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. Z 2 / A = 17 for 89 Y (yttrium). Those. fission is energetically favorable for all nuclei heavier than yttrium. Why is the majority of nuclei resistant to spontaneous fission? To answer this question, it is necessary to consider the mechanism of fission.
In the process of fission, the shape of the nucleus changes. The nucleus sequentially goes 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, a change in its potential energy is determined by a change in the sum of the surface and Coulomb energies E n + E k. The surface energy in this case increases, since the surface area of the nucleus increases. The Coulomb energy decreases as the average distance between protons increases. If, with an insignificant deformation characterized by a small parameter, the initial nucleus takes the form of an axially symmetric ellipsoid, the surface energy E "n and the Coulomb energy E" as a function of the deformation parameter change as follows:
In relations (7.4–7.5) E n and E k - surface and Coulomb energies of the initial spherically symmetric nucleus.
In the region of heavy nuclei 2E n> E k and the sum of the surface and Coulomb energies increases with increasing. From (7.4) and (7.5) it follows 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 deformations. If the deformation is so great that the nucleus takes the shape 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 graph of changes in the 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 a nucleus to split, it must be imparted an energy Q exceeding the height of the fission barrier H. The maximum potential energy of a fissioning nucleus E + H (for example, gold) into two identical fragments is ≈ 173 MeV, and the value of the energy E released during fission is 132 MeV ... Thus, when fissioning a gold nucleus, it is necessary to overcome a potential barrier with a height of about 40 MeV.
The fission barrier height H is the greater, the lower the ratio of the Coulomb and surface energies E to / E n in the initial nucleus. This ratio, in turn, increases with an increase in the fission 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 n = (a 3 Z 2) / (a 2 A) ~ A. | (7.6) |
Therefore, heavier nuclei, as a rule, need to be imparted less energy in order to cause nuclear fission.
The height of the fission barrier vanishes at 2E n - E k = 0 (7.5). In this case
2E p / E k = 2 (a 2 A) / (a 3 Z 2),
Z 2 / A = 2a 2 / (a 3 Z 2) ≈ 49.
Thus, according to the droplet model, nuclei with Z 2 / A> 49 cannot exist in nature, since they should spontaneously split into two fragments in a characteristic nuclear time of the order of 10 –22 s almost instantaneously. 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 Fig. 7.3.
Rice. 7.3. Radial dependence of the shape and height of the potential barrier and the fission energy E for different values of the parameter Z 2 / A. The value of E p + E k is plotted on the vertical axis.
Spontaneous fission of nuclei 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 up to 0.3 s for 260 Rf.
Forced fission of nuclei 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 a compound nucleus E * formed during the capture of a neutron is equal to the binding energy of a neutron in this nucleus ε n. Table 7.1 compares the barrier height H and the neutron binding energy ε n for the isotopes Th, U, Pu, 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 |
|---|---|---|---|
| 232 Th | 5.9 | 233 Th | 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 usually 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 fragments from fission of 235 U 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 zero. In the fission of 235 U by thermal neutrons, the probability of symmetric fission is approximately three orders of magnitude less 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 magic skeleton.
The ratio of the number of neutrons to the number of protons in the 235 nucleus is U N / Z = 1.55, while for stable isotopes having a mass number close to the mass number of fragments, this ratio is 1.25 - 1.45. Consequently, the fission fragments turn out to be strongly overloaded with neutrons and should be
β - radioactive. Therefore, fission fragments undergo successive β - decays, and the charge of the primary fragment can vary by 4 - 6 units. Below is a typical chain of radioactive decays of 97 Kr, one of the fragments formed during the fission of 235 U:
The excitation of fragments caused by the violation of the ratio of the number of protons and neutrons, 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 act. Their energy spectrum is continuous with a maximum of about 1 MeV. The average prompt neutron energy 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.
With 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) - about 67 MeV. Thus, the total kinetic energy of the fragments is ≈ 167 MeV. The total fission energy in this case is 200 MeV. Thus, the remaining energy (33 MeV) is distributed between 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 various products during fission of 235 U by thermal neutrons is given in Table 7.2.
Table 7.2
Fission energy distribution 235 U thermal neutrons
Fission products (NPPs) 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, 7, 49 and 343 days after the explosion, the NPP activity decreases by 10, 100, and 1000 times, respectively, in comparison with the activity an hour after the explosion. The yield of the most biologically significant radionuclides is shown in Table 7.3. In addition to the NPP, 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 unseparated part of uranium and plutonium. The role of induced activity in thermonuclear explosions is especially great.
Table 7.3
The release of some fission products in a nuclear explosion
| Radionuclide | Half life | Yield per division,% | Activity per 1 Mt, 10 15 Bq |
|---|---|---|---|
| 89 Sr | 50.5 days | 2.56 | 590 |
| 90 Sr | 29.12 years | 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 |
| 136 Cs | 13.2 days | 0.036 | 32 |
| 137 Cs | 30 years | 5.57 | 5.9 |
| 140 Ba | 12.8 days | 5.18 | 4700 |
| 141 Cs | 32.5 days | 4.58 | 1600 |
| 144 Cs | 288 days | 4.69 | 190 |
| 3 H | 12.3 years | 0.01 | 2.6 · 10 -2 |
During nuclear explosions in the atmosphere, a significant part of the precipitation (in ground-based explosions up to 50%) falls near the test area. Some of the radioactive substances are retained in the lower part of the atmosphere and, under the influence of the wind, move over long distances, remaining at approximately 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 emitted into the stratosphere (to an altitude of 10-15 km), where they are scattered globally and largely decayed.
Various structural elements of nuclear reactors have been highly active for decades (Table 7.4)
Table 7.4
Specific activity values (Bq / t of 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 |
| 89 Sr | 4. 04· 10 16 | 3. 98· 10 16 | 5. 78· 10 15 | 2. 7· 10 14 |
1. 2· 10 10 |
|
| 90 Sr | 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 | |
| 95 Nb | 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 | ||
| 134 Cs | 7. 50· 10 15 | 7. 50· 10 15 | 6. 71· 10 15 | 5. 36· 10 15 | 2. 73· 10 15 | 2. 6· 10 14 |
| 137 Cs | 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 | ||
| 140 La | 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 uranium nuclei are bombarded 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 Ghana // Kvant. - 2001. - No. 4. - S. 20-21.30.
Droplet kernel model
This fission reaction can be explained based on the droplet 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, the protons experience additional electrostatic repulsion, as a result of which they are located at the periphery of the nucleus. In an unexcited state, the forces of electrostatic repulsion are compensated, therefore the core has a spherical shape (Fig. 1, a).
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 between all nucleons, and the intermediate nucleus itself is deformed and begins to vibrate. If the excitation is small, then the nucleus (Fig. 1, b), freeing itself from excess energy by emission γ -quantum or neutron, returns to a steady state. If the excitation energy is high enough, then the deformation of the nucleus during oscillations can be so large that a constriction is formed in it (Fig. 1, c), similar to the constriction between two parts of a bifurcated liquid droplet. Nuclear forces acting in a narrow waist can no longer withstand the significant Coulomb repulsion force of parts of the nucleus. The constriction breaks, and the nucleus splits into two "fragments" (Fig. 1, d), which fly away 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 are:
\ (~ ^ (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 neutron-initiated fission produces new neutrons that can trigger fission reactions in other nuclei. 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 the kinetic energy of the fragments; the remaining 20% is accounted for by the energy of radioactive radiation from 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. Specific binding energy of nucleons in nuclei with mass number A≈ 240 of the order of 7.6 MeV / nucleon, while in nuclei with mass numbers A= 90 - 145 the specific energy is approximately equal to 8.5 MeV / nucleon. Consequently, 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 in the combustion of 3 tons of coal or 2.5 tons of oil.
see also
- A.A. Varlamov Drip model of the nucleus, Kvant. - 1986. - No. 5. - P. 23-24
Chain reaction
Chain reaction- a nuclear reaction in which the particles causing the reaction are formed as products of that reaction.
When the uranium-235 nucleus fission, which is caused by a collision with a neutron, 2 or 3 neutrons are released. Under favorable conditions, these neutrons can enter other uranium nuclei and cause their fission. At this stage, from 4 to 9 neutrons will appear, capable of causing new decays of uranium nuclei, etc. Such an avalanche-like process is called a chain reaction. A diagram of 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 with 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 with only fast neutrons with energies of the order of 1 MeV. Otherwise, the excitation energy of the formed nuclei \ (~ ^ (239) _ (92) U \) turns out to be insufficient for fission, and then instead of fission, nuclear reactions take place:
\ (~ ^ (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 isotope of neptunium \ (~ ^ (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 fissions 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. Under real conditions, not all neutrons produced during fission participate in the fission of other nuclei. Some of them are captured by the non-fissioning nuclei of foreign atoms, while others fly out of the uranium outward (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, causing the fission of nuclei 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 matter, on geometric shape the volume occupied by it. 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 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 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 D 2 O. When interacting with neutrons, ordinary water itself turns into heavy water.
Graphite, the nuclei of which does not absorb neutrons, is also a good moderator. In 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 kept constant. Such a regime 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 value, then the multiplication factor is TO> 1 and each new generation of neutrons causes all more divisions. The chain reaction is growing like an avalanche and has the character of an explosion, accompanied by a huge release of energy and an increase in the ambient temperature up to several million degrees. A chain reaction of this kind occurs when an atomic bomb explodes.
Nuclear bomb
In its 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 causing a nuclear explosion cannot be sustained under such conditions. However, if the fragments of a nuclear charge are combined together, then their total mass will become sufficient for the uranium fission chain reaction 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 size, 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 a uranium nucleus;
- consider the condition for the occurrence of a chain reaction;
- find out the factors influencing the course of the chain reaction;
- develop the speech and thinking of students;
- develop the ability to analyze, control and correct their own activities within a given time.
Equipment: computer, projection system, didactic material (test “Core composition”), discs “Interactive course. Physics 7-11kl ”(Fizikon) and“ 1C-repeater. Physics ”(1C).
Course of the lesson
I. Organizational moment (2 ').
Greetings, announcement of the lesson plan.
II. Repetition of previously studied material (8 ').
Students' independent work - performing the test ( Annex 1 ). The test must indicate one correct answer.
III. Study of new material (25 '). In the course of the lesson, we make a synopsis(Appendix 2 ).
We recently learned that some chemical elements are converted into other chemical elements during radioactive decay. What do you think will happen if a particle is sent to the nucleus of an atom of a certain chemical element, well, for example, a neutron to the nucleus of uranium? (listening to students' suggestions)
Let's check your assumptions (working with the interactive model "Nucleus fission"“Interactive course. Physics 7-11kl " ).
What happened as a result?
- When a neutron enters 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 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 becomes excited and begins to deform. Why is the core bursting into 2 parts? Under the influence of what forces does the rupture occur?
What forces are at work inside the core?
- Electrostatic and nuclear.
Okay, how do electrostatic forces manifest themselves?
- Electrostatic forces act between charged particles. In the nucleus, the charged particle 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 - forces of attraction between all nucleons.
So, under the influence of what forces does the nucleus burst?
- (If difficulties arise, I ask leading questions and lead the students to the correct conclusion) Under the influence of electrostatic repulsive forces, the nucleus breaks into two parts, which fly in different directions and emit 2-3 neutrons.
The fragments are scattered at a very high speed. It turns out that part of the internal energy of the nucleus is converted into the kinetic energy of the scattering fragments and particles. Shards fall into environment. What do you think is happening to them?
- Fragments are inhibited in the environment.
In order not to violate the law of conservation of energy, we must say what will happen to kinetic energy?
- The kinetic energy of the fragments is converted into internal energy Wednesday.
Can you notice that the internal energy of the environment has changed?
- Yes, the environment is heating up.
And whether the factor that will participate in the division will 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 uranium environment increases.
From the course of chemistry, you know that reactions can occur with both the absorption of energy and the release. What can we say about the course of the fission reaction of uranium nuclei?
- 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 available 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 would happen to the shards how will neutrons behave?
– (listening to the students' assumptions, testing the assumptions, working with the interactive model "Chain Reaction"“1C-repeater. Physics" ).
True, neutrons on their way can meet uranium nuclei and cause fission. This reaction is called a chain reaction.
So what is the condition for a chain reaction?
- The 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 over time. What can this lead to?
- To the explosion.
- The number of nuclear fissions and, accordingly, the energy released per unit of time is increasing.
But another option is also possible, in which the number of free neutrons decreases with time, the nucleus did not meet a neutron on its way. In this case what will happen 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 so that the number of neutrons remains constant over time.
How can we ensure that the number of neutrons remains constant all the time?
- (suggestions from the guys)
To solve this problem, you need 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 uranium mass ... The point 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 its way, cause its fission and thus give rise to a new generation of neutrons necessary for the continuation of the reaction. In this case, the chain reaction will stop. In order for the reaction not to stop, it is necessary to increase the mass of uranium to a certain value, called critical.
Why does a chain reaction become possible with increasing mass?
- The greater the mass of the piece, the greater the likelihood of collisions of neutrons 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 produced by fission of nuclei becomes equal to the number of lost neutrons (i.e., captured by nuclei without fission and escaped from the lump).
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 uranium mass is greater than the critical one?
- As a result of a sharp increase in the number of free neutrons, the chain reaction leads to an explosion.
And if less than 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 ... For this, a piece of uranium is placed in a shell made of a substance that reflects neutrons well (for example, beryllium). Reflecting from this shell, neutrons return to uranium and can take part in nuclear fission.
In addition to 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 influencing the course of the reaction is Availability in uranium of the so-called neutron moderator ... The fact is that uranium-235 nuclei are most likely to fission under the influence of slow neutrons. And during fission of nuclei, fast neutrons are formed. If fast neutrons are slowed down, then most of them will be captured by uranium-235 nuclei with the 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. Moreover, its radius is only 9 cm, since uranium has a very high density.
Using a moderator and a reflective shell, and 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, rapidly moving protons are formed. This meant that the nucleus of the nitrogen isotope, as a result of the collision with the alpha particle, was converted into the nucleus of the oxygen isotope:
.
Nuclear reactions can proceed with the release or absorption of energy. Using the law of interrelation of mass and energy, the energy yield of a nuclear reaction can be determined by finding the difference in the masses of the particles entering into the reaction and the reaction products:
Fission chain reaction of uranium nuclei. Among the various nuclear reactions, fission chain reactions of some heavy nuclei are of particular importance in the life of modern human society.
The fission reaction of uranium nuclei 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 enters the uranium nucleus, the nucleus is divided into two or three parts.
Fission of one uranium nucleus releases about 200 MeV of energy. The kinetic energy of the movement of nuclei-fragments accounts for about 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 yield during the fission of all nuclei of 1 kg of uranium is 80 thousand billion joules. This is several million times more than 1 kg is released when burning. coal or oil. Therefore, a search was undertaken for ways to release nuclear energy in significant quantities for practical use.
F. Joliot-Curie was the first to suggest the possibility of nuclear chain reactions in 1934. In 1939, together with H. Halban and L. Kovarski, he experimentally discovered that in the fission of a uranium nucleus, in addition to nuclear fragments, 2 -3 free neutrons. Under favorable conditions, these neutrons can enter other uranium nuclei and cause their fission. When fission of three uranium nuclei, 6-9 new neutrons should be released, they will enter new uranium nuclei, etc. A diagram of 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 that simple task how it looks in the diagram. The neutrons released during the fission of uranium nuclei are capable of causing fission of only 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, the share of uranium with a mass number of 238 is 99.8%, and the share of uranium with a mass number of 235 is only 0.7%. Therefore, the first possible way the implementation of a chain fission reaction is associated with the separation of uranium isotopes and obtaining in pure form in a sufficiently large quantities isotope. A necessary condition for 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 slightly less critical mass. The chain reaction in each of them separately cannot go. At fast connection a chain reaction develops from these pieces and colossal energy is released. The temperature of uranium reaches millions of degrees, uranium itself and any other substances in the vicinity turn into vapor. The incandescent 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 uranium fission 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. In this case, their velocities are equal to about 107 m / s, therefore they are called fast neutrons. At these energies, neutrons interact with uranium and uranium nuclei 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 the nuclei of uranium-238, but in the nuclei of uranium-235 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
Deceleration of neutrons occurs 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 atomic nuclei of the moderating 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 retarders are plain water and graphite.
The space in which the chain reaction takes place is called the reactor core. To reduce neutron leakage, the reactor core is surrounded by a neutron reflector, which rejects a significant part of the emitted neutrons into the core. The reflector is usually the same substance that serves as a moderator.
The energy released during the operation of the reactor is removed with the help of 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 heat carrier, sometimes used carbon dioxide and even liquid metallic sodium.
The reactor is controlled using special control (or control) rods introduced into the reactor core. The control rods are made of 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. By absorbing a significant part of the neutrons, they make it impossible for the development of a chain reaction. To start the reactor, the control rods are gradually removed from the core until the energy release reaches a predetermined level. With an increase in power above the set level, the automata are switched on, immersing the control rods deep into the core.
Nuclear energy. For the first time in our country, nuclear energy was put into the service of the world. The first organizer and leader of work on atomic science and technology in the USSR was Academician Igor Vasilievich Kurchatov (1903-1960).
Currently, the largest in the USSR and Europe, the Leningrad NPP im. IN AND. Lenin has a capacity of 4000 MW, i.e. 800 times the capacity 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 power is developing at an accelerated pace.
Nuclear reactors are used as power plants in sea ships... The world's first peaceful nuclear powered ship, the Lenin nuclear-powered icebreaker, 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 fission reactions of heavy nuclei, but also in reactions involving 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, a thermonuclear fusion reaction was carried out in experimental thermonuclear explosions.
The synthesis of helium from a light isotope of hydrogen occurs at a temperature of about 108 K, and for the synthesis of helium from heavy isotopes of hydrogen - deuterium and tritium - according to the scheme
requires heating to about 5 10 7 K.
When 1 g of helium is synthesized from deuterium and tritium, an energy of 4.2 · 10 11 J is released. Such energy is released when burning 10 tons of diesel fuel.
The reserves of hydrogen on Earth are practically inexhaustible, therefore the use of the energy of thermonuclear fusion for peaceful purposes is one of the most important tasks. modern science and technology.
A controlled thermonuclear reaction for the 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. On the experimental setup "Tokamak-10" 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 containing a mixture of heavy isotopes of deuterium and tritium. Plasma with a temperature of several tens of millions of degrees has already been obtained in experiments on laser installations.
