same substance that serves as the moderator.
The energy released during the operation of the reactor is removed using a coolant. Only liquids and gases that do not have the ability to absorb neutrons can be used as a coolant. Common water is widely used as a coolant, sometimes carbon dioxide and even liquid metallic sodium.
The reactor is controlled by means of special control (or control) rods introduced into the reactor core. Control rods are made from boron or cadmium compounds, which absorb thermal neutrons with a very great efficiency. Before starting the operation of the reactor, they are completely introduced into its core. Absorbing a significant part of the neutrons, they make it impossible to develop a chain reaction. To start the reactor, the control rods are gradually withdrawn from the core until the energy release reaches a predetermined level. When the power increases above the set level, automata are switched on, immersing the control rods into the depth of the active zone.
Nuclear energy. Nuclear energy for the service of peace was put for the first time in our country. Academician Igor Vasilievich Kurchatov (1903-1960) was the first organizer and leader of work on atomic science and technology in the USSR.
At present, the largest in the USSR and in Europe, the Leningrad NPP. 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 for sea 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 first surface ship in the world to reach the North Pole.
thermonuclear reaction. Nuclear energy is released not only in the nuclear fission reactions of heavy nuclei, but also in the reactions of the 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 are available in the interiors of stars. On Earth, thermonuclear fusion reaction has been carried out in 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
heating up to about 5 10 7 K is required.
During the synthesis of 1 g of helium from deuterium and tritium, energy of 4.2·10 11 J is released. Such energy is released when 10 tons of diesel fuel are burned.
The reserves of hydrogen 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 helium synthesis from heavy hydrogen isotopes 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 touching the chamber walls. At the Tokamak-10 experimental facility, Soviet physicists succeeded in heating 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, inside which is 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.
